Identifying bore scoring words
Discussion
Because so many specialists (and some privateers) now have boroscopes and owners are keen to find out about their own engine’s condition – some understandable confusion has cropped up over interpretation about bore scoring (where the word “scoring” is a dictionary definition taken to mean “a deep line cut into a surface”). To try and clear this up first we need to explain some different types of failures. This is our interpretation and explanation after numerous tests, measurements, observations and correlations over many decades and fits all the evidence we have of a great number of failures – but may not be the full story. (N.b. Photos attached to demonstrate differences).
(1) Seizing-up. This is the traditional full seizure cause by the piston expanding more than the cylinder diameter (usually caused by weak mixtures, lack of coolant, ignition problems, blocked exhausts etc).
It was very common in 2 strokes in the mid ‘70’s and ‘80’s. When the piston expands so big it tries to be bigger than the cylinder diameter it is running in - the extreme pressure breaks through the oil film leaving alloy material pressing hard against any bore material and rubbing up and down so fast the piston surface micro-melts and cools raising bumps that can micro weld for a few milliseconds to the piston and/or cylinder wall and the rough surface that results tears into both the piston and cylinder bore leaving vertical grooves on both sides of the cylinder (although you can only see those in the cylinder wall with a boroscope as it requires stripping to inspect the piston surface).
Similar to pot holes in road surfaces - the lumps elongate the grooves and dislodge more particles to join the damaged lumps making things worse. They usually go from OK to fully seized in less than 3 seconds and leave several grooves or scores in the surface on both sides of the cylinder bore.
(2) Scuffing. This occurs when a piston is travelling slowly but with a high force (in an engine delivering high torque - typically big diesel ships engines trying to disembark where revs must be kept low to avoid the propeller cavitating but torque must be high to get the mass moving – and also known in some large capacity 4 stroke thumpers like Ducati’s).
When a piston converts the linear movement of the piston to rotating movement in the crankshaft the force caused by the angle of the rod connecting them together pushes the piston into the cylinder wall on the aptly named “thrust side”. The oil film present then gets squeezed out during the stroke until the piston changes direction and encounters more fresh oil.
When the piston is moving faster (at higher revs) it has more momentum so there is less inclination to press into the cylinder wall preferring instead to continue straight movement and because the piston is barrel shaped - the oil film acts a bit like a surf-board allowing that oil film to roll-up on the leading edge and stay between the piston and the cylinder bore. Oil viscosity measurement is time based and so the slower the piston is travelling (i.e. low revs) the longer time there is for the oil film to be squeezed out leaving metal to metal contact under load. This will often leave a thin patch of alloy stuck on an iron cylinder wall but the piston is often not hot enough to melt the surface and does not always scores the bores.
If it does score the bore it will only be on the thrust side since the piston is still smaller than the bore and it is under no load on the other side where the oil has no pressure to squeeze it out of contact – it only scores as a result of the resolution of forces back from the con-rod angle pushing the piston against the cylinder wall.
(3) Scoring. This is a relatively new phenomenon particularly in the engines with Lokasil bores so we need to discuss the differences to understand cause and effect.
Because the thermal expansion of pistons is a function of the diameter – bigger pistons have greater clearances cold to hot than smaller ones and since oil film thicknesses are the same for both – cold to hot clearances can get too extreme in large bore engines – and since iron has a lower expansion rate - large cold clearances were needed to ensure sufficient clearance when hot that were noisy and did not run smoothly until pistons had expanded up to size during fast road work or racing. So around 45 years ago alloy cylinders (that expand and contract more similarly to alloy pistons than iron bores) were developed that were lighter, cheaper and had better heat transfer qualities with one weakness – the cylinder bores were too soft. So silicon was mixed with the alloy in such proportions that some of it could not be absorbed in the mix and grew into evenly spaced small very well bonded hard silicon particles throughout the crankcase casting – creating “ALUSIL”.
Because the small surface particles were so hard – and because after honing there were always some particles almost machined completely away and hardly held in the surface that easily broke loose initially - pistons only worked if they had a hard iron coating electroplated onto them (so any loose silicon particles could not impinge into the piston surface and were small enough to escape before causing any damage). Alusil worked very well (924S, 944, 944 Turbo, 968) the only problem was the cost of machining the crankcase with hard silicon particles everywhere so Lokasil was an attempt to locate silicon (Local – silicon or Lokasil) just at the cylinder walls. Larger silicon particles (than in Alusil) were pre-formed into a porous tube (looking like a rough cast in iron liner) and similarly cast into the cylinder block - except the liner was very porous and the molten aluminium flowed under pressure into the voids to entrap the silicon just near the cylinder bore area. A great idea – if it worked. We think the variation in silicon distribution that resulted was insufficiently even to be foolproof in every example.
Early 96mm bore engines suffered from bore ovality cracking cylinders because the silicon pre-formed area of the bore was not as strong as Alusil and neither was the outer tube of pure alloy surrounding it so when the 93mm bore Boxster S casting was bored out 3mm larger (without increasing the O/D) the cylinders were weaker. Later engines probably had a different silicon mix (to make them stronger) and that probably included even larger silicon particles. Unfortunately at the same time the hard iron piston electroplating process was apparently banned on health and safety grounds in Europe so piston coatings became softer plastic instead.
We believe scoring occurs when small (but larger than previously) particles of silicon break free from the surface of the cylinder bore and become trapped between the piston and the cylinder bore, rubbing up and down between them until they either escape from the top or bottom of the piston to be taken away with the oil before it has done any damage or gets stuck on the piston surface to develop a score and at the same time knock a few more particles loose in a typical “catastrophe theory” knock-on effect creating larger scores and more free particles.
We think that sometimes the particle can sit in the groove it has created causing no further damage for a while, or become imbedded deeper in the piston surface so it no longer “sticks out” – again delaying further damage until another particle becomes loose.
Like scuffing – this only ever occurs on the thrust side of the piston (because it relies on the pressure under load to press the particles into the surfaces while the oil film is thin) and because the other side of the piston is unmarked they can be driven for many mils during early bore scoring – eventually suffering high oil consumption and/or knocking noises as the piston rocks too much at TDC.
Contributory factors are therefore high loads at lower speeds, thin oil (by choice or high temperatures), higher mileage examples (where the oil has gradually washed away some surface alloy exposing more silicon particles), increased bore clearances (due to increasing ovality resulting in pistons knocking and rocking into the loaded surface freeing up particles with more momentum and squeezing the oil out faster), uneven distribution of silicon particles during manufacture (resulting in some areas have more or less silicon in some areas), poor bonding of the silicon particles in the substrate alloy in some areas.
It occurs on bank 2 long before bank 1 where the main difference between banks is that the coolest coolant enters the bottom of both banks but the thrust surface is on the bottom of bank 1 (where it will be cooler) and the top of bank 2 (where our tests shows it is hotter – especially during heat soak while stationary after a run).
Hotter means the oil must be thinner on the bank 2 thrust face and therefore more likely to suffer the consequences of loose silicon particles than in bank 1 where the oil will be thicker and form a thicker layer in which the particles can float and/or escape before causing too much damage.
Our interpretation fits all our evidence as follows.
(a) Bank 2 suffers first and is hotter.
(b) Variation in silicon distribution shown by some cylinders in which the oil has washed away patches of the surface.
(c) Different distribution shown by some areas of the bore being different sizes than other where the hones have more easily removed material.
(d) Our hard black “diamond like” coated test pistons clearly showing minute score lines of free silicon particles in the shiny black surface.
(e) The fact that hard iron coated pistons resisted scoring but softer plastic coated ones did not.
(f) That some leading specialist piston manufacturers that use plastic coatings now exclude their use in Lokasil and Alusil.
(g) Manufacturers own publicity differentiates the silicon particle size in earlier and later production types.
(h) Our internal tests on Alusil, alloy and Lokasil showed up the Lokasil as susceptible to crumbling under pressure, releasing surface particles. For this reason we do not plate Nikasil onto failed scored Lokasil bores in case the scoring damaged the underlying substrate or the pressure of thrust loads gradually breaks down the substrate that could eventually result in the Nikasil becoming detached.
(4) Nikasil is completely different to any other cylinder bore. It is an electroplated surface bonded very well to an aluminium cylinder wall forming a thin tube of homogeneous material that will not crumble or release particles – and is oleophilic. It is also very hard and difficult for anything to score into it’s surface. It is however very reflective and shows up polishing lines that have no depth (and are therefore by definition not scoring) and are caused by piston rings bedding in.
All surfaces (even finely ground ones) under enough magnification look like rough saw tooth like edges and how “smooth” something is becomes largely a matter of how much magnification is involved.
If piston rings had chamfered edges they would not work effectively but minute surface differences do initially rub on the cylinder bore slightly differently in some areas compared to others. Because the rings are not perfectly round (under microscopic measurement) performance for many years benefits from “running in” (just like many other parts of an engine and for the same reasons) but on the highly reflective bore surface this can show up as polished lines of no depth and with no deterioration of the surface – just an ability to reflect light differently making them look like scoring lines.
To help “run-in” rings the Nikasil surface (like other bore materials) needs some roughness created by honing cross hatch lines that retain oil and help that initial running in period minutely wear the ring into a compatible shape where they fit each other perfectly.
The evidence of “scoring” would therefore be if the surface had deeper grooves that cut deeper under the reflective surface or the honing marks (which would be scoring) or just simply looked different but had no depth (which would not).
(5) Gen 2 9A1 seizing of Alusil blocks.
Because the failures we have seen with this model were caused by the bore shrinking and reducing the piston clearance they create full seizures marking both sides of the piston and cylinder bore.
The difference with the GT3, Turbo and Hartech Nikasil plated cylinders is that they are plated onto strong dense aerospace alloy thick tubes (that have no surface porosity therefore support the plated Nikasil prefectly) forming a reliable hard naturally lubricated surface that holds shape, hardly ever wears and expands and contracts more in keeping with the piston enabling the piston clearances to remain tight during different running conditions and temperatures resulting in engines that can run well at both modest and high power outputs. The only downside is when the polishing marks in the cylinder bores result in the inexperienced using boroscopes interpreting them as bore scoring.
Thin alloy Nikasil plated dry liner tubes fitted into a hole machined in the original bore surface can suffer from differences in the fit and differential expansion that sometimes cause them to move or become lose.
Hartech Nikasil plated cylinders are thicker and are wet liners so do not incur any fitting tolerance problems and have full contact with coolant while are thick enough to retain roundness and size (and also incorporate ribbed exteriors to increase surface area to aid cooling rates).
CONCLUSIONS.
Generally Early Lokasil engines (from 2.5 Boxsters up to and including Boxster S 3.2’s) had hard iron coated pistons and thick cylinders so rarely cracked or scored and were actually reliable.
Early Lokasil 996 3.4’s also had iron coated pistons but thinner cylinder walls and eventually often cracked.
Later Lokasil 996 and 997 engines had thinner cylinders as well but stiffer (due to larger silicon particles and a different mix) and went oval more slowly and rarely cracked but had plastic coated pistons that didn’t resist the impact of any released (probably larger) silicon particles and scored.
Baz
(1) Seizing-up. This is the traditional full seizure cause by the piston expanding more than the cylinder diameter (usually caused by weak mixtures, lack of coolant, ignition problems, blocked exhausts etc).
It was very common in 2 strokes in the mid ‘70’s and ‘80’s. When the piston expands so big it tries to be bigger than the cylinder diameter it is running in - the extreme pressure breaks through the oil film leaving alloy material pressing hard against any bore material and rubbing up and down so fast the piston surface micro-melts and cools raising bumps that can micro weld for a few milliseconds to the piston and/or cylinder wall and the rough surface that results tears into both the piston and cylinder bore leaving vertical grooves on both sides of the cylinder (although you can only see those in the cylinder wall with a boroscope as it requires stripping to inspect the piston surface).
Similar to pot holes in road surfaces - the lumps elongate the grooves and dislodge more particles to join the damaged lumps making things worse. They usually go from OK to fully seized in less than 3 seconds and leave several grooves or scores in the surface on both sides of the cylinder bore.
(2) Scuffing. This occurs when a piston is travelling slowly but with a high force (in an engine delivering high torque - typically big diesel ships engines trying to disembark where revs must be kept low to avoid the propeller cavitating but torque must be high to get the mass moving – and also known in some large capacity 4 stroke thumpers like Ducati’s).
When a piston converts the linear movement of the piston to rotating movement in the crankshaft the force caused by the angle of the rod connecting them together pushes the piston into the cylinder wall on the aptly named “thrust side”. The oil film present then gets squeezed out during the stroke until the piston changes direction and encounters more fresh oil.
When the piston is moving faster (at higher revs) it has more momentum so there is less inclination to press into the cylinder wall preferring instead to continue straight movement and because the piston is barrel shaped - the oil film acts a bit like a surf-board allowing that oil film to roll-up on the leading edge and stay between the piston and the cylinder bore. Oil viscosity measurement is time based and so the slower the piston is travelling (i.e. low revs) the longer time there is for the oil film to be squeezed out leaving metal to metal contact under load. This will often leave a thin patch of alloy stuck on an iron cylinder wall but the piston is often not hot enough to melt the surface and does not always scores the bores.
If it does score the bore it will only be on the thrust side since the piston is still smaller than the bore and it is under no load on the other side where the oil has no pressure to squeeze it out of contact – it only scores as a result of the resolution of forces back from the con-rod angle pushing the piston against the cylinder wall.
(3) Scoring. This is a relatively new phenomenon particularly in the engines with Lokasil bores so we need to discuss the differences to understand cause and effect.
Because the thermal expansion of pistons is a function of the diameter – bigger pistons have greater clearances cold to hot than smaller ones and since oil film thicknesses are the same for both – cold to hot clearances can get too extreme in large bore engines – and since iron has a lower expansion rate - large cold clearances were needed to ensure sufficient clearance when hot that were noisy and did not run smoothly until pistons had expanded up to size during fast road work or racing. So around 45 years ago alloy cylinders (that expand and contract more similarly to alloy pistons than iron bores) were developed that were lighter, cheaper and had better heat transfer qualities with one weakness – the cylinder bores were too soft. So silicon was mixed with the alloy in such proportions that some of it could not be absorbed in the mix and grew into evenly spaced small very well bonded hard silicon particles throughout the crankcase casting – creating “ALUSIL”.
Because the small surface particles were so hard – and because after honing there were always some particles almost machined completely away and hardly held in the surface that easily broke loose initially - pistons only worked if they had a hard iron coating electroplated onto them (so any loose silicon particles could not impinge into the piston surface and were small enough to escape before causing any damage). Alusil worked very well (924S, 944, 944 Turbo, 968) the only problem was the cost of machining the crankcase with hard silicon particles everywhere so Lokasil was an attempt to locate silicon (Local – silicon or Lokasil) just at the cylinder walls. Larger silicon particles (than in Alusil) were pre-formed into a porous tube (looking like a rough cast in iron liner) and similarly cast into the cylinder block - except the liner was very porous and the molten aluminium flowed under pressure into the voids to entrap the silicon just near the cylinder bore area. A great idea – if it worked. We think the variation in silicon distribution that resulted was insufficiently even to be foolproof in every example.
Early 96mm bore engines suffered from bore ovality cracking cylinders because the silicon pre-formed area of the bore was not as strong as Alusil and neither was the outer tube of pure alloy surrounding it so when the 93mm bore Boxster S casting was bored out 3mm larger (without increasing the O/D) the cylinders were weaker. Later engines probably had a different silicon mix (to make them stronger) and that probably included even larger silicon particles. Unfortunately at the same time the hard iron piston electroplating process was apparently banned on health and safety grounds in Europe so piston coatings became softer plastic instead.
We believe scoring occurs when small (but larger than previously) particles of silicon break free from the surface of the cylinder bore and become trapped between the piston and the cylinder bore, rubbing up and down between them until they either escape from the top or bottom of the piston to be taken away with the oil before it has done any damage or gets stuck on the piston surface to develop a score and at the same time knock a few more particles loose in a typical “catastrophe theory” knock-on effect creating larger scores and more free particles.
We think that sometimes the particle can sit in the groove it has created causing no further damage for a while, or become imbedded deeper in the piston surface so it no longer “sticks out” – again delaying further damage until another particle becomes loose.
Like scuffing – this only ever occurs on the thrust side of the piston (because it relies on the pressure under load to press the particles into the surfaces while the oil film is thin) and because the other side of the piston is unmarked they can be driven for many mils during early bore scoring – eventually suffering high oil consumption and/or knocking noises as the piston rocks too much at TDC.
Contributory factors are therefore high loads at lower speeds, thin oil (by choice or high temperatures), higher mileage examples (where the oil has gradually washed away some surface alloy exposing more silicon particles), increased bore clearances (due to increasing ovality resulting in pistons knocking and rocking into the loaded surface freeing up particles with more momentum and squeezing the oil out faster), uneven distribution of silicon particles during manufacture (resulting in some areas have more or less silicon in some areas), poor bonding of the silicon particles in the substrate alloy in some areas.
It occurs on bank 2 long before bank 1 where the main difference between banks is that the coolest coolant enters the bottom of both banks but the thrust surface is on the bottom of bank 1 (where it will be cooler) and the top of bank 2 (where our tests shows it is hotter – especially during heat soak while stationary after a run).
Hotter means the oil must be thinner on the bank 2 thrust face and therefore more likely to suffer the consequences of loose silicon particles than in bank 1 where the oil will be thicker and form a thicker layer in which the particles can float and/or escape before causing too much damage.
Our interpretation fits all our evidence as follows.
(a) Bank 2 suffers first and is hotter.
(b) Variation in silicon distribution shown by some cylinders in which the oil has washed away patches of the surface.
(c) Different distribution shown by some areas of the bore being different sizes than other where the hones have more easily removed material.
(d) Our hard black “diamond like” coated test pistons clearly showing minute score lines of free silicon particles in the shiny black surface.
(e) The fact that hard iron coated pistons resisted scoring but softer plastic coated ones did not.
(f) That some leading specialist piston manufacturers that use plastic coatings now exclude their use in Lokasil and Alusil.
(g) Manufacturers own publicity differentiates the silicon particle size in earlier and later production types.
(h) Our internal tests on Alusil, alloy and Lokasil showed up the Lokasil as susceptible to crumbling under pressure, releasing surface particles. For this reason we do not plate Nikasil onto failed scored Lokasil bores in case the scoring damaged the underlying substrate or the pressure of thrust loads gradually breaks down the substrate that could eventually result in the Nikasil becoming detached.
(4) Nikasil is completely different to any other cylinder bore. It is an electroplated surface bonded very well to an aluminium cylinder wall forming a thin tube of homogeneous material that will not crumble or release particles – and is oleophilic. It is also very hard and difficult for anything to score into it’s surface. It is however very reflective and shows up polishing lines that have no depth (and are therefore by definition not scoring) and are caused by piston rings bedding in.
All surfaces (even finely ground ones) under enough magnification look like rough saw tooth like edges and how “smooth” something is becomes largely a matter of how much magnification is involved.
If piston rings had chamfered edges they would not work effectively but minute surface differences do initially rub on the cylinder bore slightly differently in some areas compared to others. Because the rings are not perfectly round (under microscopic measurement) performance for many years benefits from “running in” (just like many other parts of an engine and for the same reasons) but on the highly reflective bore surface this can show up as polished lines of no depth and with no deterioration of the surface – just an ability to reflect light differently making them look like scoring lines.
To help “run-in” rings the Nikasil surface (like other bore materials) needs some roughness created by honing cross hatch lines that retain oil and help that initial running in period minutely wear the ring into a compatible shape where they fit each other perfectly.
The evidence of “scoring” would therefore be if the surface had deeper grooves that cut deeper under the reflective surface or the honing marks (which would be scoring) or just simply looked different but had no depth (which would not).
(5) Gen 2 9A1 seizing of Alusil blocks.
Because the failures we have seen with this model were caused by the bore shrinking and reducing the piston clearance they create full seizures marking both sides of the piston and cylinder bore.
The difference with the GT3, Turbo and Hartech Nikasil plated cylinders is that they are plated onto strong dense aerospace alloy thick tubes (that have no surface porosity therefore support the plated Nikasil prefectly) forming a reliable hard naturally lubricated surface that holds shape, hardly ever wears and expands and contracts more in keeping with the piston enabling the piston clearances to remain tight during different running conditions and temperatures resulting in engines that can run well at both modest and high power outputs. The only downside is when the polishing marks in the cylinder bores result in the inexperienced using boroscopes interpreting them as bore scoring.
Thin alloy Nikasil plated dry liner tubes fitted into a hole machined in the original bore surface can suffer from differences in the fit and differential expansion that sometimes cause them to move or become lose.
Hartech Nikasil plated cylinders are thicker and are wet liners so do not incur any fitting tolerance problems and have full contact with coolant while are thick enough to retain roundness and size (and also incorporate ribbed exteriors to increase surface area to aid cooling rates).
CONCLUSIONS.
Generally Early Lokasil engines (from 2.5 Boxsters up to and including Boxster S 3.2’s) had hard iron coated pistons and thick cylinders so rarely cracked or scored and were actually reliable.
Early Lokasil 996 3.4’s also had iron coated pistons but thinner cylinder walls and eventually often cracked.
Later Lokasil 996 and 997 engines had thinner cylinders as well but stiffer (due to larger silicon particles and a different mix) and went oval more slowly and rarely cracked but had plastic coated pistons that didn’t resist the impact of any released (probably larger) silicon particles and scored.
Baz
I’m a bit unclear about scoring on the gen 2 9A1. Do they ever score in the same manner as gen 1 (particles on thrust face) or is it always due to the bore clearances being tight? I know from talking to you guys on the phone the gen 2 isn’t immune from cylinder scoring.
Once scoring starts how many miles does it take for it to be a real issue that a rebuild is needed?
Once scoring starts how many miles does it take for it to be a real issue that a rebuild is needed?
Lord Gover - sorry about my difficulty attaching potos - they are in a separate post. The first photo is a Lokasil bore score, the second is a 9A1 Gen 2 bore score (you will notice lower down), the third pgoto is looking down a 9A1 Gen 2 bore showing the scoring is a seizure both sides and low down and the final picture is a Lokasil scored plastic coated piston.
Buggyjam - scoring on 9A1 Gen 2 engines we know about or have seen is all down to shrinking bore clearances at the bottom and the photo number 3 shows the thick alloy main bearing supports at the bottom that we think contribute to the shrinkage down there.
Cmoose - all the 3.2 pistons we have seen are ferrous coated. We have had plenty of 3.4's with plastic coated pistons with scoring and Cayman S 3.4's as well.
I think it is important to recognise that for every conclusion we have reached about these engines we have probably investigated and tested several other possibilities before working out the likely explanations. For example we had many different piston coatings tested in engines, stripped and inspected and found that the early ferrous coated pistons (called Ferostan) were 10 to 13 microns thick hard iron over tin (1 to 2 microns thick) and copper, Ferroprint (the screen printed plastic coated pistons) seemed to be a polymer with stainless steel particles 20 microns thick) and we are not sure what Ferrotec is (the latest ferrous coated pistons in the 9A1 Gen 2 engines). We did get some pistons coated in Ferrotec and although better than plastic they didn't survive as well in Lokasil as the original hard iron coated pistons - hence our concern over them in Alusil.
During these tests we also coated lots of pistons in every possible available coating (at huge expense) and had discussions with the Lokasil manufacturers who stated that Lokasil will only work with iron coated pistons. The shiny hard diamond like black coated pistons finally revealed the minute silicon particles that cut tiny easy to see vertical lines in the surface before escaping.
Bearing in mins that the blocks are the same size and cylinder pitch from 2.5 to 3.8 - the space to make the cast in liners diminished with the space for coolant - so it was not possible to make the bigger engines have as thick a cylinder as the earlier engines.
Also the forces on a small engine between the piston and the bore are less and increase both with the weight of the piston, the torque delivered and the greater toque at lower speeds of the bigger engines - so dislodging silicon particles and impacting on the Lokasil surface will be a greater problem as the engines are larger.
If Porsche had fitted the alloy Nikasil plated liners they used in the GT3 and turbo versions there would have been no cylinder issues (as there are not with our similar replacements). if they has aldo fitted the larger IMS bearing the engines would have been fantastic - just small misjudgements perhaps that influenced long term reliability although in terms of numbers - still a very good sports car engine.
We even cut up Lokasil, alusil and the base M996/7 material and tested it finding the strongest and most stable was Alusil and that Lokasil was crushable, stiff and cracked easily under bending. We inspected the distribution of silicon with high resolution microscope cameras and found distribution of silicon less even that in Alusil.
Time seems to have proven that the Ferrotec coating works well in Alusil and that the only problem we have so far encountered has been due to shrinkage at the base where the integral part of the main bearing location connects one side of the bore to the other (see photos).
Baz
Buggyjam - scoring on 9A1 Gen 2 engines we know about or have seen is all down to shrinking bore clearances at the bottom and the photo number 3 shows the thick alloy main bearing supports at the bottom that we think contribute to the shrinkage down there.
Cmoose - all the 3.2 pistons we have seen are ferrous coated. We have had plenty of 3.4's with plastic coated pistons with scoring and Cayman S 3.4's as well.
I think it is important to recognise that for every conclusion we have reached about these engines we have probably investigated and tested several other possibilities before working out the likely explanations. For example we had many different piston coatings tested in engines, stripped and inspected and found that the early ferrous coated pistons (called Ferostan) were 10 to 13 microns thick hard iron over tin (1 to 2 microns thick) and copper, Ferroprint (the screen printed plastic coated pistons) seemed to be a polymer with stainless steel particles 20 microns thick) and we are not sure what Ferrotec is (the latest ferrous coated pistons in the 9A1 Gen 2 engines). We did get some pistons coated in Ferrotec and although better than plastic they didn't survive as well in Lokasil as the original hard iron coated pistons - hence our concern over them in Alusil.
During these tests we also coated lots of pistons in every possible available coating (at huge expense) and had discussions with the Lokasil manufacturers who stated that Lokasil will only work with iron coated pistons. The shiny hard diamond like black coated pistons finally revealed the minute silicon particles that cut tiny easy to see vertical lines in the surface before escaping.
Bearing in mins that the blocks are the same size and cylinder pitch from 2.5 to 3.8 - the space to make the cast in liners diminished with the space for coolant - so it was not possible to make the bigger engines have as thick a cylinder as the earlier engines.
Also the forces on a small engine between the piston and the bore are less and increase both with the weight of the piston, the torque delivered and the greater toque at lower speeds of the bigger engines - so dislodging silicon particles and impacting on the Lokasil surface will be a greater problem as the engines are larger.
If Porsche had fitted the alloy Nikasil plated liners they used in the GT3 and turbo versions there would have been no cylinder issues (as there are not with our similar replacements). if they has aldo fitted the larger IMS bearing the engines would have been fantastic - just small misjudgements perhaps that influenced long term reliability although in terms of numbers - still a very good sports car engine.
We even cut up Lokasil, alusil and the base M996/7 material and tested it finding the strongest and most stable was Alusil and that Lokasil was crushable, stiff and cracked easily under bending. We inspected the distribution of silicon with high resolution microscope cameras and found distribution of silicon less even that in Alusil.
Time seems to have proven that the Ferrotec coating works well in Alusil and that the only problem we have so far encountered has been due to shrinkage at the base where the integral part of the main bearing location connects one side of the bore to the other (see photos).
Baz
Useful and helpful post by an expert genuinely trying to help. A rare thing these days on this forum. Counters the obsession with values and petulant winging about not getting the latest GT wonder car.
So thank you Baz for reminding us all how useful this forum can be to those of us who can be bothered to acually learn something from it
So thank you Baz for reminding us all how useful this forum can be to those of us who can be bothered to acually learn something from it
The cylinder is part of the main crankcase casting. It is like a tube that half way down joins the main crankcase via a ring - all in one piece - leaving both the top and the bottom as round tubes with no support.
There is more load at the top (as the piston force is maximum well before half way down the bore) so the top stretches oval more than the bottom.
However - even new cases have some small ovality all over as a result we presume of stress relieving.
The 996 3.4 engines eventually go up to 10 thou oval at the top gradually reducing to nearer 3 or 4 thou in the middle and after that most are cracked.
The later engines we think have Lokasil 2 material which can have bigger silicon particles, a different mix and are stronger. We don't find they are cracked before they are scored - so do not yet know if they would have eventually cracked if they had not scored.
Baz
There is more load at the top (as the piston force is maximum well before half way down the bore) so the top stretches oval more than the bottom.
However - even new cases have some small ovality all over as a result we presume of stress relieving.
The 996 3.4 engines eventually go up to 10 thou oval at the top gradually reducing to nearer 3 or 4 thou in the middle and after that most are cracked.
The later engines we think have Lokasil 2 material which can have bigger silicon particles, a different mix and are stronger. We don't find they are cracked before they are scored - so do not yet know if they would have eventually cracked if they had not scored.
Baz
hartech said:
Buggyjam - scoring on 9A1 Gen 2 engines we know about or have seen is all down to shrinking bore clearances at the bottom and the photo number 3 shows the thick alloy main bearing supports at the bottom that we think contribute to the shrinkage down there.
Ah, I see on the photo what you mean about the supports.Are there any conclusions as to Driver behaviour that triggers or contributes this in gen 2 or is it just pot luck due to cylinder clearance at manufacture?
I have a gen 2 and have followed the general gen 1 scoring advice of not applying heavy loads from standing starts until well down the road. I also try and give a decent warm up.
Did the scoring on gen 2 follow the same bahaviour pattern as thrust face scoring on gen 1? Eg, XYZ thousand miles later, same symptoms as thrust face scoring?
What differences in the 981 Cayman are there, would they be similarly at risk? From an observer stance seems 2 generations of the 987 which both can score for different reasons, but the common factor seems the bore material.
Edit - just thinking about it, would the most likely scenario for the scoring on gen 2s be when temps are low say winter? The bearing supports look thick, so guessing the piston heats up quicker and expands quicker than the supports can expand to allow the bore to swell slightly in concert with the piston. As a result the clearance shrinks locally where the bore is effectively held back from expanding by the big bit of metal (the supports) taking ages to heat up. Would that be roughly half there or way off?
Edited by Buggyjam on Tuesday 3rd July 14:12
hartech said:
The cylinder is part of the main crankcase casting. It is like a tube that half way down joins the main crankcase via a ring - all in one piece - leaving both the top and the bottom as round tubes with no support.
There is more load at the top (as the piston force is maximum well before half way down the bore) so the top stretches oval more than the bottom.
However - even new cases have some small ovality all over as a result we presume of stress relieving.
The 996 3.4 engines eventually go up to 10 thou oval at the top gradually reducing to nearer 3 or 4 thou in the middle and after that most are cracked.
The later engines we think have Lokasil 2 material which can have bigger silicon particles, a different mix and are stronger. We don't find they are cracked before they are scored - so do not yet know if they would have eventually cracked if they had not scored.
Baz
So looking at those pictures where does the maximum ovality take place in relation to the scoring or does it just happen randomly around the cylinder.There is more load at the top (as the piston force is maximum well before half way down the bore) so the top stretches oval more than the bottom.
However - even new cases have some small ovality all over as a result we presume of stress relieving.
The 996 3.4 engines eventually go up to 10 thou oval at the top gradually reducing to nearer 3 or 4 thou in the middle and after that most are cracked.
The later engines we think have Lokasil 2 material which can have bigger silicon particles, a different mix and are stronger. We don't find they are cracked before they are scored - so do not yet know if they would have eventually cracked if they had not scored.
Baz
If you picture the 2 solid alloy sections at the bottom shrinking it would pull the bores in in the same direction which is the thrust direction.
Pistons are not round but an oval shape often elliptical, therefore as the bore gets smaller in the same direction that the piston is oval you end up with 2 oval shapes getting closer together. The piston is already oval and expanding and the bore was round but is now shrinking oval as well. You end up with 2 oval shapes getting closer together - and because of this the piston may touch first in the centre - or as shown in the picture nearer the 45 degree point around the face - but it will change depending upon how shrunk the bores are and how hot the piston is etc.
Baz
Pistons are not round but an oval shape often elliptical, therefore as the bore gets smaller in the same direction that the piston is oval you end up with 2 oval shapes getting closer together. The piston is already oval and expanding and the bore was round but is now shrinking oval as well. You end up with 2 oval shapes getting closer together - and because of this the piston may touch first in the centre - or as shown in the picture nearer the 45 degree point around the face - but it will change depending upon how shrunk the bores are and how hot the piston is etc.
Baz
hartech said:
If you picture the 2 solid alloy sections at the bottom shrinking it would pull the bores in in the same direction which is the thrust direction.
Pistons are not round but an oval shape often elliptical, therefore as the bore gets smaller in the same direction that the piston is oval you end up with 2 oval shapes getting closer together. The piston is already oval and expanding and the bore was round but is now shrinking oval as well. You end up with 2 oval shapes getting closer together - and because of this the piston may touch first in the centre - or as shown in the picture nearer the 45 degree point around the face - but it will change depending upon how shrunk the bores are and how hot the piston is etc.
Baz
Hi Baz. Pistons are not round but an oval shape often elliptical, therefore as the bore gets smaller in the same direction that the piston is oval you end up with 2 oval shapes getting closer together. The piston is already oval and expanding and the bore was round but is now shrinking oval as well. You end up with 2 oval shapes getting closer together - and because of this the piston may touch first in the centre - or as shown in the picture nearer the 45 degree point around the face - but it will change depending upon how shrunk the bores are and how hot the piston is etc.
Baz
What causes this shrinking that pulls the bores in?
As mentioned before - given its different from gen 1 causes - anything that causes this bore shrinking thats caused by the driver? Or can avoid?
Also, how long once scoring starts in rough miles until problems? Is it same symptoms as gen 1 thrust face particle scoring?
Thanks.
Castings are created by molten metal. Hot metal shrinks as it cools. The outside of the castings shrink before the inside which cools later. When the outside is cool it becomes settled but the inside that is hotter continues to cool and as it does so it continues to shrink. This leaves a stress from the inside to the outside leaving a "pull" inwards.
The parts on the outside are machined, as they are machined that releases the metal on the outside that was trying to be pulled inwards by the internal areas and the internal areas move inwards a bit.
The shape of the casting, the amount that is machined off, the changes in section throughout the casting and the temperature of ay heat/cool cycles afterwards all allow some areas to change shape very slightly over time.
All this is called "age related stress relieving". It is difficult to predict how it will affect castings. Some manufacturing techniques will put castings through heat cool cycles to try and relieve the stresses, sometimes after roughing out the machining the castings are stress relieved before final machining to minimise this stress relief.
If the engines we checked has smaller bores at the bottom as a result of the original machining you would expect the seizure to have occurred earlier in their life. The fact that the failures occurred after several years (and lots of heat/cool cycles) and that some bores were more shrunk than others and because the shrinkage is at the bottom and in line with the large sections you can see at the bottom - a professional opinion would conclude that the cause was indeed stress relieving.
Manufacturers would need to run engines for hundreds of full heat/cool cycles to find out if this might occur and this would takes years and hence is simply not possible to be found out quickly enough to find a solution before selling cars - so sometimes this sort of problem may emerge years later when it is too late to do anything about it.
Of course it could also only affect a small number of castings that for some reason underwent slightly different casting procedures, slightly different stress relieving processes (if indeed that underwent that) and possibly even different machining times etc.
Once the stress is relieved the shape stabilises. It could be that they have all shrunk a little by now but that as long as warming up is done over say 15 minutes before full power in cold climates on really cod days - that none will ever fail - hence our advice.
It could also be that eventually a lot more will suffer - if the shrinkage continues beyond the sizes we measured.
We found that on each bank one cylinder was still round, one was slightly shrunk and one very small as if one end of the casting cooled at a different rate to the other end.
Only time will tell the extent of the problem or if it was an isolated small number.
Baz
The parts on the outside are machined, as they are machined that releases the metal on the outside that was trying to be pulled inwards by the internal areas and the internal areas move inwards a bit.
The shape of the casting, the amount that is machined off, the changes in section throughout the casting and the temperature of ay heat/cool cycles afterwards all allow some areas to change shape very slightly over time.
All this is called "age related stress relieving". It is difficult to predict how it will affect castings. Some manufacturing techniques will put castings through heat cool cycles to try and relieve the stresses, sometimes after roughing out the machining the castings are stress relieved before final machining to minimise this stress relief.
If the engines we checked has smaller bores at the bottom as a result of the original machining you would expect the seizure to have occurred earlier in their life. The fact that the failures occurred after several years (and lots of heat/cool cycles) and that some bores were more shrunk than others and because the shrinkage is at the bottom and in line with the large sections you can see at the bottom - a professional opinion would conclude that the cause was indeed stress relieving.
Manufacturers would need to run engines for hundreds of full heat/cool cycles to find out if this might occur and this would takes years and hence is simply not possible to be found out quickly enough to find a solution before selling cars - so sometimes this sort of problem may emerge years later when it is too late to do anything about it.
Of course it could also only affect a small number of castings that for some reason underwent slightly different casting procedures, slightly different stress relieving processes (if indeed that underwent that) and possibly even different machining times etc.
Once the stress is relieved the shape stabilises. It could be that they have all shrunk a little by now but that as long as warming up is done over say 15 minutes before full power in cold climates on really cod days - that none will ever fail - hence our advice.
It could also be that eventually a lot more will suffer - if the shrinkage continues beyond the sizes we measured.
We found that on each bank one cylinder was still round, one was slightly shrunk and one very small as if one end of the casting cooled at a different rate to the other end.
Only time will tell the extent of the problem or if it was an isolated small number.
Baz
hartech said:
Castings are created by molten metal. Hot metal shrinks as it cools. The outside of the castings shrink before the inside which cools later. When the outside is cool it becomes settled but the inside that is hotter continues to cool and as it does so it continues to shrink. This leaves a stress from the inside to the outside leaving a "pull" inwards.
The parts on the outside are machined, as they are machined that releases the metal on the outside that was trying to be pulled inwards by the internal areas and the internal areas move inwards a bit.
The shape of the casting, the amount that is machined off, the changes in section throughout the casting and the temperature of ay heat/cool cycles afterwards all allow some areas to change shape very slightly over time.
All this is called "age related stress relieving". It is difficult to predict how it will affect castings. Some manufacturing techniques will put castings through heat cool cycles to try and relieve the stresses, sometimes after roughing out the machining the castings are stress relieved before final machining to minimise this stress relief.
If the engines we checked has smaller bores at the bottom as a result of the original machining you would expect the seizure to have occurred earlier in their life. The fact that the failures occurred after several years (and lots of heat/cool cycles) and that some bores were more shrunk than others and because the shrinkage is at the bottom and in line with the large sections you can see at the bottom - a professional opinion would conclude that the cause was indeed stress relieving.
Manufacturers would need to run engines for hundreds of full heat/cool cycles to find out if this might occur and this would takes years and hence is simply not possible to be found out quickly enough to find a solution before selling cars - so sometimes this sort of problem may emerge years later when it is too late to do anything about it.
Of course it could also only affect a small number of castings that for some reason underwent slightly different casting procedures, slightly different stress relieving processes (if indeed that underwent that) and possibly even different machining times etc.
Once the stress is relieved the shape stabilises. It could be that they have all shrunk a little by now but that as long as warming up is done over say 15 minutes before full power in cold climates on really cod days - that none will ever fail - hence our advice.
It could also be that eventually a lot more will suffer - if the shrinkage continues beyond the sizes we measured.
We found that on each bank one cylinder was still round, one was slightly shrunk and one very small as if one end of the casting cooled at a different rate to the other end.
Only time will tell the extent of the problem or if it was an isolated small number.
Baz
Fascinating read. Looks like Porsche never run out of novel new methods to get their bores to go wrong or turn into bananas. The Porsche bore score development lab must have a nice Christmas party.The parts on the outside are machined, as they are machined that releases the metal on the outside that was trying to be pulled inwards by the internal areas and the internal areas move inwards a bit.
The shape of the casting, the amount that is machined off, the changes in section throughout the casting and the temperature of ay heat/cool cycles afterwards all allow some areas to change shape very slightly over time.
All this is called "age related stress relieving". It is difficult to predict how it will affect castings. Some manufacturing techniques will put castings through heat cool cycles to try and relieve the stresses, sometimes after roughing out the machining the castings are stress relieved before final machining to minimise this stress relief.
If the engines we checked has smaller bores at the bottom as a result of the original machining you would expect the seizure to have occurred earlier in their life. The fact that the failures occurred after several years (and lots of heat/cool cycles) and that some bores were more shrunk than others and because the shrinkage is at the bottom and in line with the large sections you can see at the bottom - a professional opinion would conclude that the cause was indeed stress relieving.
Manufacturers would need to run engines for hundreds of full heat/cool cycles to find out if this might occur and this would takes years and hence is simply not possible to be found out quickly enough to find a solution before selling cars - so sometimes this sort of problem may emerge years later when it is too late to do anything about it.
Of course it could also only affect a small number of castings that for some reason underwent slightly different casting procedures, slightly different stress relieving processes (if indeed that underwent that) and possibly even different machining times etc.
Once the stress is relieved the shape stabilises. It could be that they have all shrunk a little by now but that as long as warming up is done over say 15 minutes before full power in cold climates on really cod days - that none will ever fail - hence our advice.
It could also be that eventually a lot more will suffer - if the shrinkage continues beyond the sizes we measured.
We found that on each bank one cylinder was still round, one was slightly shrunk and one very small as if one end of the casting cooled at a different rate to the other end.
Only time will tell the extent of the problem or if it was an isolated small number.
Baz
With a gen 2 do you still have to follow the thrust face scoring advice of taking it easy from a standing start off traffic lights and counting a fair few elephants before giving it a harsher load? When accelerating hard I always ensure the revs are nicely high in a lower gear to try and avoid burying the piston into the thrust face. But I’ve found this one required restriction re standing starts very annoying about owning a Porsche. Being a sports car and all.
Once fully warmed up I can see no reason to avoid full throttle from any revs in a Gen 2.
I should also have replied to an earlier question as follows. Lokasil seems crumbly when scored and therefore seems to make space for the engine to still run - all be it not as well. Alusil is so much more solid all the examples we have seen would be obviously damaged by a seizure and clearly need fixing straight away.
Baz
I should also have replied to an earlier question as follows. Lokasil seems crumbly when scored and therefore seems to make space for the engine to still run - all be it not as well. Alusil is so much more solid all the examples we have seen would be obviously damaged by a seizure and clearly need fixing straight away.
Baz
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