Bore Scoring - which models?
Discussion
A few answers for you.
Low revs means opening the throttle fully at 1,500 to 2000 rpm,
It is possible to plate bores but much more time consuming and expensive - but you cannot plate Lokasil which is the cylinder material involved mainly in scoring which we are really discussing.
The piston rings do touch the bore and seal the pressure while dissipating some heat but it is the action of the piston pushing against the cylinder bore that converts the pressure from burning fuel that is pushing the piston down - into rotary motion through the pressure of the piston pushing against the thrust face of the cylinder wall and the connecting rod feeling that push down onto the crankshaft - resulting in it turning.
It is very like a bicycle - imagine the pedal is not connected to anything - you push down hard on it and it simply presses into the road - but if it is connected to an arm and to the crank - that downward pressure now turns the pedal spindle and in turn the chain. The harder you push down the harder it pushes the crank around.
Older engines only provided good torque at high revs and when pistons are moving faster in the bore - they do not scuff against the cylinder wall as much - but varying the camshaft timing and valve lift has increased the torque at lower revs where scuffing (or scoring) is particularly prone to occur.
The original cast iron cylinders had good qualities of compatibility between the hard surface of the iron and the softer alloy of the piston - but needed larger cold piston clearances and didn't transfer heat as well. Better torque results from tight piston clearances and the reduction in piston blow-by improves emissions (which has become more important).
Plating alloy - initially with chrome (which needed grinding out) and then Nikasil (which could be honed) provided a step forward but was more expensive.
Alusil (in which silicon particles were mixed into the cast alloy) provided a good surface for the cylinder bore with good heat transfer and tight piston clearances - but was found to need a piston coating that was first used I think in the late '70's and for many engines since. It was also wasteful as the silicon was all over the block (when it was only needed at the cylinder wall) and being hard - made machining slower and more expensive.
That piston coating uses materials frowned on by health and safety experts now and so an alternative was sought.
Meanwhile KS came up with this great idea to only place the silicon at the cylinder wall by casting in tubular sleeves containing the high silicon - called Lokasil after "lOKAl (or local) SILicon".
This left the rest of the block just alloy - easy and quicker to machine but still required the piston coating that was moving out of favour.
But Lokasil turned out to be a little more difficult to maintain consistent quality and didn't work as well with the newer piston coatings that replaced the original type.
It seems that in future alloy cylinders will be plasma sprayed and honed and the materials used probably will not require any piston coating or certainly not an expensive one - perhaps just one to help running in etc. So there is no real incentive for manufacturers to solve what is now a short term problem by finding a suitable eco friendly and affordable piston coating and have instead reverted to the Alusil route - probably until plasma spraying has been perfected and production machinery built.
This leaves several years of production of Lokasil bores predictably vulnerable to scoring and because of the open deck design - also of eventual cracking.
Lokasil almost worked wonderfully and it seems still that bank 1 rarely fails - so it seems likely that if a few changes can be made to the coolant flow, temperature and the open deck is closed - that bank 2 should also be much improved (and this is what we do and seems to provide a perfect answer).
However there is nothing better than Nikasil and so that is how we repair damaged bores and cracked cylinders and why many customers prefer to change the whole engine with all 6 cylinders for a cost that we manage to keep little more than steel liners that are inferior.
I think - if I had to try and put a simple perspective on things - at the same time that Porsche engines produced much more torque at low revs than they ever did before - they also tried to cut costs by changing a previously successful but expensive block and piston coating system to something that on paper and under test seemed to provide a perfect answer but that in practice and over a period of time - with fluctuations in quality control and driver styles and maintenance etc - some just got too close to the failure point and the convergence of a less perfect piston to bore surface material, temperature and lubrication with simultaneous large increases in the loads that the system has to deal with - only just went too far in some examples and they failed.
I think all of this is explained on our web site so can I respectfully suggest those still interested read all that now before asking more questions that are probably already answered there.
Baz
Low revs means opening the throttle fully at 1,500 to 2000 rpm,
It is possible to plate bores but much more time consuming and expensive - but you cannot plate Lokasil which is the cylinder material involved mainly in scoring which we are really discussing.
The piston rings do touch the bore and seal the pressure while dissipating some heat but it is the action of the piston pushing against the cylinder bore that converts the pressure from burning fuel that is pushing the piston down - into rotary motion through the pressure of the piston pushing against the thrust face of the cylinder wall and the connecting rod feeling that push down onto the crankshaft - resulting in it turning.
It is very like a bicycle - imagine the pedal is not connected to anything - you push down hard on it and it simply presses into the road - but if it is connected to an arm and to the crank - that downward pressure now turns the pedal spindle and in turn the chain. The harder you push down the harder it pushes the crank around.
Older engines only provided good torque at high revs and when pistons are moving faster in the bore - they do not scuff against the cylinder wall as much - but varying the camshaft timing and valve lift has increased the torque at lower revs where scuffing (or scoring) is particularly prone to occur.
The original cast iron cylinders had good qualities of compatibility between the hard surface of the iron and the softer alloy of the piston - but needed larger cold piston clearances and didn't transfer heat as well. Better torque results from tight piston clearances and the reduction in piston blow-by improves emissions (which has become more important).
Plating alloy - initially with chrome (which needed grinding out) and then Nikasil (which could be honed) provided a step forward but was more expensive.
Alusil (in which silicon particles were mixed into the cast alloy) provided a good surface for the cylinder bore with good heat transfer and tight piston clearances - but was found to need a piston coating that was first used I think in the late '70's and for many engines since. It was also wasteful as the silicon was all over the block (when it was only needed at the cylinder wall) and being hard - made machining slower and more expensive.
That piston coating uses materials frowned on by health and safety experts now and so an alternative was sought.
Meanwhile KS came up with this great idea to only place the silicon at the cylinder wall by casting in tubular sleeves containing the high silicon - called Lokasil after "lOKAl (or local) SILicon".
This left the rest of the block just alloy - easy and quicker to machine but still required the piston coating that was moving out of favour.
But Lokasil turned out to be a little more difficult to maintain consistent quality and didn't work as well with the newer piston coatings that replaced the original type.
It seems that in future alloy cylinders will be plasma sprayed and honed and the materials used probably will not require any piston coating or certainly not an expensive one - perhaps just one to help running in etc. So there is no real incentive for manufacturers to solve what is now a short term problem by finding a suitable eco friendly and affordable piston coating and have instead reverted to the Alusil route - probably until plasma spraying has been perfected and production machinery built.
This leaves several years of production of Lokasil bores predictably vulnerable to scoring and because of the open deck design - also of eventual cracking.
Lokasil almost worked wonderfully and it seems still that bank 1 rarely fails - so it seems likely that if a few changes can be made to the coolant flow, temperature and the open deck is closed - that bank 2 should also be much improved (and this is what we do and seems to provide a perfect answer).
However there is nothing better than Nikasil and so that is how we repair damaged bores and cracked cylinders and why many customers prefer to change the whole engine with all 6 cylinders for a cost that we manage to keep little more than steel liners that are inferior.
I think - if I had to try and put a simple perspective on things - at the same time that Porsche engines produced much more torque at low revs than they ever did before - they also tried to cut costs by changing a previously successful but expensive block and piston coating system to something that on paper and under test seemed to provide a perfect answer but that in practice and over a period of time - with fluctuations in quality control and driver styles and maintenance etc - some just got too close to the failure point and the convergence of a less perfect piston to bore surface material, temperature and lubrication with simultaneous large increases in the loads that the system has to deal with - only just went too far in some examples and they failed.
I think all of this is explained on our web site so can I respectfully suggest those still interested read all that now before asking more questions that are probably already answered there.
Baz
The biggest problem in fitting dry sleeves is that the top of the block usually distorted at the age of the car and no longer flat. Dry sleeves will not sit as perfectly flat as they need to be so additional cylinder head sealing via say Wills rings is beneficial. If we did that job (which we are too busy to do right now) we would remove the studs and fit the sleeves permanently and skim the top again but often removing the studs damages the threads and larger replacements are needed - so it becomes quite expensive - and if oversized pistons are available this can be an easier answer.
I know of no problems with Nikasil plated cylinders as the thickenss and quality control is higher than trying to plate into a solid block.
Baz
I know of no problems with Nikasil plated cylinders as the thickenss and quality control is higher than trying to plate into a solid block.
Baz
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