my 996 engine blew up.
Discussion
dave, i talked to several engine rebuilders around here and they all told me that they have had so much problem with rebuilding this particular engine with the standard parts there are no longer doing anymore. two shops told me to buy a new engine and one shop said to go with 4.0 L with what they say "upgraded" parts. that true you think or are they just not wanting to do my engine rebuild???
dave, i talked to several engine rebuilders around here and they all told me that they have had so much problem with rebuilding this particular engine with the standard parts there are no longer doing anymore. two shops told me to buy a new engine and one shop said to go with 4.0 L with what they say "upgraded" parts. that true you think or are they just not wanting to do my engine rebuild???
oh, about the metal... the dealer service tech who called me just told me that the metal is likely due to bearing and just told me large bits. i don't know how to go after the source of the problem other than paying them 1500 bucks to open the engine. i don't want to do that since they told me the engine is no good...
oh, about the metal... the dealer service tech who called me just told me that the metal is likely due to bearing and just told me large bits. i don't know how to go after the source of the problem other than paying them 1500 bucks to open the engine. i don't want to do that since they told me the engine is no good...
Edited by daegucb on Sunday 23 July 21:03
Autofarm have good reputation over here, if i was in your position i would do their 3.9 or 4litre conversion, after all you will have 60 more horses?? thats some power hike, for sensible money as well really.
Now consider this, with the $$$ you will have saved by not paying Porsche to rebuild your car you could easily add an exhaust system and headers.....WOW that would give you a smile for sure!!!!!!!!!!!
390 odd horses!!!!! IMO that would add serious value to your car as well, when you come to sell you would recoup at least 50% of what you pay out now..
Bad luck by the way, but how does the saying go? every cloud has a silver lining!!!
All the best what ever you decide
...rob
Now consider this, with the $$$ you will have saved by not paying Porsche to rebuild your car you could easily add an exhaust system and headers.....WOW that would give you a smile for sure!!!!!!!!!!!
390 odd horses!!!!! IMO that would add serious value to your car as well, when you come to sell you would recoup at least 50% of what you pay out now..
Bad luck by the way, but how does the saying go? every cloud has a silver lining!!!
All the best what ever you decide
...rob
Edited by 911silverback on Sunday 23 July 22:23
The most likely causes of your blow up are as follows.
(1) Intermediate shaft failure (bearing or shaft fracture) = if block OK not too expensive to rebuild unless the engine has the early chain drive when a new crankshaft may also be neccessary to run with the replacement intermediate shaft.
(2) Big end shells and crankshaft damaged - similar problem to above - if an early engine - may need another intermediate shaft as well.
Usually small pices of solid metal are the intermediate shaft bearing and small particles of bronze indicate a big end.
These failures have taken several years to materialise and newer versions may have improved re-designs to cope - so a new engine may be a better option although I think you only get one replacement under the Porsche warranty scheme - if it goes again (perhaps someone could check and confirm this or ot may vary with the Country the car is in).
Without solving the problems that lead to these failures - it seems risky to me to increase the load that the same parts have to handle in future by increasing capacity - but if better parts are replaced at the same time (different big ends etc) then it may be worth considering. Or if extra performance is worth paying for then even if the eventual life is a little less - perhaps it is worth it anyway!
Baz Hartech
(1) Intermediate shaft failure (bearing or shaft fracture) = if block OK not too expensive to rebuild unless the engine has the early chain drive when a new crankshaft may also be neccessary to run with the replacement intermediate shaft.
(2) Big end shells and crankshaft damaged - similar problem to above - if an early engine - may need another intermediate shaft as well.
Usually small pices of solid metal are the intermediate shaft bearing and small particles of bronze indicate a big end.
These failures have taken several years to materialise and newer versions may have improved re-designs to cope - so a new engine may be a better option although I think you only get one replacement under the Porsche warranty scheme - if it goes again (perhaps someone could check and confirm this or ot may vary with the Country the car is in).
Without solving the problems that lead to these failures - it seems risky to me to increase the load that the same parts have to handle in future by increasing capacity - but if better parts are replaced at the same time (different big ends etc) then it may be worth considering. Or if extra performance is worth paying for then even if the eventual life is a little less - perhaps it is worth it anyway!
Baz Hartech
rob and hartech, thanks both of you for the info. but, still can't decide. it seems rebuild should cost less (which i want to do) but i can't seem to find a guy to do it unless they bore to 4.0 L . that costs as much as the new engine and it's done with after market stuff the porsche isn't going to take care of if problem occurs later on. so, if i can find a guy to just get rid of what's wrong/damaged and replace with new porsche parts so i'd keep the same engine and save some money would be great. but, local rebuilders say they don't want to just replace the parts because they had bad experience doing this. i am miffed as to why i can't find a local mechanic who is willing to take care of this problem...
The problem rebuilding the engines comes in different areas.
(1) To do it the way Porsche instruct in their manual requires expensive special tooling and is unfamiliar to tradditonal Porsche technicians. Who will invest in it when the result is being questioned? Is there a better way?
(2) Some parts have been superceeded so have to be replaced - even if they are OK - so they can match up with a part that you did need to replace = unexpected extra costs.
(3) Some parts don't seem to always like being put back together a second time (like the con rods with the fractured joint face).
(4) Some failures are caused by the bores becoming oval and there being too much piston clearance for "thin" oils. The Lokasil preformed - cast in - liners have little tensile strength to resist hoop stress (brittle) so can crack when the unsupported cylinder distorts oval under piston loads plus cylinder pressures. Some of this may well be improved as the new Lokasil technology is no doubt developed over the years - I mean a simple bridge or two - cast into the cylinders opposite the cylinder pressure faces would have cured that particular problem straight away - perhaps something similar is in the pipeline or we may find thicker cylinder walls if they are determined to maintain a free standing cylinder tube (for reducing expansion distortion).
(5) However - you can replace one crankcase half (or both halves) if there is cylinder damage or wear/seizure - (or intermediate shaft bearing location damage) as the crankshaft is supported in a seperate housing and the main bearings are not in the crankcase part that forms the cylinder block. So you could rebuild with new "PORSCHE" parts and have new cylinder bores anyway.
(6) The whole engine rebuild concept (from all manufacturers) is moving towards IT BEING CHEAPER TO REPLACE THAN REBUILD and technology is changing accordingly - influenced by increases in labour rates.
(7) However they must crash Porsches in the USA - so presumably a lower mileage/newer engine should be available from a crashed car breakers yard and could provide a viable escape route.
(8) For any brave and competent USA mechanics/technicians - capable of trying to rebuild these engines - there should be a long term benefit providing they are prepared to work things out for themselves and try different methods and tooling - because we are only dealing with engines that are at most 10 years old and the cars must be in use for many more years than that when residual values (still high with a Porsche compared to family saloons) make less expensive options essential. Boxster and 996 rebuilds are the same and use common parts, tooling and technology. However if no one has done so yet - you would be under increased risk until they have sorted it all out - so perhaps the best options for you - over there - are a used crashed car engine or a new replacement.
(9) Some aspects of the design lend themselves to the engines being rebuildable for many years (like the steel reinforced crankshaft housing - following Japanese technology to stop main bearing creep damaging bearing locations).
(10) Perhaps it is the fear of legal come back if the rebuild fails that puts off potential technicians in the USA - because it is becoming more of a problem in the UK with so many would be lawyers getting their cars repaired all the time and expecting far too much for their money. If it is a "cost less option" then there should be some added risk associated with it.
Good luck making your decision.
Baz Hartech
(1) To do it the way Porsche instruct in their manual requires expensive special tooling and is unfamiliar to tradditonal Porsche technicians. Who will invest in it when the result is being questioned? Is there a better way?
(2) Some parts have been superceeded so have to be replaced - even if they are OK - so they can match up with a part that you did need to replace = unexpected extra costs.
(3) Some parts don't seem to always like being put back together a second time (like the con rods with the fractured joint face).
(4) Some failures are caused by the bores becoming oval and there being too much piston clearance for "thin" oils. The Lokasil preformed - cast in - liners have little tensile strength to resist hoop stress (brittle) so can crack when the unsupported cylinder distorts oval under piston loads plus cylinder pressures. Some of this may well be improved as the new Lokasil technology is no doubt developed over the years - I mean a simple bridge or two - cast into the cylinders opposite the cylinder pressure faces would have cured that particular problem straight away - perhaps something similar is in the pipeline or we may find thicker cylinder walls if they are determined to maintain a free standing cylinder tube (for reducing expansion distortion).
(5) However - you can replace one crankcase half (or both halves) if there is cylinder damage or wear/seizure - (or intermediate shaft bearing location damage) as the crankshaft is supported in a seperate housing and the main bearings are not in the crankcase part that forms the cylinder block. So you could rebuild with new "PORSCHE" parts and have new cylinder bores anyway.
(6) The whole engine rebuild concept (from all manufacturers) is moving towards IT BEING CHEAPER TO REPLACE THAN REBUILD and technology is changing accordingly - influenced by increases in labour rates.
(7) However they must crash Porsches in the USA - so presumably a lower mileage/newer engine should be available from a crashed car breakers yard and could provide a viable escape route.
(8) For any brave and competent USA mechanics/technicians - capable of trying to rebuild these engines - there should be a long term benefit providing they are prepared to work things out for themselves and try different methods and tooling - because we are only dealing with engines that are at most 10 years old and the cars must be in use for many more years than that when residual values (still high with a Porsche compared to family saloons) make less expensive options essential. Boxster and 996 rebuilds are the same and use common parts, tooling and technology. However if no one has done so yet - you would be under increased risk until they have sorted it all out - so perhaps the best options for you - over there - are a used crashed car engine or a new replacement.
(9) Some aspects of the design lend themselves to the engines being rebuildable for many years (like the steel reinforced crankshaft housing - following Japanese technology to stop main bearing creep damaging bearing locations).
(10) Perhaps it is the fear of legal come back if the rebuild fails that puts off potential technicians in the USA - because it is becoming more of a problem in the UK with so many would be lawyers getting their cars repaired all the time and expecting far too much for their money. If it is a "cost less option" then there should be some added risk associated with it.
Good luck making your decision.
Baz Hartech
jonny996 said:
How mauch would it cost to put it in a container & get it shipped to UK to get Hartech or Autofarm to replace it & ship it back, cant be that much more than the $1500 they want to open engine up.
Just thinking outside the box.
Just thinking outside the box.
...was thinking the same thing, so long as you could tolerate any potential customs hold-ups. Mind you, for the cost of a brand-spanking new engine, it'd still prob work out better to a) get engine shipped to UK for Baz to fettle, and in the meantime b) buy a cheap run around car then re-sell it in a few months...
daegucb
You might find a solution nearer to home if you look on the Rennlist 996 forum
http://forums.rennlist.com/rennforums
You might find a solution nearer to home if you look on the Rennlist 996 forum
http://forums.rennlist.com/rennforums
polarexpress said:
carreraboy said:
Mates GT3 Blew Up just out of warranty...argued got it done under warranty.....perhaps you could get a thermostatic cut off as per my Porsche Design Kettle.........
Des - that's not something you hear everyday... what happened?
Went in for a service they mentioned a noise from the engine was ok prior to service.......maybe someone dropped a spanner down it!
thanks for the info. everyone. but, i want to ask baz a question. Is there an easy way to expose the crankshaft to find out what the problem is (so that I can determine whether to replace the engine or to fix it)??? For a regular V6 engines, you can just remove the oil pen to expose the crankshaft. is it much more complicated with theis flat six???
In a word - no - the engines are easy to strip though - just difficult to re-assemble.
You cannot obtain re-grind shells so any damage to crank requires a new one.
The big end shells seem very skimopy in size and the size determines how much load they can take while supported by the oil pressure - you know - load over area etc - the same oil pressure over a 944 big end (for example) provides perhaps about twice the support pressure and I think this may be what is behind the problem I have seen with some shells curling around the crankshaft journal over time.
A similar problem emerged with the 944 in about 1986 but I think the shell material was changed as later cars were OK (although Porsche also increased the sump capacity, and provided a sump level indicator at about that time).
The intermediate shaft bearing sounds like the other possibility. This is a strange idea - a grease filled ball bearing with seals imersed in oil - the splash oil gradually filters past the seals and fills up the intermediate shaft hollow tube but doesn't get circulated or replenished. I cannot understand why it was not just left open for splash lubrication - but then again - what do I know! I think the grease breaks down and eventually does not do it's job (as the roational speed - although less than the crankshaft speed (but not by half) is close to design limits for a grease filled bearing anyway.
If it is the intermediate shaft bearing - then as along as the plastic oil pick up tubes are stripped - cleaned out (or replaced) and the crank is OK - it should be rebuildable.
The problem is how to get the last three pistons fitted and the solution is somewhat hit and miss (in my opinion) - according to Porsche it needs work down a long hole in the side of the engine to feed the pins and circlips - not a nice prospect - but it can obviously be done - the problem is being sure it is all in place at the end - otherwise many bits fly out within a few miles!
Baz Hartech
You cannot obtain re-grind shells so any damage to crank requires a new one.
The big end shells seem very skimopy in size and the size determines how much load they can take while supported by the oil pressure - you know - load over area etc - the same oil pressure over a 944 big end (for example) provides perhaps about twice the support pressure and I think this may be what is behind the problem I have seen with some shells curling around the crankshaft journal over time.
A similar problem emerged with the 944 in about 1986 but I think the shell material was changed as later cars were OK (although Porsche also increased the sump capacity, and provided a sump level indicator at about that time).
The intermediate shaft bearing sounds like the other possibility. This is a strange idea - a grease filled ball bearing with seals imersed in oil - the splash oil gradually filters past the seals and fills up the intermediate shaft hollow tube but doesn't get circulated or replenished. I cannot understand why it was not just left open for splash lubrication - but then again - what do I know! I think the grease breaks down and eventually does not do it's job (as the roational speed - although less than the crankshaft speed (but not by half) is close to design limits for a grease filled bearing anyway.
If it is the intermediate shaft bearing - then as along as the plastic oil pick up tubes are stripped - cleaned out (or replaced) and the crank is OK - it should be rebuildable.
The problem is how to get the last three pistons fitted and the solution is somewhat hit and miss (in my opinion) - according to Porsche it needs work down a long hole in the side of the engine to feed the pins and circlips - not a nice prospect - but it can obviously be done - the problem is being sure it is all in place at the end - otherwise many bits fly out within a few miles!
Baz Hartech
hartech said:
The big end shells seem very skimopy in size and the size determines how much load they can take while supported by the oil pressure - you know - load over area etc - the same oil pressure over a 944 big end (for example) provides perhaps about twice the support pressure and I think this may be what is behind the problem I have seen with some shells curling around the crankshaft journal over time.
maybe the 944 pistons are a lot heavier?
Biggest driver of big end loading is piston and rod mass and engine speed - peak load is tdc on the exhaust stroke, and it's all inertial.
Ref the additional loading from the 4.0 upgrade, it all comes down to the piston mass.
bluesatin, thanks. but, i am in us. buying an engine oversea isn't practical. but, thanks anyway for the thought. i decided to buy a new engine from porsche. HOWEVER, I JUST FOUND OUT ABOUT WHAT CAN HAPPEN WITH THE NEW ENGINE FROM READING "996 engine problems and porsche customer torture" postings. wow. scary. i feel like i've been just kicked in the groin...
Hi NDT (not short for "non destructive testing" - is it?) look I am sorry - but I don't have time to start a technical debate about big end failure (and I am not a tribologist) - and you are of course right about maximum dynamic load on the big end bearing is inertial and at top and bottom dead centre - so if the rod is going to snap or metal fatigue - those are the most likely factors to influence the outcome and the heavier the piston, the higher the revs, the longer the stroke etc - the greater the load. But I find there are often more factors involved in problems than just one simple explanation and we have to base what we think upon what we see at this early stage in assessing the problems of these engines.
We don't usually have a huge technical debate with ourselves when we observe things - we just have time for a quick assessment of what is the likely cause based upon experience. After all we are not representing Porsche we are simply interested in assessing as soon as possible what needs to be changed when we rebuild and engine - even if it looks OK at the time - because we regard it as a vulnerable area - which is what good engineering is all about.
Although the inertia loads are maximum at top and bottom dead centre (becasue direction reverses), the loading is only high for a very small number of rotational degrees (after which it is low due to the slow piston acceleration/deceleration) and at neither top nor bottom dead centre is it added to by the maximum pressure of combustion which occurs some time later in the burn cycle (and not at all at bottom dead centre). It is also high when the maximum linear acceleration and deceleration occurs later in the cycle (depending upon stroke and rod lengths etc).
Now if the problem was caused only by inertia loads it would just as likely occur at low mileages as high ones - but - in my limited experience of these engines so far - the failures occur after 50 to 80 K and seem to be wear related. I.e. most failures are not being caused by broken rods instead - in my experience - being cause by big end shell wear rates being excessive in some examples (and don't forget we see many that have not yet failed but are clearly well worn out prematurely - sometimes just one shell in the whole engine with all the others perfectly OK).
The oil film in the big end under a supply pressure will leak out at a rate determined by time and loads. i.e. a sudden very brief load for just a few degrees of rotation - will not have time to squeeze out the oil film and cause surface friction damage in the same way as a more prolonged excessive load/area over several degrees - would.
Now we have observed - in some instances - that the shell bearing surface wears through in one localised place because the rod/and/or/shell is not perfectly round and instead of supporting the loads accross the whole face and diameter - only supports it locally -- so the dynamic and frictional load on that area is massivley increased and the oil film under pressure is a critical factor. In this scenario a wider shell would support the load better and keep the frictional surface apart and better lubricated (because the oil pressure works though an area and the larger the area the greater the support and the slower the leak-out rate) and makes it last longer whereas a small contact area will not support the rod as well and the frictional surface will wear away quicker into a vicious circle whereby the clearance increases, the oil film under pressure escapes more quickly and the actual supporting oil pressure film becomes less and less effective until the bearing finally overheats and fails (often welding the shell to the crankshaft journal). Now the 944 has about 50% more shell surface area - which is a lot and of course is manufactured in a more expensive but more repeatable way.
Although the rod and big end loading are maximum through inertial loads at top and bottom dead centre - maximum bearing roational speed does not occur then because the shell relative speed compared to the crankshaft journal is at its slowest at this point - almost sationary at top and bottom dead centre compared to the shell (and piston) speed when the rod/crank angle is nearer 90 degrees - when it is also under a relatively high driving load.
So if the shell is going to fail because the oil pressure cannot support it well enough to keep the surface friction down while it is rotating (i.e. working like a bearing) to a minimum - then the narrower the shell the greater the friction load and the poorer the oil film will keep the shell and journal seperated and the quicker it will wear out.
The excessive wear rates we have observed in the big end shells that have not yet failed have all been roughly at around 45 degress of crankshaft rotation - which leads me to believe that it is more likely to be the lack of surface oil film through reduced areas that are resulting in the problem. I could be wrong but all it means at the moment is that we always replace the big end shells (yes I know we would do anyway - but we are also cautious about increasing piston diameter and capacity until we are sure of the causes). Yes if the bigger diameter piston was no heavier then the inertia loads may be the same but the load on the shell at the rotational angle that we observe maximum wear would be higher as the torque has increased - so the oil film will squeeze out quicker and will not support the bearing surface so well.
This increase in wear and shell clearance is why eventually any engine wears out the big end shells - because over time - the surface gradually wears away - the clearance increases, the support reduces and finally (if left long enough) the shell surface starts to touch the journal for more than the split second at top and bottom dead centre but also at the points of maximum shell surface friction speed - until it finally fails. This is also why they are OK for many thousands of miles and then deteriorate quickly once a certain clearance is exceeded.
Usually maximum wear is noticed near the centre of the shell (therefore related more to inertia loads) but in this model we have so far observed a slightly different position that needs an explanation.
We have also found some shells that have deformed so that although they sit perfectly round when they are trapped in the closed rod, as soon as they are free (during strip down) they curl around the crankshaft journal having been deformed with an excessive internal stress in the centre and this is caused by sudden excessive inertial shock loads. So far none of these had local wear anywhere - the shells were all evenly worn and still OK but just the shell was deformed in the free state. This means that - in our limited experience - there seem to be 2 potential problems but both could be explained by the oil film that supports the rod on the crankshaft being closer to its critical pressure/leak out rate - than previous models - increasing the shock loading effect and reducing oil film support pressures.
So when I offer to this forum an opinion that merely expresses a concern about things I have observed - it is just for everyones potential benefit - not an edict - and my intentions are merely to advise caution where I see it beneficial to help people make the right decisions. We already machine out and replace our own manufactured liners - so increasing capacity with bigger pistons would not be a problem - but I cannot increase the big end shell width - so I will not follow that route yet - until I fell more confident about the causes of the big end failures I have observed - but I could be wrong and my concern could be uneccessary.
I never claim to be right anyway - just offer an opinion - I have been around too long for such arrogance - but I have more often than not seen things in the right direction. We were (for example) the first to publicise the weak 944S and S2 (now also 968/Boxster/996 etc) cam chain system and suggest regular checks and replacements that have saved hundreds (perhaps thousands) of customers from needing new cams and cylinder heads Worldwide etc.
In this instance I think that the big end shells are on the narrow side and unless everything else is spot on (oil pump and pressure, oil grade, rod dimensions, shell quality) etc I think that may be a vulnerable area in the future - thats all - and so we are checking rods - dimensions, roundness etc more carefully than before - changing them if they are even slightly out of spec or the cracked joint line is at a poor angle or uneven - but then I could always be wrong.
Baz Hartech
We don't usually have a huge technical debate with ourselves when we observe things - we just have time for a quick assessment of what is the likely cause based upon experience. After all we are not representing Porsche we are simply interested in assessing as soon as possible what needs to be changed when we rebuild and engine - even if it looks OK at the time - because we regard it as a vulnerable area - which is what good engineering is all about.
Although the inertia loads are maximum at top and bottom dead centre (becasue direction reverses), the loading is only high for a very small number of rotational degrees (after which it is low due to the slow piston acceleration/deceleration) and at neither top nor bottom dead centre is it added to by the maximum pressure of combustion which occurs some time later in the burn cycle (and not at all at bottom dead centre). It is also high when the maximum linear acceleration and deceleration occurs later in the cycle (depending upon stroke and rod lengths etc).
Now if the problem was caused only by inertia loads it would just as likely occur at low mileages as high ones - but - in my limited experience of these engines so far - the failures occur after 50 to 80 K and seem to be wear related. I.e. most failures are not being caused by broken rods instead - in my experience - being cause by big end shell wear rates being excessive in some examples (and don't forget we see many that have not yet failed but are clearly well worn out prematurely - sometimes just one shell in the whole engine with all the others perfectly OK).
The oil film in the big end under a supply pressure will leak out at a rate determined by time and loads. i.e. a sudden very brief load for just a few degrees of rotation - will not have time to squeeze out the oil film and cause surface friction damage in the same way as a more prolonged excessive load/area over several degrees - would.
Now we have observed - in some instances - that the shell bearing surface wears through in one localised place because the rod/and/or/shell is not perfectly round and instead of supporting the loads accross the whole face and diameter - only supports it locally -- so the dynamic and frictional load on that area is massivley increased and the oil film under pressure is a critical factor. In this scenario a wider shell would support the load better and keep the frictional surface apart and better lubricated (because the oil pressure works though an area and the larger the area the greater the support and the slower the leak-out rate) and makes it last longer whereas a small contact area will not support the rod as well and the frictional surface will wear away quicker into a vicious circle whereby the clearance increases, the oil film under pressure escapes more quickly and the actual supporting oil pressure film becomes less and less effective until the bearing finally overheats and fails (often welding the shell to the crankshaft journal). Now the 944 has about 50% more shell surface area - which is a lot and of course is manufactured in a more expensive but more repeatable way.
Although the rod and big end loading are maximum through inertial loads at top and bottom dead centre - maximum bearing roational speed does not occur then because the shell relative speed compared to the crankshaft journal is at its slowest at this point - almost sationary at top and bottom dead centre compared to the shell (and piston) speed when the rod/crank angle is nearer 90 degrees - when it is also under a relatively high driving load.
So if the shell is going to fail because the oil pressure cannot support it well enough to keep the surface friction down while it is rotating (i.e. working like a bearing) to a minimum - then the narrower the shell the greater the friction load and the poorer the oil film will keep the shell and journal seperated and the quicker it will wear out.
The excessive wear rates we have observed in the big end shells that have not yet failed have all been roughly at around 45 degress of crankshaft rotation - which leads me to believe that it is more likely to be the lack of surface oil film through reduced areas that are resulting in the problem. I could be wrong but all it means at the moment is that we always replace the big end shells (yes I know we would do anyway - but we are also cautious about increasing piston diameter and capacity until we are sure of the causes). Yes if the bigger diameter piston was no heavier then the inertia loads may be the same but the load on the shell at the rotational angle that we observe maximum wear would be higher as the torque has increased - so the oil film will squeeze out quicker and will not support the bearing surface so well.
This increase in wear and shell clearance is why eventually any engine wears out the big end shells - because over time - the surface gradually wears away - the clearance increases, the support reduces and finally (if left long enough) the shell surface starts to touch the journal for more than the split second at top and bottom dead centre but also at the points of maximum shell surface friction speed - until it finally fails. This is also why they are OK for many thousands of miles and then deteriorate quickly once a certain clearance is exceeded.
Usually maximum wear is noticed near the centre of the shell (therefore related more to inertia loads) but in this model we have so far observed a slightly different position that needs an explanation.
We have also found some shells that have deformed so that although they sit perfectly round when they are trapped in the closed rod, as soon as they are free (during strip down) they curl around the crankshaft journal having been deformed with an excessive internal stress in the centre and this is caused by sudden excessive inertial shock loads. So far none of these had local wear anywhere - the shells were all evenly worn and still OK but just the shell was deformed in the free state. This means that - in our limited experience - there seem to be 2 potential problems but both could be explained by the oil film that supports the rod on the crankshaft being closer to its critical pressure/leak out rate - than previous models - increasing the shock loading effect and reducing oil film support pressures.
So when I offer to this forum an opinion that merely expresses a concern about things I have observed - it is just for everyones potential benefit - not an edict - and my intentions are merely to advise caution where I see it beneficial to help people make the right decisions. We already machine out and replace our own manufactured liners - so increasing capacity with bigger pistons would not be a problem - but I cannot increase the big end shell width - so I will not follow that route yet - until I fell more confident about the causes of the big end failures I have observed - but I could be wrong and my concern could be uneccessary.
I never claim to be right anyway - just offer an opinion - I have been around too long for such arrogance - but I have more often than not seen things in the right direction. We were (for example) the first to publicise the weak 944S and S2 (now also 968/Boxster/996 etc) cam chain system and suggest regular checks and replacements that have saved hundreds (perhaps thousands) of customers from needing new cams and cylinder heads Worldwide etc.
In this instance I think that the big end shells are on the narrow side and unless everything else is spot on (oil pump and pressure, oil grade, rod dimensions, shell quality) etc I think that may be a vulnerable area in the future - thats all - and so we are checking rods - dimensions, roundness etc more carefully than before - changing them if they are even slightly out of spec or the cracked joint line is at a poor angle or uneven - but then I could always be wrong.
Baz Hartech
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