996 Ovality Cracks?? What are they?
Discussion
Most engines I have seen that have covered over 60 K generally have bores that are going oval. This is similar to bore wear but seems to be distortion as the dimension side to side gets less as the dimension front to back gets bigger. As a result - if it is measured with a 3 pronged micrometer (bore guauge) it reads right and it is only noticed using a 2 pronged bore mic in 2 directions.
Some have seized due to the blow-by on the thrust face slipping past the rings (because they cannot adjust to the ovality) burning away (or overheating) the oil that is supposed to support the piston and pulled out parts of the lining face. Others carry on trying to expand and crack.
The centre sleeve is silicon rich and is not as strong in hoop stress (circumferential tension) as the base aluminium sorounding it which eventually seems to crack on some engines but not others.
In an attempt to find out why I have machined away the outside of some liners to reveal a centre "cast in" sleeve not fitted very central but largely seems bonded well to the outer aluminium (something I was hoping to fault to explain the reason for the problem - but couldn't).
I guess that this new system may have thrown up a few that failed in production and as Porsche often get the engines back - I guess (or hope) they have diagnosed the problem as we have and altered some casting controls or manufacturing techniques/pressures - to improve them as time has gone by.
I am a little concerned about the process in which I believe frozen cores (like liners) are fitted in the mould and molten aluminium low pressure poured in. I don't know how you would tell if there was a patch in one contact face that wasn't joined quite as well (or perhaps with the same pressure between the cooled faces) so that over time that area gave a little allowing the weaker core to crack just there or how you would know that the flow and temperatures were always the same (but they may be as I never was involved in low pressure casting only gravity casting).
With shrunk in liners the interference fit is predetermined and with iron or steel cast in liners the liner is strong enough in its own right to resist the hoop stress delivered which is also controlled by the interference fit chosen.
It is actually a very clever and cost effective process that I am sure is improving and anyway by far the highest % of engines seem quite capable of covering over 100K before problems emerge.
If they do all fail eventually they may need the bores fixing one day and if so there is more than one option available which by then will be little different in cost than the typical rebuild an air cooled generation engine may have experienced - done properly.
So overall - not a wide spread problem. In time - perhaps one that needs a fix (that will be available by then) and meanwhile it is not a big enough issue to worry too much about (and Porsche warranty that covers the engine is a good safeguard).
Baz Hartech
Some have seized due to the blow-by on the thrust face slipping past the rings (because they cannot adjust to the ovality) burning away (or overheating) the oil that is supposed to support the piston and pulled out parts of the lining face. Others carry on trying to expand and crack.
The centre sleeve is silicon rich and is not as strong in hoop stress (circumferential tension) as the base aluminium sorounding it which eventually seems to crack on some engines but not others.
In an attempt to find out why I have machined away the outside of some liners to reveal a centre "cast in" sleeve not fitted very central but largely seems bonded well to the outer aluminium (something I was hoping to fault to explain the reason for the problem - but couldn't).
I guess that this new system may have thrown up a few that failed in production and as Porsche often get the engines back - I guess (or hope) they have diagnosed the problem as we have and altered some casting controls or manufacturing techniques/pressures - to improve them as time has gone by.
I am a little concerned about the process in which I believe frozen cores (like liners) are fitted in the mould and molten aluminium low pressure poured in. I don't know how you would tell if there was a patch in one contact face that wasn't joined quite as well (or perhaps with the same pressure between the cooled faces) so that over time that area gave a little allowing the weaker core to crack just there or how you would know that the flow and temperatures were always the same (but they may be as I never was involved in low pressure casting only gravity casting).
With shrunk in liners the interference fit is predetermined and with iron or steel cast in liners the liner is strong enough in its own right to resist the hoop stress delivered which is also controlled by the interference fit chosen.
It is actually a very clever and cost effective process that I am sure is improving and anyway by far the highest % of engines seem quite capable of covering over 100K before problems emerge.
If they do all fail eventually they may need the bores fixing one day and if so there is more than one option available which by then will be little different in cost than the typical rebuild an air cooled generation engine may have experienced - done properly.
So overall - not a wide spread problem. In time - perhaps one that needs a fix (that will be available by then) and meanwhile it is not a big enough issue to worry too much about (and Porsche warranty that covers the engine is a good safeguard).
Baz Hartech
Yes, we have done this and so have Autofarm. I think they are ahead of us on road trials.
Ours is alloy with a Nikasil bore - but we have not covered enough miles yet to say if it is OK or not.
However you can buy crankcases if the bores have gone oval - so even if one has cracked - they really all need replacing then new crankcases is a viable option (costs no more and better in other ways - all new etc and probably made with new process control anyway).
Re-linering is probably only worthwhile if only one bore is damaged and the rest are OK for size and shape - so probably a low mileage failure.
The design of the block does not lend itself to re-linering except with good machinery and accurate work - as it is not like a conventiuonal block.
Afterwards the original type of head gasket may not be good enough long term and an additional ring seal may be neccessary - time will tell.
If using different bore material different piston rings may be neccessary.
The main thing is - yes it is possible.
There are some photos on our web site (www.hartech.org) that may be of some help explaining further.
Baz Hartech
Ours is alloy with a Nikasil bore - but we have not covered enough miles yet to say if it is OK or not.
However you can buy crankcases if the bores have gone oval - so even if one has cracked - they really all need replacing then new crankcases is a viable option (costs no more and better in other ways - all new etc and probably made with new process control anyway).
Re-linering is probably only worthwhile if only one bore is damaged and the rest are OK for size and shape - so probably a low mileage failure.
The design of the block does not lend itself to re-linering except with good machinery and accurate work - as it is not like a conventiuonal block.
Afterwards the original type of head gasket may not be good enough long term and an additional ring seal may be neccessary - time will tell.
If using different bore material different piston rings may be neccessary.
The main thing is - yes it is possible.
There are some photos on our web site (www.hartech.org) that may be of some help explaining further.
Baz Hartech
I do not know of one single car, using short skirt pistons, that revs to 7k+, that will not develop oval bores, eventually.
This is all very sensationalist and not something that should really be worrying anyone.
As Baz says, a new crankcase would be the best solution, but I doubt it would be a problem on any cars with less than 125-150k.
This is all very sensationalist and not something that should really be worrying anyone.
As Baz says, a new crankcase would be the best solution, but I doubt it would be a problem on any cars with less than 125-150k.
I am not sure if Raven Flyer has got the point of my posting. Firstly - I think I know enough about engines to have noticed bores wearing oval especially with short skirt pistons - but the key point to my posting is that the bores have not gone oval through bore wear but through distortion and that is unusual and I cannot see how that is sensationalising a serious technical problem that I imagine is what readers want experienced engineers like me to post. Imagine taking a tube and squashing it in a vice so it is narrower in one direction and wider in the other (and smaller in the narrow direction than it was when it was made). It seems that the pressure from the thrust face of the piston is gradually stretching the bore in that direction. This is what I have measured and we fit rings in the top to prevent further movement but we cannot return them to the original shape or size.
This is what makes the prospect of re-linering dubious if the other sound bores are already oval unless all 6 are done and then the cost is similar to new cases anyway (and we have measured clearances of + and - 5 thou giving a front to back bore clearance much to high allowing blow by and beyond what a piston rings can distort itself to seal fully.
I suppose I cannot win here - if I keep such things to myself I am not informing others about a serious problem that could affect their decision making in the future when eventually engines need rebuilds (which is not too far away in some higher mileage cars) and then if I do forewarn readers - I am "being sensational".
It is not my fault that the bores go oval - and I am trying to do something to help.
Baz Hartech.
This is what makes the prospect of re-linering dubious if the other sound bores are already oval unless all 6 are done and then the cost is similar to new cases anyway (and we have measured clearances of + and - 5 thou giving a front to back bore clearance much to high allowing blow by and beyond what a piston rings can distort itself to seal fully.
I suppose I cannot win here - if I keep such things to myself I am not informing others about a serious problem that could affect their decision making in the future when eventually engines need rebuilds (which is not too far away in some higher mileage cars) and then if I do forewarn readers - I am "being sensational".
It is not my fault that the bores go oval - and I am trying to do something to help.
Baz Hartech.
Regarding warranties - our scheme is available for cars we have not sold and fully explained on www.hartech.org - and basically it involves a thorough service/check over and then it can go on the scheme. Faults found during the check may have to be fixed first or if not most can be excluded from being covered. So if for example the drive shafts are worn out but working OK they could be excluded from being fixed in the future under the scheme.
Our "Lifetime Maintenance Plan" involves a monthly payment covering free future servicing & Mot's (parts and labour - varies with mileage) and free labour to repair most faults including wear and tear items.
So you bring it in for it's next service and nothing else needs doing - you pay nothing extra.
If it needs discs, pads, steering rack, clutch etc you just pay for parts.
It came about from realising over the years that cars that we looked after long term rarely cost a lot to keep on the road - even after many years and huge mileages - but this only works if we are looking after the car - so we included the service cost in the price of the scheme.
We benefit from regular monthly income and repeat business and the customer benefits from steady monthly outgoings, good workmanship when the car is in here (because we would be the losers if we had to subsequently keep fixing it before the next service) and us sharing the cost of any major problem - if it ever occured. We are always provide the best service and workmanship possible anyway but new customers do not know this until they have put their faith in us for a while - so this is a way of perhaps giving them confidence to come to us
knowing that we are (I think) still the only Porsche business in the UK offering such a broad and long term commitment to their customers satisfaction through a contracted maintenance scheme. We place no time limit on cover, no mileage limit, no claims limit (value or quantity) and many owners have been on the scheme for over 7 years and still keep coming back. I cannot remember there ever being a problem about a claim. One example was a 996 that came in with no compression on one cylinder - found to be a broken valve spring. It was covered under the scheme and the customer only paid for the spring, oil, coolant, gaskets etc. On this occasion he followed our advice and also paid a little extra for replacing all the springs and doing the other bank as well - as a good preventative measure and of course he paid a lot less than he expected after the initial diagnosis.
I think the costs are very reasonable - currently for example £65.00/month for 12K miles/year - providing 2 - 6 monthly services (minor and major) - and an annual MOT.
It puts us and the customer in the same position - both trying to render the car as reliable as possible - with the minimum costs - without coming back in between services. If we then recomend a new steering rack (for example) the customer knows it must be neccessary - because we are paying to fit it. Parts prices may be less than main agent or similar to it (if we have to source it there).
So far it is working well for both parties although a modest price increase is imminent having kept it static for 18 months and with all garage costs rising fast.
Cars we sell benefit from a slightly cheaper price, that all the work is already done beforehand at our cost to make the car OK and the first three months cover are free.
Baz Hartech
Our "Lifetime Maintenance Plan" involves a monthly payment covering free future servicing & Mot's (parts and labour - varies with mileage) and free labour to repair most faults including wear and tear items.
So you bring it in for it's next service and nothing else needs doing - you pay nothing extra.
If it needs discs, pads, steering rack, clutch etc you just pay for parts.
It came about from realising over the years that cars that we looked after long term rarely cost a lot to keep on the road - even after many years and huge mileages - but this only works if we are looking after the car - so we included the service cost in the price of the scheme.
We benefit from regular monthly income and repeat business and the customer benefits from steady monthly outgoings, good workmanship when the car is in here (because we would be the losers if we had to subsequently keep fixing it before the next service) and us sharing the cost of any major problem - if it ever occured. We are always provide the best service and workmanship possible anyway but new customers do not know this until they have put their faith in us for a while - so this is a way of perhaps giving them confidence to come to us
knowing that we are (I think) still the only Porsche business in the UK offering such a broad and long term commitment to their customers satisfaction through a contracted maintenance scheme. We place no time limit on cover, no mileage limit, no claims limit (value or quantity) and many owners have been on the scheme for over 7 years and still keep coming back. I cannot remember there ever being a problem about a claim. One example was a 996 that came in with no compression on one cylinder - found to be a broken valve spring. It was covered under the scheme and the customer only paid for the spring, oil, coolant, gaskets etc. On this occasion he followed our advice and also paid a little extra for replacing all the springs and doing the other bank as well - as a good preventative measure and of course he paid a lot less than he expected after the initial diagnosis.
I think the costs are very reasonable - currently for example £65.00/month for 12K miles/year - providing 2 - 6 monthly services (minor and major) - and an annual MOT.
It puts us and the customer in the same position - both trying to render the car as reliable as possible - with the minimum costs - without coming back in between services. If we then recomend a new steering rack (for example) the customer knows it must be neccessary - because we are paying to fit it. Parts prices may be less than main agent or similar to it (if we have to source it there).
So far it is working well for both parties although a modest price increase is imminent having kept it static for 18 months and with all garage costs rising fast.
Cars we sell benefit from a slightly cheaper price, that all the work is already done beforehand at our cost to make the car OK and the first three months cover are free.
Baz Hartech
Hi Baz,
Really interesting write-up, always enjoy reading your posts, and *personally* find them very valuable.
With regards to Raven Flyer, I don't *think* the "sensationalist" comment was referring to your post, more the original post about "I read in a recent magazine that this is a problem that will eventually cause the failure of 996 engines??"....may be wrong...
Really interesting write-up, always enjoy reading your posts, and *personally* find them very valuable.
With regards to Raven Flyer, I don't *think* the "sensationalist" comment was referring to your post, more the original post about "I read in a recent magazine that this is a problem that will eventually cause the failure of 996 engines??"....may be wrong...
Baz
It all sounds totally plausible, but I have mates with 80k, 90k 996's that don't have compression problems (look at them babies go!), so it's still all a bit sensationalist.
I don't doubt you've seen some but then we are talking about hand made boxes of cogs, at the end of the day. With that in mind, there will be some 'Friday afternoon' cars out there.
I believe that also applies to Bentley, Aston, Lambo, Ferrari etc
It all sounds totally plausible, but I have mates with 80k, 90k 996's that don't have compression problems (look at them babies go!), so it's still all a bit sensationalist.
I don't doubt you've seen some but then we are talking about hand made boxes of cogs, at the end of the day. With that in mind, there will be some 'Friday afternoon' cars out there.
I believe that also applies to Bentley, Aston, Lambo, Ferrari etc
Great information Baz. I've been toying with the idea of getting a 996, but put off by the engine issues, especially in earlier models - but yuor warrantee scheme sound very reasonable and fair. If you have a moment to reply can you give me your feedback on:-
a) whether the engines you've seen gone oval or cracked were all from 97-99 models (as suggested on some other posts here), or have you seen higher mileage 2000-2001 3.4 engines also suffer the same ? (or for that matter 3.6 996 engines in later models)?
b) if a complete engine rebuild is required, on a 3.4 996 engine and the car is not covered by your scheme, what price (ballpark) are we talking about?
Cheers,
a) whether the engines you've seen gone oval or cracked were all from 97-99 models (as suggested on some other posts here), or have you seen higher mileage 2000-2001 3.4 engines also suffer the same ? (or for that matter 3.6 996 engines in later models)?
b) if a complete engine rebuild is required, on a 3.4 996 engine and the car is not covered by your scheme, what price (ballpark) are we talking about?
Cheers,
Gassing Station | Porsche General | Top of Page | What's New | My Stuff