Timing Belt Failure 1992 968
06-05-2012, 08:04 PM
#16
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The truth is that I have no idea what brand it was. Three years is a long time. Something says Continental but maybe because I read others posts. This remains a mystery as to why the belt is so decomposed. Also suspect that the cam journal (oiling) failed independently of the timing belt and caused the belt to fail when it did. But judging from the condition of the belt I was long overdue for a belt change. Of course, this could have been a bad belt but that is pretty random and less likely than the heat and high running temps and mileage all contributing to belt decomposition. I guess to be safe one really needs to change these belts often. It is not that bad to do yourself, especially compared to the work of removing the head and then reinstalling the head, not to mention the head work and replacing valves etc that some machine shop will end up doing.
D
D
06-06-2012, 10:46 AM
#17
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Don, you can safely run these belts for atleast 4 yrs if not more. The key is to use Porsche belts only. I have compared Dayco and Gates "oem" belts to the ones that come in the Porsche box. Porsche belts are much higher quality. I guess Porsche specs are much higher and the manufacturers don't sell the same quality belts independently.
Contitechs just don't last. Contitech is an oem supplier to BMW and I compared Contitech belts bought from a reseller to the ones from BMW. Same thing, the BMW belts were much higher quality even though they are both manufactured by Contitech.
It is cheap insurance considering the cost of fixing a head with bent valves.
Contitechs just don't last. Contitech is an oem supplier to BMW and I compared Contitech belts bought from a reseller to the ones from BMW. Same thing, the BMW belts were much higher quality even though they are both manufactured by Contitech.
It is cheap insurance considering the cost of fixing a head with bent valves.
06-06-2012, 03:36 PM
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By looking at the belt, it looks like something stopped turning and broke the belt. (all the broken ribs close to the breaking point) The fact that it looks like there are stretched ribs it may have been a build-up of a load on the belt and finally the pulley stopped turning and the belt broke. Did you try to turn your cam after head removal? was it seized up in the head?
Just my $.02 worth.
Just my $.02 worth.
06-07-2012, 12:03 AM
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Machinist and I met today buy he had not had time to actually examine the cam journal yet- says he will do tonight/tomorrow. We shall see. Honest guy and does a great business in Houston here and business is booming here in car crazy Houston.
Interesting observations on the failure mechanism of the belt and the difference in quality in belts. Ryan, the machinist would probably agree with your diagnosis of the failure mechanism, he thinks oil starvation caused the cam journal to grab but not totally stop or seize the exhaust camshaft, putting stress on the belt and causing it, in its deteriorated state to fail. When I get this back together I will be using only Porsche OEM belts, clearly the premium is worth it.
I did put a timing belt back on the engine, before pulling the head, just to see how bad the damage was and I was able to turn the engine over (with ratchet/socket wrench on crankshaft pulley nut) pretty easily with no internal sounds (had the plugs out) and I got compression on cylinders 1 and 3- predicting that I had problems on 2 and 4. This proved to be dead on as I had 4 bent exhaust valves- 2 on 2 and 4. I have never seen a timing belt break before so it is interesting to hear that other breaks were much cleaner.
I still don't have any idea what caused the oil starvation- and that worries me about reinstalling my head even if the machinist says he can fix it. He did tell me that while the technology/machines exists in some shops to do line boring on camshaft journals- the practical side is that it is almost never done as the setup to do one line bore might be 12 machinist hours and noone is willing to invest that kind of money in the heads that most machine shops see day in, day out. So, if the damaged journal is out of tolerance and requires additional material (alum) to be deposited (welded on), that will demand line boring which would not be economically feasible even with a head like ours that might be worth $1k- you can buy used heads for that kind of money with no repairs. So, stay tuned.
Interesting observations on the failure mechanism of the belt and the difference in quality in belts. Ryan, the machinist would probably agree with your diagnosis of the failure mechanism, he thinks oil starvation caused the cam journal to grab but not totally stop or seize the exhaust camshaft, putting stress on the belt and causing it, in its deteriorated state to fail. When I get this back together I will be using only Porsche OEM belts, clearly the premium is worth it.
I did put a timing belt back on the engine, before pulling the head, just to see how bad the damage was and I was able to turn the engine over (with ratchet/socket wrench on crankshaft pulley nut) pretty easily with no internal sounds (had the plugs out) and I got compression on cylinders 1 and 3- predicting that I had problems on 2 and 4. This proved to be dead on as I had 4 bent exhaust valves- 2 on 2 and 4. I have never seen a timing belt break before so it is interesting to hear that other breaks were much cleaner.
I still don't have any idea what caused the oil starvation- and that worries me about reinstalling my head even if the machinist says he can fix it. He did tell me that while the technology/machines exists in some shops to do line boring on camshaft journals- the practical side is that it is almost never done as the setup to do one line bore might be 12 machinist hours and noone is willing to invest that kind of money in the heads that most machine shops see day in, day out. So, if the damaged journal is out of tolerance and requires additional material (alum) to be deposited (welded on), that will demand line boring which would not be economically feasible even with a head like ours that might be worth $1k- you can buy used heads for that kind of money with no repairs. So, stay tuned.
06-07-2012, 01:49 PM
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I will bet that this is a question that Raj will be able to answer.
The machinist measured the clearances in the damaged cam journal as between 1.5 and 3 thousandths. His research is telling him that stock clearances are supposed to be between 1.5 and 2 thousandths. He cannot find a wear tolerance in his research- ie is 3 thousandths acceptable in a running cam. He says many engines have larger clearances than this but he acknowledges that Porsche engines may not allow that sort of tolerance. He just cannot find any info on this. Does anyone have any info that could help?
We still cannot find a failure mechanism that caused the cam journal to be oil starved which we are feeling pretty sure led to the timing belt's premature demise. All of this is leading towards a replacement head I am feeling. Will keep you posted.
Don
The machinist measured the clearances in the damaged cam journal as between 1.5 and 3 thousandths. His research is telling him that stock clearances are supposed to be between 1.5 and 2 thousandths. He cannot find a wear tolerance in his research- ie is 3 thousandths acceptable in a running cam. He says many engines have larger clearances than this but he acknowledges that Porsche engines may not allow that sort of tolerance. He just cannot find any info on this. Does anyone have any info that could help?
We still cannot find a failure mechanism that caused the cam journal to be oil starved which we are feeling pretty sure led to the timing belt's premature demise. All of this is leading towards a replacement head I am feeling. Will keep you posted.
Don
06-07-2012, 10:50 PM
#23
OkRider
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Don, tolerances and wear limits section in my copy of the Porsche 968 Technical specifications state that the camshaft bore inner diameter to be 28mm +0.021 -0, or in inch dimension equals 1.10236 +.0008 -0
Camshaft diameter is 28mm -0.04 to -0.055, or in inch terms 1.10236 -0.00157 to -0.00217 about but that is really splitting hairs.
So, we wind up with a total of 0.00297. Lets just call it 0.003 total wear limit combined of the camshaft and the camshaft bore. Another way to look at this is if you had a minimum spec camshaft of 28mm and a minimum camshaft bore diameter of 28mm -0.04, this would be -0.00157 inch total clearance between the two. Just a little over a thousandth of an inch allowance for wear.
I hope this helps, BTW, I,m a machinist too.
Doug
Camshaft diameter is 28mm -0.04 to -0.055, or in inch terms 1.10236 -0.00157 to -0.00217 about but that is really splitting hairs.
So, we wind up with a total of 0.00297. Lets just call it 0.003 total wear limit combined of the camshaft and the camshaft bore. Another way to look at this is if you had a minimum spec camshaft of 28mm and a minimum camshaft bore diameter of 28mm -0.04, this would be -0.00157 inch total clearance between the two. Just a little over a thousandth of an inch allowance for wear.
I hope this helps, BTW, I,m a machinist too.
Doug
06-09-2012, 05:31 AM
#24
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As cheap as the Cam Belt and Balance Belt is they are replaced by the 944 Gents every 3 years or 30K miles whichever comes first. Replacing the belts at those intervals is a no brainer. The Water pump is changed every other belt change and the rollers as needed. After market Dayco and Gates belts are the only ones, IMHO, I'll use.
Cheers,
Larry
Cheers,
Larry
06-09-2012, 09:46 AM
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I just removed my timing belt after one year and 10k road miles. The belt was a Contitech. The belt showed no cracking, but the tensioner was fully extended which to me indicates unacceptable belt stretch. I'm with Raj on the OEM Porsche belts for my next change
06-11-2012, 12:02 AM
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Timing belt failure 1992 968
Update, and need a bit of help.
Machinist tells me that the maximum clearance in the damaged journal is .0003 (he used plastigage) Thanks Dougs968 for the info. We could not find the wear spec as it was not listed in the workshop manual or in any of his manuals.
Probably would not have made much difference in my decision to buy spare head from Raj, who just happened to have one all done up and ready to go. Must admit that I was a bit skittish about putting a bunch of head work into my head (getting 4 new exhaust valves and all new seals and a valve job) only to find out that it was something the head that I cannot see that caused the cam journal to lose oil which I am now pretty sure led to the timing belt failure.
Anyway that decision led to me deciding to take off the oil pan and oil pump etc to clean them up and check for debris or anything else that might have caused the failure. A lot of work stripping all of the pieces and parts. To get the oil pan off is quite a chore. Remove power steering rack (badly leaking anyway so need a new one) remove cross member and clean it up, covered in oil from 20 years of leaks. Lower balance shaft badly leaking so that's off too. Oil pan was clean as was the pump so I am thinking things are OK.
Had a bad oil leak on the drivers side at the oil filler assembly- two O rings that seal the assembly to the block were leaking. That is off as well and I found a hole or boss on the block that was obscured by the filler assembly- I don't remember taking anything off this boss but it has a hole in it. Anyone know what this this hole could be. 1st picture shows the hole in question with a short yellow screwdriver in it. Second shows the hole with a mirror pointed so as to get a look in the hole. Third shows the approximate location of the oil filler assembly so one can figure out where this is on the block. This hole was obscured by the filler assembly and I am sure I did not disconnect anything from it. What is it? Is it a hole into the engine? Is there a cap needed? The hole was clean somehow
Also when dropping the oil pan I had to remove some foam like padding between the end of the oil pan and the bell housing cover. Do I need to replace this foam padding? Does it stop vibrations? Does anyone replace it?
Dropping that oil pan is a lot of work! Do not underestimate the labor needed to get it off. Old rusted bolts did not help. Also need new motor mounts as both were broken and leaking oil
Machinist tells me that the maximum clearance in the damaged journal is .0003 (he used plastigage) Thanks Dougs968 for the info. We could not find the wear spec as it was not listed in the workshop manual or in any of his manuals.
Probably would not have made much difference in my decision to buy spare head from Raj, who just happened to have one all done up and ready to go. Must admit that I was a bit skittish about putting a bunch of head work into my head (getting 4 new exhaust valves and all new seals and a valve job) only to find out that it was something the head that I cannot see that caused the cam journal to lose oil which I am now pretty sure led to the timing belt failure.
Anyway that decision led to me deciding to take off the oil pan and oil pump etc to clean them up and check for debris or anything else that might have caused the failure. A lot of work stripping all of the pieces and parts. To get the oil pan off is quite a chore. Remove power steering rack (badly leaking anyway so need a new one) remove cross member and clean it up, covered in oil from 20 years of leaks. Lower balance shaft badly leaking so that's off too. Oil pan was clean as was the pump so I am thinking things are OK.
Had a bad oil leak on the drivers side at the oil filler assembly- two O rings that seal the assembly to the block were leaking. That is off as well and I found a hole or boss on the block that was obscured by the filler assembly- I don't remember taking anything off this boss but it has a hole in it. Anyone know what this this hole could be. 1st picture shows the hole in question with a short yellow screwdriver in it. Second shows the hole with a mirror pointed so as to get a look in the hole. Third shows the approximate location of the oil filler assembly so one can figure out where this is on the block. This hole was obscured by the filler assembly and I am sure I did not disconnect anything from it. What is it? Is it a hole into the engine? Is there a cap needed? The hole was clean somehow
Also when dropping the oil pan I had to remove some foam like padding between the end of the oil pan and the bell housing cover. Do I need to replace this foam padding? Does it stop vibrations? Does anyone replace it?
Dropping that oil pan is a lot of work! Do not underestimate the labor needed to get it off. Old rusted bolts did not help. Also need new motor mounts as both were broken and leaking oil
06-11-2012, 12:32 AM
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Dandy,
I saw the same hole under the oil filler, of course mine is still covered with oil/dirt
I'm not sure what that is for.
I'm sure someone will.
I'm planning on tackling the oil pan next year. Looks like fun.
I'm in the middle of belts waterpump and cam chain/pads. I was impressed at how clean the inside of my head was.
I have seen a belt break like that and it was definitely due to one of the rotating items failing to rotate (non P-car)! Sorry to hear about yours.
I saw the same hole under the oil filler, of course mine is still covered with oil/dirt
I'm not sure what that is for. I'm sure someone will.
I'm planning on tackling the oil pan next year. Looks like fun.
I'm in the middle of belts waterpump and cam chain/pads. I was impressed at how clean the inside of my head was.
I have seen a belt break like that and it was definitely due to one of the rotating items failing to rotate (non P-car)! Sorry to hear about yours.
06-13-2012, 06:31 PM
#28
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Sorry to hear about your timing belt breakage.
I got real lucky when mine broke during the engine start up.
Only broke one exhaust valve.
I found a really good machine shop and they fixed the bad valve, the guide and did a port polish for $450. I noticed an increase in hp and an overall better running engine afterwards.
I recommend having a similar top end clean up to anyone thinking of doing there own timing belt. Well worth the money.
I got real lucky when mine broke during the engine start up.
Only broke one exhaust valve.
I found a really good machine shop and they fixed the bad valve, the guide and did a port polish for $450. I noticed an increase in hp and an overall better running engine afterwards.
I recommend having a similar top end clean up to anyone thinking of doing there own timing belt. Well worth the money.
06-15-2012, 12:11 AM
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Well, if anyone ever had a thought to question Raj Datta's bona fides, let me absolutely refute those thoughts. I just received tonight a reconditioned cylinder head from Raj that I had bought sight unseen last week along with some gaskets etc. It cost me quite a bit more than I really wanted to invest in the car but the value received was far in excess of what I bargained for. It was everything that Raj said it was- it appears to my amateur eye to be a completely rebuilt head- new valves, valve seats, valve job, decked, totally ready to install. More than a few people tend to exaggerate either the condition of their cars, the condition of what they are selling etc- none of this applies to Raj. Raj and I settled on a price for this totally redone head that was just a bit above what a junkyard head (from a running engine but with 100k miles or over) would have cost.
I guess in retrospect that I probably should have asked for pictures of the head prior to sending Raj the money but the truth is that I never once felt that Raj was anything other than exactly who he said he was or that he was anything but totally honest. He shipped it on the same day he promised he would and it arrived in perfect condition because he spent the time to package it properly.
Have a big shipment of seals, replacement motor mounts, hoses, etc coming in tomorrow from Paragon as well as an OEM Porsche timing belt from Sunset Porsche out in Oregon that Raj suggested I get. With that I should have everything I need to put the engine back together again this weekend. Machinist has given the cams a clean bill of health, so the rest is up to me. It is a lot of work as I have it stripped down pretty far and I refuse to put back dirty parts on the car. Easy to underestimate the time it takes to get rid of 20 years of dirt and oil and put this thing back together. At least I won't be worrying about stripping out a bolt head or snapping something taking it apart. I will post a few pics of the engine in various stages of the rebuild. Wish me luck!
I guess in retrospect that I probably should have asked for pictures of the head prior to sending Raj the money but the truth is that I never once felt that Raj was anything other than exactly who he said he was or that he was anything but totally honest. He shipped it on the same day he promised he would and it arrived in perfect condition because he spent the time to package it properly.
Have a big shipment of seals, replacement motor mounts, hoses, etc coming in tomorrow from Paragon as well as an OEM Porsche timing belt from Sunset Porsche out in Oregon that Raj suggested I get. With that I should have everything I need to put the engine back together again this weekend. Machinist has given the cams a clean bill of health, so the rest is up to me. It is a lot of work as I have it stripped down pretty far and I refuse to put back dirty parts on the car. Easy to underestimate the time it takes to get rid of 20 years of dirt and oil and put this thing back together. At least I won't be worrying about stripping out a bolt head or snapping something taking it apart. I will post a few pics of the engine in various stages of the rebuild. Wish me luck!
06-15-2012, 07:20 AM
#30
Three Wheelin'
Raj's expertise and honesty is second to none - glad you were able to work out a deal! Good luck getting the car sorted, it's the rebuild part that's usually the most fun!
Steve
Steve
