odb812

It looks like most of yours broke further towards the base of the tooth than mine. The small tooth in your hand is about the same amount of material as I had missing from each tooth.

dougs968

I am thinking about how this happened. Each chain wheel has 19 teeth. The cam is made of cast iron, which seems to be brittle in this case. From my way of looking at it, there are 8 or 9 teeth engaged at anytime. The exhaust cam lost teeth in a random way but the intake lost none.

It is a fact that severely worn pads can lead to engine failure. Could other things be going on also? Such as, improper heat treatment of the exhaust cam, leaving it in a more brittle condition? I have no idea how they did it at the factory. I am pretty sure they could have differentially heat treated the camshafts sprockets leaving them in more of a tough/hard state before finish grinding.

Another thing. If the variocam actuator is a little "sluggish", could there be a little chain slack that would cause the chain to get a little "whippy" and want to start jumping teeth? Or in this case the chain rides up a tooth, can't make it over, binds then breaks the tooth? Maybe hard on/off throttle?

If this all sounds like crazy talk just ignore, I was thinking out loud.

Doug

odurandina

the 968 is a low production car. and not to beat this to death, but we're not carrying the mantenance schedule of a $10 k 968 from the guy across town who was kind enough to sell it to us.... we're carrying the maintenance schedule and costs more typical of a $60, 70, or 80 k car. we could replace our chains and pads (and other expensive parts) more often. and the cam teeth would probably still fatigue and break..... therefore, we should be replacing the cams at some point... these types of problems are all part of the price we pay for trying to get 250 horsepower out of 4 cylinders. 250 hp from 4 cylinders (no turbine) is might not seem unreasonable, but it's still a lot. 550~600 hp from 8 cylinders is less.... due to (the imbalances, stresses and vibrations from the 2 up/2 down design and 60 pound crank).

maybe some of the engines rattle themselves to death.


.

Eric_Oz_S2

The theory goes the chain stretches over time and the chain therefore does not fully engage each tooth leading to stress on the leading tooth. Cyclists change their chains regularly to avoid wearing the cassette teeth for this reason.

wpb968

Eric,

I think you may be correct that chain stretch is the issue. The internals on my spare 968 motor were remarkably free of wear after 135k street miles. Even the rod bearings were in almost new condition. The teeth on the cams I sold show no wear. The teeth on the cams are cast which will make them very brittle and prone to fatigue.

I have been a cyclist for years and chain stretch is easy to detect with a ruler. All you do is measure up the links. I will measure up my 135k vario cam timing chain to see what wear it shows versus the new chain spec. I believe that chain stretch would shear off the teeth. The pads on my vario cam assembly were original and were worn. They had several pock marks and chips out of the upper and lower pads, but no grooves or cracks in the pads. None of the chips were found in the engine and I believe they were all small enough to get trapped in the oil filter. I assume that Don replaced the pads when he installed my used vario cam tensioner assembly in his motor after his timing belt failure.

I will report back on my old chain measurement. Perhaps someone has a new chain that they can measure up or find the new chain length spec in the service or technical manual.

Thanks,
Steve

sydneyman

im sure its a combination of a ton of things, including those you've listed.. unfortunately nothing acts the way its supposed to given enough time/mileage... that goes for bodies too

i agree its an exotic motor with a lot of cool gadgets that were pretty new back then. not so crazy nowadays. i think we're going to see crazy engine problems in the next 20 years on new porsches with all of their compliated systems... and forget about electronics...


i think you're exactly right. hopefully replacing the chain with the pads as a regular maintenance item(maybe every other timing belt?) will keep things in spec... short of anything like in Doug's post goes on...

id love to see that comparison. i'll see if Pete can rustle up some numbers or measure my new chain before it goes on.

Dandy_don

Before you spend $1,600 or more on a new tensioner, please do yourself a favor and call/email Parts Heaven in California. They will see you a used tensioner for $75 and a brand spanking new one in the Porsche box for (ready for this) $175 yeah that's righ $175. I found their deal right after I had agreed to pay Steve $150 for his used tensioner- it works great by the way Steve- thanks. So, get in contact with them and they will fix you right up. I questioned him closely on this because I thought he was talking about a new timing belt tensioner- but I was told no it is the tensioner under the valve cover and it costs $175. Also Raj put me on to a stronger racing chain (made by Iwis who made the OEM part) available at Paragon Porsche in Corpus Christi. I had the exhaust cam almost seize on me a month ago while driving home one evening from Dallas- snapped the timing belt- 4 bent valves- no damage to pistons- bought a totally reconditioned head (new valves, guides, and seals ) from Raj (thanks Raj) and it now runs like a champ. Worked my way through a strange "brapping" noise coming from the tensioner solendoid when the throttle was blipped- that caused me to buy Steve's used tensioner and now all is well in 968 land. So give Parts Heaven a jingle and prepare to be pleasantly surprised.

odurandina

..............

sydneyman

thanks for the heads up! actually my tensioner is fine, it turned out to be just the cam teeth... but if they find my tensioner to be due for replacement then i will give them a call. thanks!

wpb968

I cut and measured my old 135k timing chain from my spare 968 motor. Stretched out under load, 32 links measured 13 1/16th, which equals 10.34 mm per link.

Anybody know the new timing chain spec?

sydneyman

one thing that the shop mentioned about this was that the tensioner should adjust automatically for any slack... makes sense to me.


and just a quick update, ive been back on the road for about a week and even made it to a pca concours event. (didn't register in time though for the judging!)
Oh they also gave me a little low end-midrange bump using my cam timing! haven't noticed a real drop in high end but i definitely could feel some more low end power! I'll ask to find out exactly what was done.


i'll see if i can get some more pics. Thanks for everyone's gestures and opinions!

sydneyman

So its happened again

exact same noise along the intake cam under the valve cover... i haven't taken the cover off yet to see. at this point im wondering if i should do the work myself... maybe get some headwork done, new headgasket preventatively, maybe performance cams etc...

the only thing stopping me is if it was the indy's fault... then at least something would be covered if i have it towed back to him... ugh.

any advice? commiserations?

dougs968

Well, shoot. Sorry to hear that. If it were me, I'd take the valve cover off and have a look. You know what it looked like the last time. If the camshaft sprocket failed this quickly again, it would lead me to believe that something else is going on. Not sure what though at this point.

I know you will let us know what you discover. Best of luck.

sydneyman

Thanks Doug, I'll report back with my findings. if i get a wild hair, i might take a look tomorrow...

i think you're right though, its most likely something else going on. From what i hear, it seems the camshafts themselves grow weak over time and i installed used cams so maybe i just need to step up to a regrind or new performance cams... in any case we'll see once i get the cover off...

BikePilot

Sorry to hear it. I'd also pop the cover off and see what's up. I doubt the indy will provide a warranty 2yrs out.