Cam shaft change

[Neil Haughey]

....or as was said above some bad batches of cams. If you guys recall Fen used to bang on about this several years back as I believe he was one of the guys who got stung by this problem,,,,, and that on an S2! However there are cars out there like my S2 race car motor with big mileages that still run sweet as a nut, make a lot of power and/or show little signs of wear.

 

[pauljmcnulty]

If you guys recall Fen used to bang on about this several years back as I believe he was one of the guys who got stung by this problem,,,,, and that on an S2

Indeed, it was his S2 cab. In fact, his indy used to do the cams cheaper than anywhere else at the time, I seem to remember he bought a bulk lot as many were failing.

I wonder if it was before people realised that you need to change the timing chain on S2s when the cars were getting to over 100K, and because of the publicity on forums like this most cars had the chain changed before there was a problem? Or, just that there are fewer 968s so it seems like a higher proportion failing. [8|]

 

[bazhart]

Yes just repeating lots that has been said before (from the perspective of the first business to report this potential problem in a 911 and Porsche World article over a decade ago).

Cams are chill cast (therefore hard full depth) cam lobe wear is usually because of lubrication failure.

When a sprocket wears on the driving flank - the chain roller sits nearer to the next incoming tooth while at the same time the chain has stretched so the tip of the next incoming tooth meets the next roller at the tip instead of down the flank and as it is chill cast (and relatively brittle) eventually it breaks off a tooth.

Replacement cams were machined to a non standard tooth profile that thinned the tip of the tooth more than usual so it delayed the interference between the roller and the tip as things wear - so will last longer - the earlier sprockets are more susceptible.

The right way to measure sprocket wear is with the right sized rollers and a micrometer (which we do) and a chain with a vernier calliper (for stretch).

After checking hundreds of cams and chains this way that have totally failed, just got some teeth missing or quite new it was easy for us to draw a graph with areas indicating OK, marginal and failed and then set limmits so the techician just measures them (and the chain stretch) records it on the service sheet and the sizes are there to confirm if it is still oK, marginal or about to become a problem and then we report accordingly. We have done this for over a decade without problems.

If the sprockets are not too worn then a new chain will extend their life (because it is shorter and will not hit the tip of the incoming tooth as hard). But if the sprockets are badly worn, the new chain will stretch quickly and start the whole damage proceedure sooner.

Porsche listed it as a service for life item probably because the earlier std 928 16 valve engine had similar cams (but much softer timing. lift and profile) and the engines didn't see such large or fast rev changes (especially as most were automatic) - so they do last much longer fitted to them and I think they came out first (or at least they based their expectations on those).

One thing to watch out for is that the thick washer that you tighten the bolt against to lock the timing can bell concave and then only touch the end of the camshaft in a very narrow ring resulting in it often slipping the timing you have set before you lock it in place.

If in doubt check it out!

Baz