jim morehouse

Thought I'd report back on the final results. I decided to replace both
cam gears. Bill Ball was absolutely right, the passenger side was
noticably worse than the drivers side. I followed Marc's advice
in the thread and carefully marked the postion of the gear w/r
to the bolt heads (that hold down the distributor rotor) and also
the position of the gear 'notches' w/r to the notch on the rear cover
while at 45 degrees. Then I transferred these markings to the new
gears. The slots in the gear allow about 2 1/2 teeth of adjustment. No problems on re-assembly..thanks to all for your advice. Everything went well and last night I finished up my first timing belt job (video of dance available on request but it's not a pretty sight )...everything lines up just as before I started and the car seems to run fine although I haven't had a chance to drive it because of the *&^% snow. THANK YOU!
Jim
---------------orig post---------------------
'88 S4 5 spd, 145K miles. I'm in the middle of my first timing belt job and just got the belt off (thanks to Mssrs Pirtle and Plumley for their writeups!). Looking at the cam sprockets, the top of the teeth have shiny areas on them. I've attached a pic of one below. I know this indicates wear but I'm not sure if this warrants replacing them or not. I tried putting a metal straight edge across one tooth and shining a small light behind it....I can see a small slit of light from the other side, indicating some 'bowing' but would be hard pressed to hazard a guess as to the actual measurement. Do I need to replace them? If I do, is there a good writeup somewhere (I haven't done a search yet of either the FWSM or the archives and will if I need to replace them...just thought if anyone knows of one 'off the top of their head' it be good to know).

TIA,
Jim

IcemanG17

Jim
I would replace them. Worth the piece of mind knowing that they are new.

ErnestSw

Once the hard anodizing is worn off and the softer metal underneath is exposed the wear rate accelerates. I'd replace them.

worf928

The WSM has a quite adequate write-up on the steps required.
Those sprockets are borderline at best. You might be able to get another year or two (say 10,000 miles) out of them before they are totally polished - assuming you don't over-tighten the belt. But, since they will wear to the point of requiring replacement before the belt is due for R&R it would seem that now is the time to replace them. And then you get another 145k miles of not worrying about them.

Big Dave

You may need to adjust valve timing when you install the new gears unless someone knows a way around it. I'd suggest checking the oil pump and crank gears while you're at it.

jim morehouse

Thanks guys, I appreciate the input. My brother was over (no 928 experience) and we were looking at them and he thought the wear was minimal and didn't see that they'd be a problem. My response to him was I was checking w the list but "would you bet the top end of your motor on that?". Guess I'll place the order.

thanks,
Jim

FlyingDog

Has anybody tried re-anodizing the factory gears? It's probably a waste of time but I am curious.

borland

It's hard to tell by your photo, but the upper teeth look fine. Sure the hard coating shows some wear, which would be typical for about 80-100K miles.

If you don't put more than 5K miles a year, then it would be a waste of money to install new cam sprockets, cause you'd be looking at a new belt in 20K miles anyway.

Otherwise they are not too difficult to replace, however the valve timing is an issue if you replace them.

UKKid35

You don't have to adjust valve timing when you replace the sprockets. You will have to fit the three 5mm bolts and mark precisely where they locate and then transfer those marks to the new sprockets.

Bill Ball

These are far from the worst I have seen, even on my own car. Tell me, is the passenger side worse than the driver side? True in all the cases I have seen. If so, you can leave them be and examine the passenger gear at each timing belt tension check (15K miles). I have no argument with those who say replace them for peace of mind and I have done that myself, but you are quite safe as is, as long as you check the gear at fair intervals.

slate blue

I'm sorry but yes you do, they are not expensive either, I bought them last year for $100 AUD that is $78 USD.

heinrich

Agreed with Bill, but I would replace them.

WallyP

Jim,

If I saw that wear on the sprockets on my '90 GT, I would put the new belt on and drive.

The drive surface is the round indentations, not the top of the teeth. The drive surface looks fine to me.

worf928

If you leave the old sprockets on just make sure that the belt tension is correct and not overly-tight. The tighter the belt the quicker the sprockets will wear.

Some may recall a thread I started a few months ago "Photo without Caption" that showed a belt missing 10-12 teeth. The most obvious cause for the failure was a way-over-tight belt that ate the sprockets in about 2500 miles.

T_MaX

Dido what Wally P said (belt&drive)!

Exception: "The drive surface is the round indentations, not the top of the teeth."

The drive surfaces are the leading flanks of the sprockets teeth.