Timing chain IS gear wear

[tharbert]

I was cleaning up my intermediate shaft to start test fitting my engine internals. There is no telling how long it was driven with failed tensioners but when I got the car, it was obvious one or both had failed. Both chain gears show signs of scuffing and wear on the sides, presumably where the chains were slapping. Teeth and groves look uniform.

Is there a test, measurement or rule of thumb for replacing the gears? Is it simply a while-you're-in-there thing?

I'm dreading my lash measurement. I don't think the IS gear lash will be in spec but I don't know yet...

Engine is getting:
New-to-the-car in-spec crank
Case worked over by Ollies - Decked, line bored, oil mod, case serts a plenty
Mag 4 rib pump
new p/c's
polished/re-bushed rockers
New crank, rod and IS shaft bearings
new cams
rebuilt heads

[tharbert]

This is the worst:




These aren't cleaned to the point of installation. I merely wiped them down with some WD40 and an obviously non-lint free rag.

[Flat6pac]

That has to do with the alignment of the chains with the cam drives. You have to do the measurement each time you build and keep it to .25 mm +/-.
Bruce

[tharbert]

I do know the engine has been apart for a reseal which may explain the mis-alignment. Would the gears still be acceptable for continued use, making sure to re-shim the cam appropriately?

The gears aren't terribly priced but from what I've read, replacement is a bear, one of the hard jobs of an engine rebuild besides forking out all the cash.

...and I still have to measure the lash.

[Flat6pac]

Once the measurement is correct there shouldn't be any more side wear.
I see side wear all the time, very seldom is there no wear, even the .25 can eat the gears.
If he has dropped tensioner like you think there would be eaten up metal where the chain passes into the chain box.
Bruce

[tharbert]

Thanks, Bruce. This is but the first of a bunch of questions I'm gonna have. This shyt isn't for the timid...

[KTL]

Are there other signs of tensioner failure like chain slack banging on the upper side of the case/chain housing where the chain enters the engine case? You shouldn't see any lower chain slack contact since the chain ramps will catch the chain as it sags downward.

Reason I ask about slack evidence is because you can also have chain sprocket side wear due to the chains being too tight. The chains can get too tight when the tensioner arm binds on the post. This happens on narrow tensioner arms that are unbushed. It happened on my engine.

coating rocker pins & idler gear pins

[tharbert]

I'll check the chain box tonight but I don't seem to remember any shavings in the oil, sump or inside the housings. When I installed the Carrera tensioners, I chose to stay with the narrow arms with bushings. I was comfortable with the fit. The arm pivoted easily with no room to wander on the shaft.

[eastbay]

I replaced all the chain gears on my rebuild, IS shaft not too difficult if you have a press. If you don't have a press, it is a good opportunity to get one. There are allot of press use opportunities on an old 911.

The chain wear you are really concerned about is on the flat side of the teeth, down in the curve. IMO with all your other $$ pieces going in the motor I would definitely change all the gears and chains as they are relatively cheap and easy at this point.

[NICE 69 S]

Try wrapping a new chain 360 degrees around the gear and pinch the chain together where they meet. If you can pull the chain away from the gear, or the chain feels loose around the gear, probably time to replace the gear.
Bob B