Re: IMS sprocket slippage on a 997?

A good friend sustained engine damage due to his engine cams going out of time due to the IMS sprocket slipping on the shaft--first time I've heard of it. The cause was determined during the engine tear down to see if it was repairable.
Does anyone know how this sprocket is attached to the shaft at the factory?
I ask since I've seen youtube videos that show the sprockets being attached via drilling and tapping holes in the shaft and then attaching the sprocket with a number of set screws, but this looked to be part of the engine rebuilding process.

Quote
MikenOH
A good friend sustained engine damage due to his engine cams going out of time due to the IMS sprocket slipping on the shaft--first time I've heard of it. The cause was determined during the engine tear down to see if it was repairable.
Does anyone know how this sprocket is attached to the shaft at the factory?
I ask since I've seen youtube videos that show the sprockets being attached via drilling and tapping holes in the shaft and then attaching the sprocket with a number of set screws, but this looked to be part of the engine rebuilding process.

Don't think I have ever heard/read about the IMS sprocket slipping on the IMS. It is a rare event but the chain can skip time due to a tensioner failure or possibly from some debris from something else that gets caught between the sprocket and the chain.

The only reports of a chain sprocket slipping or coming loose are just one or two -- it is a very rare event -- of a cam sprocket slipping. Don't think it was with a 997 N/A car though, it was with the 997 Turbo engine.

(The 997 Turbo, those models withe Mezger engine, can have the gear on the layshaft come loose the bolts work loose or the key works loose and the gear can spin.)

I believe the sprocket is a press/shrink fit. Pinnng is what is done to prevent the sprocket from coming loose. This is AFAIK not done by the factory. I am not sure if this is done by dealers if the engine is repaired in-house (that is if it isn't replaced under warranty).

Thanks for the reply, Marc.
I've seen reports of this type of failure on internet searches, but the frequency would seem very small relative to the IMS bearing failures.
FWIW, when the engine was inspected, the IMS bearing assy. looked fine.

The IMS bearing press fits onto the IMS shaft., and it would matter little if it moved.
The IMS shaft has a set of drive gears - from the crank, to the cams. I believe they are in fact one locked unit.

So what likely happened is that the IMS wobbled enough, or a tensioner failed enough, that the chains could jump time.

I suppose the entire unit could move on the IMS, but i have never heard of that either. I don't recall how its attached - pressed, keyed or ????

While interesting, one point out of 10s of 1000s hardly qualifies as a pattern. Stuff happens. Sometimes weird stuff.

Grant

gee-lenahan-at-gee-mail-dot-com

Keep in mind, that when these engines fail, not all diagnosis are correct. When my 2nd engine failed, my indy, and experience Porsche mechanic said it was without a doubt and IMS failure. Grant pulled the IMS and it was fine. There is quite a bit of conjecture out there and the only way to know for sure what happened, is tear the engine down.

The first question I would ask your friend is "how did they know?".

When my first engine blew, the mechanic at the dealer asked me to start it after it came off the flatbed. It turned and sounded like hell with metal clanking and such. The mechanic said it didn't "sound" like an IMS failure but he just didn't know. Diagnosis requires tear down unless the failure is exposed.

Peace
Bruce in Philly

Bruce--thanks for the reply. I had forgotten that you had two and Grant brought one of them back to life, so you two guys have a lot of experience with bad Porsche motors.
From the reports I got, there was no gushing oil or loud metal on metal noises, just a loss of power and he turned off the engine quickly. The car was taken to an experienced Indy that pulled the motor and started going through it, pulling one of the heads first, which had all the valves bent after hitting the pistons. The lifters, apparently, took real beating and sent metal through the motor. The cam timing was checked and was found to be way, way off--causing the damage.
The IMS was inspected and found whole and in good condition.

Grant

gee-lenahan-at-gee-mail-dot-com