901 Clutch Disc Exploded - Why?

[andrew15]

We were at a Chump Car race in spokane last week and after only 1hr, the clutch packed it in. We thought it was a throwout bearing initially, but on removal of the engine and transaxle trackside, we found the friction surface of the clutch disc had come apart in dramatic fashion.

Luckily, we were able to get a 912 setup form a guy 2 pits over who happened to have a spare engine and transaxle 5 min from the track.

Any idea what would cause this? I wouldn't think it's a power issue as we are running a stock 2.4T engine that is probably down on power due to a pulled head stud or two.

I'm not sure of the brand of the disc - I've never seen the green paint before - maybe a relining of substandard material?

Thanks
Andrew M

[db_cooper]

Looks like a remanufactured clutch disc. The green paint is for quick identification in the production line for boxing. Obviously it did not hold up.

[Grady Clay]

Andrew,

That doesn’t appear to be a Sachs clutch. What brand?
Was there high-torque stress or overheating?
What pressure plate?
What max rpm? My ‘69S Sachs survive nicely to 9000 rpm.

Please include us in your forensic analysis.
Clutch disc failures are rare and everyone needs to know ‘why?’

Best,
Grady

[andrew15]

Thanks Guys -

While it was a track situation, the driver is experienced with early 911s and being an endurance race we were mindful of the clutch and transaxle.
Max RPM was set to 7,000, and power was probably only about 125-130hp - nothing too taxing!

I looked at the pressure plate and it has a small spray of green paint as well - I'm thinking it may be a rebuilt plate with higher clamping force. The disc material looks to have been 'compressed' and then cracked around the rivets. As well the fingers of the pressure plate spring are a bit grooved where the release bearing touches it.

That said - it's all marked F&S or Sachs - so I'm guessing a remanufactured unit of some ill repute.

Thanks
AM

[Grady Clay]

Andrew,

Please keep us in the loop as you diagnose this problem.
You have a wealth of experience available here.

Yes, you will install a new disc.
What are you going to use for pressure plate? Same?
If not same, what?

What are the measured dimensions of the flywheel?

Please tell us more about your ‘set-up’ including any deviation from stock.

Best,
Grady

[911pcars]

Another possibility. An incomplete mis-shift could have caused this - as in a 5th to 2nd or equivalent money shift, but without engaging the clutch and causing the engine to spin at max revs. In this brief state, the clutch disc and input shaft spins at the same speed as would the engine if fully engaged. Centrifugal forces can weaken it and additional revs/miles can finish it off.

Sherwood