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924RACR

Hey all, we're hoping someone can help us with some reference dimensions off a spare or otherwise accessible flywheel. Working on Karl's '80 US-spec 931, and we're having problems with what appears to be the pressure plate height stack-up for clutch release fork positioning.

Symptom is the clutch fork, at rest position, is further to the front of the car than it ought to be; this end up limiting the travel of the fork when the pedal's pressed, as the end of the clutch fork hits the far (front) side of the bellhousing inspection hole, maybe 11mm away.

From the 944 shop manuals, reference travel for a properly-bled clutch hydraulic system is 15mm, indicating we're a bit short.

I'm pretty sure this is enough travel to allow shifting and the car would be driveable... but we're trying to understand where this came from and what the problem could be, if we should be looking at machining or parts might be wrong.

Flywheel was machined previously when a new clutch kit was installed - the correct 931 clutch kit from Sachs, p/n KF20002 (no longer available).

So all should be well, except when all assembled the clutch fork appears to be out of position. My thinking at this point is that, unless the pressure plate delivered was incorrect, the most likely explanation is that when the flywheel was machined they took some/too much material off the mounting surface for the pressure plate. And photos confirm they did touch that surface; there is a step, but maybe it's not as big as it needs to be.

So if anyone has a flywheel they can measure, and provide some relative depth dimensions on the friction surface vs. the pressure plate mounting surface for us to compare to, that'd be really helpful...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Fifty50Plus · Posted: Sun Apr 05, 2026 3:09 am Post subject:

Sounds like a fat pressure plate or ToB. Machining the flywheel would increase the distance from a PP to the ToB. Or, can the throw out arm be installed backwards?
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1979 924 NA race car H-Prod SCCA
1982 924 NA race car - Sold
1981 924 Turbo sold
1982 924 Turbo sold
1972 911 E race car - traded for Cayenne Diesel
1975 914 1.8 Building for H-Prod SCCA

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

I might still have one from my turbo days. I’ll try to find it tomorrow. Not sure if its an S1 or S2 though, i’ll check the pn on it.
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Supercharger and EFI kits
https://www.the924.com

924RACR · Posted: Sun Apr 05, 2026 9:09 pm Post subject:

Thanks Ciprian, would greatly appreciate it!! I think these dimensions would be the same S1 vs. S2, as it looks like all the rest of the parts are the same - pressure plate, clutch, and release bearing...

Impossible to install the clutch fork backwards - but yeah, almost makes one think something like that is the case...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

924RACR · Posted: Sat Apr 11, 2026 6:05 am Post subject: Anyone else?

Looks like Ciprian's flywheel was the wrong one... anyone else have a spare Turbo flywheel laying around and can measure?
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

924RACR · Posted: Sun Apr 12, 2026 10:54 pm Post subject:

Looking around online - I see reference to exactly the needed dimension for the 911, at 22.5mm. I believe in fact this probably also applies to the 924 Turbo clutch, since there's commonality with the pressure plate and clutch disc...

Gonna see about re-measuring ours again here soon, maybe also try experimental shimming it up, see what we get for dimensions and geometry...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

barnwerks · Posted: Sun Apr 12, 2026 11:23 pm Post subject:

I pulled out some 931 parts I have, the first flywheel I found is 931 102 208 01, it has no step. I believe this was from a late turbo. I'll dig around some more to see if there may also be an early version and what the differences may be.
Other things to check are condition of the fork, cracks in the arm or wear at the bearing contact pads could cause this. Also, check that the release bearing is installed / shimmed correctly.

924RACR · Posted: Mon Apr 13, 2026 12:31 am Post subject:

Thanks for those reminders, will check them. Release bearing and fork bearings were all replaced... but will have a closer look at the fork itself, good point on wear there for one.

We definitely double- and triple-checked the assembly of the release bearing, since that was the easiest/most obvious point of error. Some shims would certainly help, but we didn't have any before or after... and I don't know that we'd have enough to make up the difference, it would be pretty substantial I think - probably more than a tenth of an inch or so I'd expect a shim would close up...

Though that does also speak to a possible good option for recovering this setup to a good state...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

barnwerks · Posted: Mon Apr 13, 2026 3:02 am Post subject:

Here are a couple pages from the FSM that may be of interest.
Note the different flywheel depth (with a step) for use with a rubber centered drive disc.
The image of two flywheels from my stash, one early, one late, neither have a step and both were spring centered discs.

Sorry, I am not able to upload images easily to this site. Vaughan, I think I have your email and will send you the images.
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