G50 Clutch Fork boss failure

Dauner · Dauner's Garage

Hi everyone,

Car is a 1989 911 3.2 with G50 transmission. I'm in the process of rebuilding the engine and keeping questions related to that in the engine build forum.

This is a different problem. I have the apparently not so rare situation of the clutch fork boss breaking off of the transmission. I am going to try to have a friend that is a seasoned professional tig welder repair it similar to what was done in the thread below.

G50 release fork boss failure

My question is in the clearance in the boss around the fork shaft. when I set the shaft in the remaining part of the bore, it seems like there is quite a bit of clearance. Does anyone know that bore diameter? the shaft itself is a tad under 0.750".

I know the bore only holds the shaft from swinging out with the fork as it's actuated by the slave cylinder piston. The car had the bushing update done at some point.

Oddly, I never knew I had a problem in 10K miles of ownership and driving. Only when I separated the engine transmission did I see the failure. I think the tiny tab that is attached to the end of the shaft that prevents the shaft from rotating was holding it back. not ideal, but seemed to work. I am guessing the boss broke off a while back, as the chunk was not floating in the bottom of the bell housing.

Couple of pics below with and without the fork assembly.

We also have machinists at work. I may review with them what they think.

Has anyone else done the weld repair?

Thanks in advance,

mark

Cory M · 07-02-2019, 02:41 PM

I had the repair done by California Motorsports in Lake Havasu. They cut off the old bosses, machined the case flat, and welded on new ones. The new ones were designed with a needle bearing instead of the factory bushings. It's been working for several years in a race car. I'm happy with how it turned out.

Marc Bixen · Marc Bixen's Garage

+1 for CMS. They repaired a 993 case for us.

Dauner · Dauner's Garage

Thanks guys,

I live by Minneapolis. Sending the transmission case to CMS is a major cost investment. If I lived within driving distance, I would probably do it, but looking for other options...

hcoles · 07-02-2019, 06:10 PM

The factory setup was needle bearings. The needles got crudded up with clutch dust/etc. and dug into the shaft. I figured there had to be a better way, I replaced my bushing setup with IGUS plastic bushings (high temp.) and hard chromed the shaft after I saw heavy scoring after running the bushing "fix" kit. I don't know what it looks like now - my clutch pedal is smooth and lite.
That was maybe 7 years ago. There should be a thread on here someplace.
IMHO - the "ears" are a weak spot - when the bushing/bearing friction increases the force on the ears increases. The failure seems to be rare. There are a few I've seen here that have had this happen.

Quicksilver · 07-02-2019, 11:59 PM

Quote:

Originally Posted by hcoles

The factory setup was needle bearings. The needles got crudded up with clutch dust/etc. and dug into the shaft.
. . .

The needle bearings were a completely incorrect solution to the application. The motion is a very short repeated back and forth motion. That is a very bad use for a needle bearing. A bushing is a much better solution for a short repeated motion.
____________________________

Back to the original question. Shipping it is expensive but California MotorSports is THE solution. They are the G50 people. The alternative is replace the transmission.

Cory M · 07-03-2019, 07:07 AM

looks like the new CMS repair procedure has changed from welding to bolting in new bosses:

https://californiamotorsports.net/collections/porsche-g50-transmission/products/cms-g50-993-gt3-clutch-fork-pivot-shaft-boss-repair

Marc Bixen · Marc Bixen's Garage

Yes Cory, That's the procedure they did on our 993. Also once done, future repairs, if necessary, can be done by just bolting a new pair on. A lot of guys are over powering the drive train, using very heavy PP's and seeing this regularly. Go CMS!