Shuddering Clutch 915

[kucharskimb]

Gents

I have a 72 911 with a 915. I use a lightweight Patrick Engineering Flywheel and a spring-style OEM replacement clutch. The flywheel has about 2K on it. the pressure plate and clutch have unknown mileage.

Autocrossed the car for the first time a couple weeks ago. today was the first day driving the car again after parking it for some work.

On the drive to work, i had normal starting/gear engagement, normal shifting and so on. A few stop signs from work, under normal acceleration, i experience a little shudder. I had experienced this sort of shudder when i had a leaking crank seal. At the last stop sign before parking, the car shuddered violently (to where i looked in the review mirror to see if the engine/trans were in the street). with the car still running, i had no problem selecting gears in my parking spot. (pushing the clutch pedal still works as advertised)

I thought, well, by lunch time, i should have a couple quarts of oil on the ground under the flywheel where the crank seal let go. good news is: it is dry, no leaks...

I've read some of the archives suggesting checking clutch cable release, engine/trans mounts, CV joints... I will do this today.

Is the next step to replace the clutch/pressure plate/resurface flywheel and associated parts? I would plan to replace with stock type products, as it is 90% street use.

Is there a thickness check for these items to ensure there is enough material to engage all of the components?

Appreciate your help.
Matt

[john walker's workshop]

check all the engine to trans nuts and bolts for tightness.

[kucharskimb]

john, thanks, i'll do so when i check the rest.

another question:

The clutch had oil leak on it from the previously mentioned crank seal leak. i cleaned everything to the best of my ability with parts cleaner prior to reassembly.

Would the oil have contaminated the friction surfaces, ruining them? if so, what parts should be replaced?

Thanks
Matt

[Grady Clay]

Matt,

It doesn’t take much oil on the clutch to cause judder. Not necessarily enough to dip on the ground. One of the common symptoms is that it is more prevalent (or more juddery) when hot.

If it turns out that the main shaft seal is leaking transmission lubricant onto the clutch, the ’72 915 transmission input shaft seal is installed from the inside of the transmission and requires disassembly. Do some Pelican searches on this subject.

Best,
Grady

[kucharskimb]

everything was tight after inspection. looks like i'm getting a little drip down from the thermostat. i'll reseal and reevaluate.

would the previous oil bath have ruined the clutch? or does it "burn off" after some driving?

Matt

[Jim Smolka]

Also check the CV joint bolts while under the car.

Also measure the amount of travel in the clutch cable. Total travel should be about 25mm.

[Grady Clay]

Matt,

No, you cannot “decontaminate” an oily clutch disc. The good news is it is only the disc that is ruined by oil leaks.

On the other hand there is another issue with oil on the disc. That is it sometimes doesn’t release easily. The oily (and polished) disc sticks to the pressure plate or flywheel friction surface. This is very bad because the grip of the stuck clutch can overwhelm the synchronizing forces in the transmission and cause major damage to syncros, sliding sleeves, and gear dogs.

Best,
Grady

[kucharskimb]

CV joints were good.

The clutch will disengage from the engine. not the problem. problem is releasing the clutch pedal and initiating forward propulsion... that's when i get the shudder.

when i rode the clutch a little longer when taking off from a stop, it was smooth. shudder only occured on more abrupt releases/quicker takeoffs.

Matt

[kucharskimb]

Grady
Thanks for the info. Would your suggestion be replace just the clutch disk, or get the whole kit while i'm in there?

Matt

[john walker's workshop]

warped flywheel or oil on the disc are the likely culprits if the mounting hardware is tight. you can get a bit of a shudder if the engine mounts or clutch cable are old and worn, but yours sounds much more violent. if there's a bunch of race stuff in there, perhaps a complete street setup would be best if that's where the majority of your driving is. be sure to replace the mainshaft seal if there's any oil in the guide tube, or if you suspect it may be coming from there. oil from the mainshaft is much more likely to get slung out onto the disc than any flywheel seal or thermostat o ring leak.

[juanbenae]

was the car jacked up out of level for an extended amout of time? i have had the front of my car elevated for a couple weeks and after that i experienced the same thing. it did go away after a while, but it sounds like that may be the deal.

[Grady Clay]

Matt,

I assume you checked the engine and transmission mounts to the chassis based on JW’s asking about the engine-to-transmission mounting nuts.

Are the large mounting washers there and in the proper orientation?

Are the engine and transmission mounts firmly attached to the engine and transmission?

Is the engine and transmission properly tight to the chassis? Grab the tail pipe and give it a wiggle. Have someone watch the engine (deck lid open) while you induce the judder.

If above checks out OK, probably clutch linkage is next

Best,
Grady

[kucharskimb]

all good info... i can't find anything that would suggest something is loose or broken as far as mounting goes.

K9
yes, the car was jacked up in the front for about a week at varying elevations. lets just say this is it. how long do i give it to go away? or is the disc bad now?

grady
washers are installed so they cup toward the bolt heads (away from the rubber) is this correct? shaking on the tail pipe moves the whole car

if i have to replace the disc, do i replace the pressure plate for matching purposes? does the flywheel need to be resurfaced?

as far as the light weight stuff, the flywheel is still steel. it just has some material removed between the bolt flanges around the outer circumference.

you guys are great. thank you for taking some time to point me in the right direction.

matt

[frankc]

I had a very similar shuddering problem once. Checked for leaks and replaced clutch cable, but didn't help. Ultimately the culprit was a hairline crack in one arm of the clutch fork which did not show up until there was enough pressure applied to disengauge the clutch.

Frank

[Bobboloo]

I'll put money on a bad mainshaft seal. These get negleted with 72' trannies because unfortunently you have to disassemble the tranny to replace them.

[kucharskimb]

i rebuilt the tranny about 3K ago. i hope it hasn't let go in that amount of time...

these things are always timely... i'm getting shoulder surgery tomorrow, so i'll have to wait to drop then engine for a couple weeks. i guess i wouldn't be driving with my right arm in a sling anyway.

keep throwing out ideas. i'll let you know what i find.

matt

[Early_S_Man]

Matt,

Once oiled, the clutch disk is ruined, and must be replaced or relined!

While you have the engine and transaxle out to replace the clutch disk ... you will be able to tell if the input shaft is wet with gear oil. Excessive runout of the input shaft can ruin the seal in fairly quick order ...

[john walker's workshop]

i you have one of those lightened flywheels with the large scallops cut out between the bolt holes, it may have warped. i would replace it with a new stock unit. it's also best to replace the entire clutch assembly, if you're chasing a shudder problem, rather than just one part like the disc. if you choose to save the flywheel, be sure it's true, by having it machined. tell the machinist not to bother if it's warped, due to the possibility of creating an imbalance problem.

[kucharskimb]

got it. i'll pull it in a couple of weeks and replace the entire clutch and have the flywheel looked at.

thanks for all of the information and advice.

Matt

[asphaltgambler]

The clutch friction surface is like a sponge when it comes to oil. You can clean it but there is more down in the pores. When the disc heats up the remaining oil will come to the surface.

If all is tight you may need to 'go back in thar' with the intention of replacing the setup. Check the flywheel for hotspots, or heat-checking.