I just rebuilt my L98 and have issues with clutch non-release. The only thing I changed was I had my flywheel surfaced, stupidly I did not measure the thickness before cutting but I did measure after, it is about .970 measuring from the crank flange to the friction surface, with the friction surface resting on a flat surface.

I dropped the trans and sure enough, one of the bolts for the pressure plate was bottoming in its counter-bored hole. This caused the pressure plate diaphragm to be uneven. I machined the counterbore 1 thread deeper and that fixed the diaphragm problem, but the clutch engagement still seems too low.

I am in the process of changing out all the hydraulics and rebuilding clutch mechanicals to make everything like new. I thought for the price I would order a flywheel, bought a SACHS flywheel. It measures .960 thick, thinner than my flywheel which is already machined!

I'm wondering what the original thickness of the OEM flywheel was? I know it's a long shot but does anyone have a NOS flywheel handy they could measure? GM Performance still sells the same flywheel (16lb). The flywheel P/N is a 14088646. Casting number is 14088647N.

 

Either I'm measuring wrong or they machined a chunk off that flywheel, I come up with 1.26 and IIRC I think I had a machine shop clean it up slightly a looong time ago

Edit: I just read on one of my "internet notes" that a 4+3 flywheel 1 1/16" thick. (?) (That's 1.0625)

 

Friction surface to the bench and measure through center to the mounting flange to the crankshaft.

Does yours have the correct casting number on the back-side?

 

I hope it's not 1.26, but you could have a different flywheel. I ran across a write up that mentions the 1-1/16 dimension after tons of searching, it was for using the 4+3 flywheel to replace the dual mass FW on a 6 speed. When I measured mine I set the friction side on my drill press and dropped the depth guage end of my dial calipers through one of the bolt holes. Measure a few times if you can to make sure it's repeatable. Sure would help me get this figured out!

 

OK I measured just as you did and I mine comes in at .968

I don't know what I did last night but in my defense is was late, dark, cold and I might have been at the far end of a six pack

Ya that's the same article I got 1 1/16" from. Your measurement sounds good/normal as I believe I buddy I was working with had it surfaced when we pulled it out to repair the OD. I think we had to come up with a longer fork push-rod to compensate too.

 

Thanks for checking your flywheel DanZ51. I emailed the techs at Fidanza and Ram, the Ram 16lb steel flywheel flange to friction surface is .690, the Fidanza 11 lb aluminum is .965. The Fidanza tech was most helpful and stated that they based the thickness of these off stock applications. If there is a way to get in touch with GM Performance I'd like to know...It could be only a few thousands were removed off my flywheel and the Mr. Gasket pressure plate bolts were not quite correct for my application. I'm not sure I want to spend the $330 for a GM performance flywheel to solve this little mystery... so I'm going to mess around with ball stud height to get things to work correctly. If someone does have a stock flywheel that hasn't been surfaced I still would be interested in knowing the thickness.

 

A fellow I asked got back to me today and mentioned .964 for one that he has. No known history of was it surfaced or if it was how much.

Call a local GM dealer and ask them to check the "LOCATOR" for a dealer maybe in the vicinity that has a flywheel "on the shelf". Give them the GM # 14088646. Contact that dealer and ask if they'll check the height for you and mention you want it "face down on the counter and through the center to the crankshaft mating flange". Just explain your issue and I'd think they'd help. You might need to give them an hour or so to gather the info and either call you OR you call them back.

 

Thanks for checking that flywheel.

I did some testing with an adjustable ball stud and could not get an improvement, so I no longer think the flywheel is the issue. With the standard ball stud the fork geometry seems good and is roughly parallel with the bell housing flange at mid travel. So I tried swapping the old parts back in. Put the old pressure plate in, no difference. Yanked that and put the old clutch disc in...bingo! I am able to rotate the disc. It seems like the air gap is still not that good, maybe 5 thou or so. I measured the thickness of each disc, free and compressed and got the following:

Luk:
.3310 free
.2935 marcel spring compressed
.0375 difference

Sachs:
.3295 free
.306 marcel spring compressed
.0235 difference

So the Luk clutch disc needs an additional .014 lift to release over my old Sachs clutch disc. There is a visual difference in the gap that I didn't notice before. I see Ram's clutch discs are in the .010-.015 range for clutch disc compression, this would give a reasonable air gap.

Now the question is why is the system so marginal on air gap? With the slave locked out, I have about 1" of pedal free play at the top of the stroke. After that play is removed I have a very firm pedal. Could be I still have some air. I'm getting about .75 slave stroke with no load, which is about the max theoretical stroke. Pushing on the clutch I get about .625 stroke at the slave. In all cases I'm pushing the pedal all the way to the floor, past the dead pedal.