C4racing

The hardware: '89 Corvette with a recently installed SPEC Stage III carbon clutch, lightweight pressure plate, and their aluminum flywheel. Master cylinder, slave cylinder, and connecting tube/hose are less than 2 years old. Although the clutch hydraulics were never opened, after the recent install about 4 master cylinder reservoirs worth of fluid were bled through the slave cylinder to make sure there was no air in the system.

The observations: Clutch engagement/disengagement point is in the top half of travel, maybe 1/3 from the top. Some chatter on engagement (this clutch seems to want to be 'on' or 'off'). Pedal effort seems normal. The warmer the car/clutch get, the more difficult it is to shift gears! After a recent engine rebuild the car was driven for over 400 miles with NO hard acceleration and never over 4,000 rpm. Then off for chip programming and dyno tuning. Now that the car is (otherwise) mechanically ready to be driven(!) I can't even USE the new engine because I can't shift the transmission. :smash:

The questions: Have any of you had difficulty in getting your transmission into or out of gear with a SPEC Stage III clutch? If so, what did you do about it?

Other bits of info: An '89 comes with an aluminum bell housing but GM doesn't sell these anymore. GM says to use the magnesium bell housing used on '91 and later Corvettes. So I replaced my aluminum bell housing with the newer magnesium one. The slave cylinder, slave cylinder rod, pivot ball and clutch fork are all stock.

I'd appreciate anyone's input on this as I don't know what to do. I've called Dave at SPEC and he said "it would be impossible for the clutch disk not to disengage from the flywheel" (as is my suspicion). I'm thinking of making longer slave cylinder rods and trying them in increasing lengths until I can get the disk to be fully disengaged, especially when things warm up.

Thanks for reading and for any feedback you might have.
:seeya

bogus

I don't really wanna go here...

but SPEC clutches are not the best option. I had NOTHING but problems and crappy customer service from them with my Stage I.

For your case, I would suggest perhaps an adjustable clutch fork pivot.

pablocruise

Hey.
89 here too. SPEC carbon fiber III, camaro FW, VALEO pressure plate, new magnesium BH, new pilot bushing, new TO bearing, new fork, new stud, new slave, 92 up braided steel hyd line, new master also.
Sounds familiar huh?
Can you verify the integrity of your fork? I had clearanced the stock one in order to clear the PP when converted over to SM. It eventually weakened and a stress cracked developed at the rivet that holds the flat metal spring. Your new BH has sight holes on the side. See if ya can look at the fork move in relation to someone's leg while they depress the clutch.
Another thing I did was to have a shop with accurate measuring instruments measure distances from the surface the clutch disc contacts in relation to the surface the crankshaft contacts. I kept everything as close to the stock DM set-up. Thus, that whole clutch assembly maintained close to stock geometry to the BH/pivot/fork/rod.
So I'm thinking your FW is not the right thickness. Have you had that measured? I also suspect your fork or the pilot bushing.
BTW, my new fork did not need clearancing. In addition, I welded a small stregnthening rib to it in the area that is the "fulcrum".
I'm not having any probs with mine<knock wood>

C4racing

I'm aware that a lot of people have had problems with SPEC products and service in recent years. But I did a lot of research (including reading all the archived posts here on the Forum) before I chose SPEC. The research included reading about and calling every well-known supplier of clutches and flywheels, including GM. All brands seemed to have their +/-. RAM tech personnel said they didn't have anything for my application that would handle 500 hp or 500 lb*ft of torque. GM was out for the same reason. McLeod was virtually impossible to communicate with even though I left messages, sent faxes, and called numerous times. Centerforce fell out of contention early in the running (but honestly I don't remember why). Most of the SPEC problems seem to have been addressed and I thought that by buying EVERYTHING from them I'd minimize the chance of component incompatability. We'll see.

Ok, enough about why I have what I have......I'd rather chat more about how to solve my problem.

Paul, the fork is brand new and I checked it for clearance against the bell housing before assembly. Ground off the mag casting flash near the fork in case of contact between fork and housing.....all nice and smooth. I wasn't there during installation but I'm sure the shop used the roller bearing supplied by SPEC. The flywheel is a brand new aluminum one supplied by SPEC.

I couldn't make any thickness comparison measurements because what I had before was a real cludge-job. The last shop used an '88 SM Corvette flywheel, extended the pivot ball, and made a longer slave cylinder rod in order to get the whole assembly to work. So I had nothing to compare the SPEC thicknesses/distances to.

Since all it takes is one or two moderate throttle upshifts to heat the assembly and get it to "stick", I still think it's a clearance issue. Like you said, flywheel is too thick or something needs to be changed to create more clearance between flywheel and clutch disk (at least during disengagement). I wish. I sure hope I don't have to take the whole thing apart again...... :(

Maybe I'll make those longer slave cylinder rods today unless someone has a better suggestion......

tpi 421 vette

I think the clutch itself should be fine once you get the disengagement problems straightend out. I have a SPEC stage 3 clutch in mine... and it doesn't slip with slicks on it, and it disengages just fine.

SPD DMN

I run a SPEC Stage 3 and Steel FW. It does have some chatter when trying to drive it slow, but it has subsided somewhat with a few miles. You sould like you have some miles on it though if I read correctly.

I too would suspect the thickness of the flywheel. I am not a big fan of the aluminum flywheels because I have read of three coming apart when using the SPEC III disk with them. Not trying to scare you but a good tranny blanket would be a wise purchase.

h rocks

That newer style bellhousing is something like .170" longer than your old one. I believe that most of it is from the steel perimeter spacers that prevent the galvanic action between the iron block and the magneisium/aluminum casting. The stacking height is a pain in the asz on these things. The "clutch authority" (not) is of little help. Pete is MUCH more helpful there at Spec. Ask me how I know. $$$ The FW thickness on a one piece rear main unit is about 1.475". This is the measurement from where the FW bolts to the crank to the friction surface. An F body FW is about 1.540". That's why everyone that knows what's going on cuts them by about .075"

C4racing

Thanks guys. Before I installed the new mag housing I put it side by side on the floor with the OEM aluminum one. Put a straight edge across one and it mated up exactly with the other one (same height), with the spacers installed on the mag housing. Btw, the spacers on mine only measured about .030" each end.

At worst that might mean that the bell housing, pivot ball, clutch fork, T/O bearing, and slave cylinder are all .030" farther away from the OEM crankshaft end. Also that would put the trans .060" farther away......IF the spacers weren't accounted for by the GM engineers. Doubtful, but I'm sure stranger things have happened...... :)

However, even though the trans might be .060" farther from the OEM crank end, in this scenario it would only be .030" farther from the pivot assembly and T/O bearing.

My hope in buying everything (including the flywheel) from SPEC was that they would take all of this into account in determining the right dimensions for their "package". Still have to get to the root of the problem so stay tuned........
:seeya

C4racing

Well, my frustrations continue....... :smash:

Since my last post I've talked to a lot of people and done a lot of things to the car, but the problem is still there and no one knows what it is or how to fix it. :cry

The car now has a new master cylinder, a new slave cylinder, new GM hydraulic fluid and has been bled WAY more often than I'd like to remember. The transmission has been removed and reinstalled a total of 3 times this past week. I even bought a borescope (couldn't find an inexpensive rental) so I could look inside the bell housing access hole to see if the pressure plate was disengaging from the flywheel when the clutch was depressed.

I've heard a lot of theories and have tried to explore most of them. Unfortunatley the problem is somewhat intermittent and sometimes worse than other times. Sometimes a LOT worse!

When the car was warm I put it up on a rack and had someone depress the clutch pedal while I looked inside with the borescope. I noticed what looked like a reasonable gap between the pressure plate and clutch disc. Since I couldn't measure it in place I compared the gap width to the width of a portion of the clutch disc. Later when I removed everything I measured that portion of the clutch disc and estimated that the gap was about .050-.060". Dave at SPEC told me that the disc only needs about .010" (.005"/side) to break free, so .050" should be plenty.

After replacing the master and slave cylinder and bleeding it for the ump-teenth time I'm 99.9999999% sure that the problem is not in the hydraulic system. The last time I bled it I even put the car up on jack stands with the rear higher than the front and the right side higher than the left to insure the bleed fitting was the highest part of the slave cylinder.

The shop foreman (who has been the one working on the car) talked to Dave at SPEC at length and both seem convinced that everything about the installation is correct. Extra attention was paid to virtually every clearance we could think of that might make one component contact another. Everything is supposed to be "fine".

It appears that the clutch disc IS separating from the flywheel/pressure plate....at least when I looked at it. Maybe it doesn't separate all the time but short of mounting a mini camera looking into the bell housing access hole I don't know how I would ever know. About the only thing I can think of is that the problem is transmission related, not clutch related. I've thought that the new bell housing might be inaccurately machined and that it might be forcing the trans into a position where the input shaft end is binding against the pilot bearing in the end of the crank. Yes the input shaft slides in, but is it being jammed against the side of the bearing????? Maybe this would cause the trans input shaft to be spun by the crank even if the clutch disc was free. Also, I wonder if the synchros in the trans were wiped out would this 'sticky shifting' be the result????? For the record, the trans shifts fine when it's cold.....

At this point I'm about at my wits end. The problem still exists. Namely, all it takes is one acceleration (in virtually any gear) to make the trans difficult to shift. The more I get on it (ie, more rpm, more hp input, more heat?) the worse the problem gets. It got so bad once that I was completely clutchless. If you want to know the feeling of hurting your car, just take a ZF trans with a Stage III SPEC clutch and jam it into 1st gear at a stoplight without putting your foot on the clutch pedal......that's what it felt like. :(

So if anyone knows what could make a ZF with SPEC clutch difficult to shift *sometimes*, getting progressively worse with rpm/power, I'd really like to have your input.

Sorry for the long post but I'm getting desperate..... :seeya

pablocruise

Was the flywheel machined or remains as it was?
Bill, where in CA are ya?

dizwiz24

Sorry to hear about your problems. Replacing the clutch, like I did on mine with a SPEC stage II/lightweight aluminum flywheel combo, is a huge job.

You will get chatter on acceleration under load (shaking rocks in a can noise) with the lightweight flywheel. When I first swapped mine in, I did long tube headers....and I was scared to drive the thing because I thought the rattling roise was detonation !

Dont know what to say about the disengaging issue. Ive had good luck with mine. I checked all the clearances to make sure everything disengaged when an assistant pressed the pedal. Eerything disengaged fine, and it has been handling the 100+ extra HP i have added to it with no problems. That was 3000 miles ago..This was on my 93 vert 6spd

C4racing

Paul, flywheel is a brand new SPEC aluminum unit....it was not machined after I received it. Sent you an email with other details so as not to make these posts too long.
:seeya

pablocruise

got it. Don't worry bout long posts. It's good to log in the info so it goes into archives and others can see it if need it. I see you mention the BMW dealer. I assume that means you put in new fluid in the tranny.
Mind if I sidetrack alitte?-----> How is the new engine balanced? Is the new flywheel zero/nuetral balanced?
I'm game to cruise down and swap my tranny out in your drive way and swap it into your car. That way you could rule out one thing or the other. Wouldn't be a big deal to me as I can do the tranny in my sleep now :(