Thanks for the info dizwiz24.

I have a few throwout bearings and pressure plates and a couple of ZF transmissions, so I was wondering about the mix an match components. Your explanation of the size difference between the black tags and blue tags matches to what I have heard. I was thinking more about swapping throwout bearings rather than swapping out input shaft guide tubes on the transmission. I thought the guide tubes might have a seal or gasket and I didn't want to worry about changing them out. It is good to hear that the throwout bearings are the same OEM or aftermaket (except for the ID for the different size transmissions)

From SJW's pictures and your description, my disc was the stage 2 kevlar. Mine is pretty well trashed. the flywheel side is scorched bad and the pressure plate side has a step near the inside from wear. I also noticed one side friction material looks a little bit shifted off center. At the outer edge one side is further out than the other side friction material, and at a 180° it is further in. However, I suspect that is from chatter and the abuse it took, but not the original condition.

Interesting to hear of your clutch changes and issues.

I also talked to Jim and he had similar comments to me about SPEC. Although I didn't bring it up in this thread, I believe I have a balance problem with my SPEC setup too. A vibration at about 3k rpm and up. I have read a lot about it and it seems to be a fairly common problem.

I will take a look at the fingers on the pressure plate. By hand I don't know how much difference I would be able to feel though. I'll see.

Thanks for your offer on parts, but I will put in one I have for now. Possibly by next spring I will have a better idea of what I will do for a permanent set up.

Since you used the aluminum flywheel it may be different, but did SPEC provide the flywheel bolts? and were they similar to the ones I pictured with the washers?

 

Yes, that is a Spec Stage 2 clutch disc in your (and my) photos.

Judging from your pix, and the low mileage on that car, if the clutch wasn't slipping, I'd say it had been abused by the PO.

Yes Spec supplied replacement bolts for the new SMF they sold me. Unless my memory is letting me down, the DMF bolts are not the same size as what's required for their extra-mass SMF, so re-using them wasn't even an option.

Maybe I just got lucky, but I have no vibration issues. I did not have the balance of the pressure plate checked before installing it. I did take the old DMF and the new SMF up to the House of Balance, and was pleased to see that the match was almost perfect out-of-the-box. HOB did take off just the smallest amount of material from the SMF (just barely kissed it with a drill bit), but I can't imagine it would have vibrated enough to have been noticeable in-service if I hadn't had them touch it with the drill, so little was removed.

Best of all, with the extra-weight SMF, I have no gear rattle, which was a high priority for me. The car is essentially stock, and is a ragtop, so it's not a track car. I had no desire to end up with it sounding like a diesel ford truck at idle. At the time I did the clutch job, I hadn't even learned of the idea of shimming the countershaft in the ZF to try to minimize gear rattle. I suspect that Bill Boudreau hadn't even discovered it yet, as I was in contact with him while I was doing the work, and he didn't mention it, and there was no mention of it on his website, either. Fortunately, in my car, with the extra-mass SMF, it wasn't necessary to shim the CS.

Live well,

SJW

 

I called the SPEC tech support line today and got some thoughts from them.

Although I think the flywheel marks look bad, they recommended to check them because the kevlar disc leaves black marks pretty commonly. He though it might not be as bad as it looked and might clean up with a scotchbrite pad. If not, it may only take a light cut to refinish it.

Other compound friction discs do not leave as much black marking, but may show more of a wear pattern on the flywheel. For the least chatter, he recommended the stage 2+ disc.

He could not determine cause but suspects a pressure plate problem. It could be cracks or from improper tightening. He also said he knows of no way to check them to see if the pressure plate is bad. (except with the equipment that SPEC has)

He confirmed that SPEC sends the flywheel bolts with hardened washers with their flywheels.

He confirmed that the SPEC SC05S-2 flywheel will work with the OEM pressure plate, but should use a sprung hub disc. (F-body) The SCO5S-2 flywheel is the same thickness as the OEM dual mass flywheel.

He confirmed that the OEM throwout bearing will interchange with theirs.

 

I would go find some OEM pieces and throw the SPEC stuff in the trash. You already know Jim, see if he has anything left. Not much last I got stuff from him.

 

I have a nearly new OEM dual mass flywheel, OEM pressure plate with throwout bearing, and OEM friction disc. I am putting that in the car now to get the car going again. I will keep that setup until at least next spring.

I agree with you on sourcing some things from Jim. I have a couple of other flywheels that I have been talking to him about refinishing and balancing. He seems to know this stuff pretty well. I expect to be working with him this winter on parts.

My SPEC pressure plate and disc are trash. My SPEC flywheel may be salvageable if I want to go to a single mass at some time.

 

Just an update.

I put in the nearly new parts (dual mass flywheel, OEM pressure plate, OEM disc, OEM throwout bearing, OEM pilot bearing).

The car runs great. No chatter at all, and no vibration at all. It is a lot more fun taking an LT4 over 3k rpm.

I had the SPEC flywheel checked. It was out of balance by 20 grams. That was my vibration problem. The surface was discolored and looked bad, but with a scotchbrite pad it cleaned up to look nearly new and checked to be flat. I had the flywheel rebalanced and now have it ready for use if I go back to a single mass on this car or another.

I never did find out for sure if the SPEC pressure plate is bad or not. However, I am pretty confident that it was the problem. Just no way for me to confirm it.

Thanks to everyone for helping, and providing pictures, etc.

 

What's out of balance reason? Bad instalttion or wear?

 

Just curious. What was the balancing charge for the flywheel? I've not had one done for many years, maybe more years than many. I don't have one planned in the near future either but I am curious. The local shop that I would use has changed hands I understand and I'd rather avoid a "nuisance" call for them.

Did they mention the additional charge to do a clutch in combination with the flywheel?

Good to see your job is "done" and it appears that the car turned out to be a good buy.

 

Installation and/or wear were not a factor in the bad balance. The flywheel was not balanced correctly. I do not know if it came from SPEC that way or if the previous owner had it balanced after they received it from SPEC. The flywheel had a couple of balancing holes that did not seem to be the normal SPEC balancing, so I don't know if SPEC did it or if it was done later.

I paid $50 for the flywheel balance. I did not ask about in combination with the clutch.

I am glad it is done too although I am not sure if I will get into it again in the spring. I wanted to try out some other parts too. I put an early black tag ZF in to check if the transmission was good. It is, so now I don't know if I will swap it back to the original blue tag from the '96 or just leave the black tag in there.

However, it is looking like the car was a good buy for me and it is a lot of fun. This is the first LT4 I have had, and it really does run good.

 

FWIW, I had a Spec extra-weight SMF match-balanced to the OEM DMF that I had removed from the car, and the cost was $50. This was five years ago.

This was probably not necessary in my case, as the SMF as-received from Spec was almost perfectly matched to the old DMF. The technician at House of Balance only removed a tiny dimple of material with a drill from the SMF to make it perfect, which leads me to think that I'd have never felt any vibration if I'd skipped this step, but I did it in the interest of avoiding having to pull the car apart again to solve a preventable vibration.

Live well,

SJW

 

An LT4 C4 is one sweet car. You're going to love it!

Live well,

SJW

 

Is it right: in pull style corvette clutch there's no pressure plate? Clutch contact is on flywheel??

I'm right??

 

Push or Pull style both have a flywheel, friction disc, pressure plate and throwout (or release) bearing.

The only difference is the direction the throwout bearing pushes (towards the engine) or pulls (away from the engine) on the pressure plate fingers.

 

after reading all the above, this is almost embarrassing, but I installed a spec liteweight billet steel flywheel and stage 3 plus clutch and had no problems (other than being shipped the wrong flywheel which was swapped out for the correct one at no cost); no vibration issues, some noise initially but it pretty much went away; So maybe things might go south later, but I pulled the engine for a rebuild, don't know the exact mileage on the new clutch but it was enough to reveal any obvious problems, and the clutch parts parts all looked perfect.

Any time someone damns a new clutch, I have to ask about the installation. I did one clutch job on a ford falcon that had been to two shops and one well intentioned boyfriend, turned out that clutch job was bad because of excessive fore and aft play on the crankshaft, that "bad clutch" was cured with a new crankshaft thrust bearing; another time on my 61 fuelie I installed the flywheel and checked the axial run out at the outer flywheel edge, just inboard of the ring gear; turned out there was a runout in excess of 0.040" of an inch which I am certain would have adversely affected clutch performance / life. That potentially bad clutch was cured by removing a tiny burr on the crankshaft / flywheel mating surface.

So my point is, how often are these basic things, crankshaft end play, flywheel run out, flywheel / pressure plate flatness, etc., checked before the clutch gets installed?

And then there's all the other little things, pilot bearing, clutch fork, clutch fork pivot that aren't normally included in a clutch "kit" (and usually reused) which can affect clutch performance.

As someone once said, the Devils in the details.

 

I checked all that on my recent clutch job. Car drives better than new at this point. Perfect. One cannot be a good mechanic/technician and NOT be a detail oriented person. When you check the details the big picture takes care of itself when it comes to cars.

I think people do not check things due to simply being lazy, ignorant, or a combination thereof. Possible lack of tools.

 

There was a question in a PM about the clutch fork and pivot stud. I took a picture to show them from inside the bell housing.

 

I'm experiencing a little bit of wining noise from trans when cold trans only. I Suppose to be pilot or throw out bearing. In your opinion is possible to grease spray them through the bell housing hole on the driver side??

 

I don't think grease spray is a good idea.

The bronze pilot bearings are not supposed to be greased. The bronze is impregnated with oil so it is permanently lubed. I have heard that adding grease on them can cause failures.

The throwout bearings need lube inside the bearing, but spraying on the outside would have no effect.

Spraying grease could also get on the friction surfaces which can cause chatter/hot spots/slipping and in general not a good idea.

In short, don't spray grease inside the bell housing.

 

I left this forum 5 years ago because spec robbed me of the enjoyment towards owning a corvette. I had the worst experience with their product. My flywheel was scorched after 300 miles. I sent it back, they sent it back, telling me it's normal. It's very easy to see that things are not normal with their product when it comes to corvettes. I never waste my time complaining about things but to give you an idea of how bad the experience was I contacted BBB who of course, sided with the company. Long story short 900 dollars down the drain not including labor, and i ended up putting the stock clutch back in. Oh and the chatter from that spec clutch. God, take care

 

this is why i dont ever trust a pArt to be balanced!

i always double check (with my own machine shop guy) that a flywheel and clutch pressure plate are balanced to each other - regardless of what the person that sent me the part said.

its well known that spec's so called balanced flywheel/ pressure plate assemblies are frequently out of balance.

all this said i am currently having good luck with their spec stage 3+ / lightweight alum flywheel setup