Mismatched clutch parts???

[Ed Jennings]

The PO of my 75 had a new clutch installed in the car shortly before I bought it. Apparently a "new" (or probably used)bellhousing was also installed. These two events may not have occured simultaneously, but both were in the past few months.
Anyway, the clutch cannot be adjusted properly. Even with zero free play, the pedal has to be flat on the floorboard in order to release the clutch. With any free play at all, the clutch frequently won't sufficiently release to shift gears.
I am speculating that one (or more) of three condions exists: Wrong throwout bearing, wrong fork stud, and least likely, the wrong bolts in the flywheel (full heads, dragging on the clutch plate).
Before I pull the transmission, which I don't particularily relish, is there any other area I'm overlooking? All the linkage appears to be stock and in good condition.
I believe that I remember reading that all Corvettes use the 1-3/4"(long) throwout bearing and the "long" stud. Could someone please confirm the dimensions of both? I strongly suspect one or both may be incorrect.

[Joe L.]

Re: Mismatched clutch parts???

Ed-----

Clutch adjustment in a Corvette is always a tricky proposition. I've always found that with the specified free-play at the pedal, you won't get full disengagement with the clutch pedal at the floor. About the best that I've ever been able to do is to get full disengagement with the clutch at the floor in combination with MINIMAL free play at the pedal. Minimal free play at the pedal should be enough and is FAR better than specified freeplay at the pedal and incomplete disengagement with the pedal at the floor. Even to achieve what I've described, EVERYTHING in the system has to be JUST RIGHT. That's ONE of the reasons that I won't even use a surfaced flywheel in a Corvette.

Anyway, the cluth fork ball stud that you require is GM #3729000 and it's 1-1/2" in length. Actually, early 1975 Corvettes used ball stud GM #3887159 which is 1-3/8" in length. This ball stud was discontinued years ago and was replaced by the 3729000 which was used in PRODUCTION on later 1975 Corvettes.

ALL 1956 through 1981 Corvettes use a SHORT clutch release bearing which is 1-1/4" long. Often times, parts countermen will supply a long clutch release bearing which is 1-7/8" long. This bearing ABSOLUTELY WILL NOT WORK on ANY 1956 to 1981 Corvette. You ABSOLUTELY MUST use the SHORT clutch release bearing.

[Mark #28455]

Re: Mismatched clutch parts???

Are you sure that stock type parts were used? I own several 1969 - 70 vettes and the stock replacement clutch/pressure plates all work great! I have used those by Borg Warner, Luk, and Ram with good success.

In my race car, I found that some racing three finger pressure plates did not work well as they had a different lever ratio than stock. In addition, the fiberglass clutch discs had a thicker marcel (the wavy flat spring in the middle of the clutch disc that reduces chatter on engagement) and required more pedal travel than stock.

If you need more clutch pressure for a modified motor, just use a stock 427 or 454 setup. I have used those for years behind modified big blocks without trouble (even my 532).

Good luck!

[Ed Jennings]

Re: Mismatched clutch parts???

Thanks, Joe. I knew it was either all long or all short. I guess I was thinking the longer one would give more throw. It is distressing to hear that the clutch is that difficult to get to release properly. The linkage setup looks just like the design in my 67, though the individual parts may vary.

I don't like to try to outengineer Chevy, but would the addition of an adjustable fork rod give the option of changing the throw?I know it wouldn't increase the throw, but I'm trying to visualize if it would get the clutch release point off the floorboard.

[Ed Jennings]

Re: Mismatched clutch parts???

Mark, The only real info I have at present is that a "clutch kit" was installed by some "Corvette Shop" in Atlanta. I don't think it's any kind of particularily high performance part, nor do I need that. I do know the clutch takes SIGNIFICANTLY greater effort than the clutch in my 67, and more than the clutch in the 71 LS5 I owned years ago. As much as I dislike the notion, I guess I'm going to have to pull the clutch and see what's going on with it.

[G B.]

Aftermarket parts

My experience is that only Luk and Sachs new diaphragm clutches consistently work properly in '63 - '81 Corvettes.

I've tried rebuilt pressure plates and found they all required excessive pedal pressure. I also got chatter in about half the installations. The local auto parts stores started selling Korean-made new clutch kits several years ago. These have different diaphragm lever arm dimensions from the original GM pressure plates, and they will not release fully unless the pedal is mashed into the carpet. The hot rod clutches I've tried were a mixed bag. The Centerforce diaphragm type worked fine, but the pedal pressure is higher than stock. The Schiefer, Weber, and Zoom Borg & Beck style clutches all required very high pedal pressure.

[Harmon C.]

Re: Mismatched clutch parts???

Ed I have found the same problem as Joe described. I had the old parts and the complete package they gave me the first time was way off. I had the trans. in and out three times to get it right. Last detail has a good prce on the delco kit but I bought the last one I needed at AutoZone and it was OK the first time but I did not resurface the fly wheel. On the one I had all the problems I had resurfaced the flywheel. Lyle

[Joe L.]

Re: Mismatched clutch parts???

Ed-----

I don't know if an adjustable fork push rod would help, or not. Over the 63-68 period GM revised the overall clutch linkage several times. Other than the change of the pedal push rod in 1968 to accomodate the new body, the linkage remained pretty much the same from 1967 to 1981.

The exception was the change of the clutch fork ball stud. GM used the 1-1/2" GM #3929000 from 1955 through 1965. Then, for 1966 through early 1975, they went to the 1-3/8" GM #3887177 ball stud. In later 1975 they went back to the 3729000 and used it through 1981. As I mentioned earlier, the 3729000 later replaced the 3887177 for all 1966-75 SERVICE. There were some problems with the 3887177 stud on 1966 Corvettes involving "clutch release" and "high pedal effort" and many 3887177 studs were replaced with the 3729000 under a service bulletin. However, as far as I can tell, they still used the 3887177 in PRODUCTION through early 1975, eliminating the 1966 problem by revising the machining for the bellhousings. In 1975 they gave up on the 3887177 and went back to the 3729000 for all PRODUCTION and SERVICE.

I expect that the various revisions made during the 63-75 period were an attempt to solve exactly the problem you, myself and others are experiencing today. They never got it right, though. I'll bet that an adjustable fork pushrod was one of the things considered, so I doubt that it's addition would solve anything today. After all the effort that I'll bet that GM engineering put into this without ever getting it right, I doubt that we'll be able to find a "field solution" to it today.

[Ed Jennings]

Re: Mismatched clutch parts???

Well, it makes me feel a little better to know I'm not alone. I owned two sharks in the 70's, a 69 350/350 and a 71 LS5. Both were my everyday transportation. That was close to 30 years ago, but I don't remember the clutch being an issue on either one of them. I was a lot younger, and may not have been as sensitive as I am now to high pedal effort. I really don't mind the pedal effort as much as I mind the clutch having to be flat to the floor. That's why I was wondering about lengthening the clutch fork rod.

[Craig D.]

Joe , more fork stud confusion..........

Joe,

I read through the clutch section of my 1981 corvette shop manual, which is an original GM book, and is corvette only, not mixed models like some are.In the component replacement category of the clutch chapter it still makes reference to two different ball studs for the fork. It does not state anything about length but that they are different. Here is quote in the manual: " Be sure to use the correct ball stud. The L48 engine and L82 engine each use a distinctive ball stud" . That quote is next to a blown up drawing of the bellhousing and stud setup. Now I realize that in 1981 there were no longer L48s and L82s , just the L81 , but it seems as if this quote was pulled from prior year text on such equipped cars. Has anyone ever confirmed any truth to that statement. Just seems a little strange.Craig

[Joe L.]

Re: Joe , more fork stud confusion..........

Craig------

The only ball studs that were ever used on 1956 through 1981 Corvettes were the GM #3729000 and the 3887159. I am about 99% sure that the use of the 3887159 ended about mid-1975 and, after that, only the 3729000 was used for all Corvette applications.

I checked a 1979 Corvette Shop Manual and the clutch section does not have the same reference that you quoted out of the 1981 manual. So, I think that this must have been some sort of error which found its way into the 81 manual. It is curious, though, since it specifically mentions L-48 and L-82. For 78 and 79 Corvettes there were differences between the clutch set-ups for L-48 and L-82 equipped Corvettes. However, as far as I know, those differences did not include any differences in the ball stud, bellhousing, or clutch linkage. The differences related to the flywheel, pressure plate assembly, and friction disc only.

In any event, the GM #3887159 ball stud was discontinued from SERVICE in May, 1996 and is replaced by the 3729000 for all 1956-81 Corvette applications. The 3887159 is not, as far as I know, reproduced nor is an equivalent dimension part available in the aftermarket. So, regardless of what part might have been originally used, the only thing available for replacement today is the 1-1/2" GM #3729000 or aftermarket equivalent.