C1 Diaphram or 3 finger

**Joe L.** · October 21, 2003, 02:58 PM

Re: C1 Diaphram or 3 finger

Don-----

The problem that you described is a sometimes-encountered problem when a diaphragm clutch is used on a C1 and under certain operating conditions, especially the one that you described. I don't know of a "cure" for it other than replacement with a 3 finger or "tame" driving.

As far as I know, the same clutch fork and clutch release bearing are used with either the 3 finger style or the diaphragm style. Since GM never used a diaphragm clutch on a C1, there's no way to know for sure, at least from the standpoint of OEM engineering. However, as far as the clutch release bearing goes, the same one was used on C1s with 3 finger clutch as was used on C2 and C3 with diaphragm.

As far as the clutch fork goes, if a different one were required for a C1 using a diaphragm clutch, then I don't think that anyone could use a diaphragm clutch in a C1. As far as I know, the stock design is the only one available.

**Robert Willis** · October 21, 2003, 04:17 PM

Re: C1 Diaphram or 3 finger

The chatter in low gear could be week springs in the clutch disc, I also have had my 57 do that and found if I tightened the front mounts down real tight the chatter went away.

Bob's Chevy Corner

http://members.kos.net/willisb/

**Mike M.** · October 21, 2003, 04:35 PM

Re: C1 Diaphram or 3 finger

agree with joe: diaphragm press plates are dangerous to ankle health. only good diaphragms are found in ladies pocketbooks. mike

**Tom P.** · October 21, 2003, 06:37 PM

Re: C1 Diaphram or 3 finger

The SPECIFIC reason as to why your pedal stays down, especially hi revs, is MOST likely because you have the "flat" diaphragm rather than the "cone" shaped diaphragm style pressure plate. The flat style uses the long throwout bearing and the cone style uses the short throwout bearing. When installed and bolted up to the flywheel, is your diaphragm flat? If so, then the only cure (if you want to stay with a diaphragm style) is to install the cone style and short throwout bearing. In my 56 with a SB400 and Muncie, all that I have ever used is a diaphragm style clutch. NEVER a problem with the cone style pressure plate-------------at any rpm!
My 51 Chevy, with a 350 and Muncie has the flat style, and if I turn it over about 5500, the pedal stays down until the rpm drops (It's a long story, but I HAVE to use a flat diaphragm in the 51).

**Joe L.** · October 21, 2003, 06:46 PM

Re: C1 Diaphram or 3 finger

Tom-----

Now, that's interesting information. I didn't realize that the flat style diaphragm units were available for these applications. ALL of the GM OEM clutches used for the 63-81 Corvettes were the cone style diaphragm units. However, I believe that a lot of C4s use the flat style and, perhaps, these are available in the aftermarket for retro-fit applications for C1s (or,even C2's and C3's). Maybe that's the root of the diaphragm clutch problems.

**John H.** · October 21, 2003, 07:12 PM

Re: C1 Diaphram or 3 finger

I've used the LuK "RepSet" (bent-finger) diaphragm clutch (from Paragon) on two solid-lifter C1's, and Centerforce II diaphragm clutches on four solid-lifter C2's plus my Grand Sport, and my '69 Z/28 still has its original GM bent-finger diaphragm clutch in it. Have never had any high-rpm problem with any of them; it's essential on a C1 to ditch the "Godzilla spring" on the clutch cross-shaft when using a diaphragm clutch and replace it with a conventional return spring like C2's use. The over-center "Godzilla spring" arrangment on C1's was a crude attempt at combining a return spring with a "pedal pressure assist" device for the 3-finger Borg & Beck clutch, and isn't necessary with a modern bent-finger diaphragm clutch. All use the same 1-1/4" throwout bearing.

**Tom P.** · October 21, 2003, 07:34 PM

Re: C1 Diaphram or 3 finger

I learned these differences, because a good friend of mine owns a clutch/brake rebuilding business, Okla Clutch. Brakes and clutches are both friction/load bearing devices. As explained to me, it is quite common for low output/low rpm engines to use the softer touch of a flat diaphragm, whereas the higher output engines commonly use a cone type diaphragm. Routinely, with a flat style diaphragm, the clutch will stay disengaged above high revs (commonly above 5000-5500rpm). And, when the rpm drops back down, BAM! the pedal pops out and the clutch suddenly engages! When you go to the local parts store (NAPA, etc) and buy a replacement pressure plate, take a look at it before walking out. In the box, a flat style will have a slight cone shape to the diaphragm fingers. But once installed, they become flat. So, if possible, compare a flat and cone style diaphragm pressure plate side by side so that you can see the difference.

**Dick W.** · October 21, 2003, 09:36 PM

Re: C1 Diaphram or 3 finger

Old drag racing trick for diaphragm clutches. Adjust clutch free travel so that clutch will just barely release with pedal on floor. Usually cures problem.

**Christopher R.** · October 22, 2003, 07:37 AM

Re: C1 Diaphram or 3 finger

Tom,

Can you post the manufacturer and part number for the "cone" style diaphragm pressure plate and short throwout bearing you have in your C1?

**Tom P.** · October 22, 2003, 07:37 PM

Re: C1 Diaphram or 3 finger

Unfortunately, there is no brand name. It is just your basic 11in rebuilt diaphragm clutch. The various places that rebuild clutches are numerous. Many of them supply to stores such as NAPA, O'Reilly, etc. My friend that owns Okla Clutch buys the basic parts for rebuilding clutches, brakes, etc, such as diaphragms, brake/clutch lining material, rivets, etc, etc and then does the rebuilding. He is (by today's standards) a very small time business which was started by his dad MANY years ago. I have no problem using a rebuilt clutch, because the parts that wear or go bad, get replaced when rebuilt. Just as brake shoes really don't go bad. The lining just wears down and when it does, the shoes can usually be re-lined with new material and thus, they are good as new. When a pressure plate is no longer serviceable, it gets disassembled, the plate itself is trued up on a surfacing machine, the diaphragm (or springs if it is the finger type clutch) and other parts get replaced with new parts and it is ready to go.

**Doug Flaten** · October 23, 2003, 01:24 PM

Re: C1 Diaphram or 3 finger

I agree that the pressure plate is serviceable. I do wnder about some of the clutch discs though. As a younger lad, I had a couple of discs fail on my '61, and I don't even get rough with the clutch. One time the lining & rivets disbonded. Another time the bent tab holding the Dampener spring broke off releasing the metal tab and the spring. Now if at all possible, I buy a new clutch disc.

**Don Whitehead** · October 23, 2003, 09:20 PM

Re: C1 Diaphram or 3 finger

Well Thanks to all for all the ideas. It seems my flywheel was shot. It had heat cracks. Resurfaced once already. Replaced it.
Friend swore to me a Centerforce 1 Diaphram would work, if not, he would replace it. It works well. Didn't stick, even over 5500. we'll try over 6000 after it breaks in a while.
BEATS ME, GO FIGURE.

C1 Diaphram or 3 finger

C1 Diaphram or 3 finger

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