Chick clutch?

[G B.]

At one time, GM listed two different stock 11" pressure plates for seventies Corvettes. The parts book called one a "bent finger, Belleville spring" and the other a "diaphragm spring type". I think only the lower performance 454's and the base 350's were listed with the Belleville design.

I'd like to install a clutch pressure plate that gives a soft pedal feel. Can anyone confirm that the L-48 Corvette clutch during the seventies required less pedal pressure than the L-82 clutch? Or, is anybody familiar with the advantages offered by a typical bent finger, Belleville design? I've seen such a pressure plate, but I've never installed one.

[Joe L.]

Re: Chick clutch?

Jerry-----

Actually, I believe that the "bent finger bellville spring" and "diaphragm type" are, basically, synonymous terms. In any event, virtually all 66-74 big blocks and virtually all 1969-80 small blocks used the same pressure plate assembly. This pressure plate assembly was GM #3884598 and it's still available from GM. This is a pressure plate assembly designed for use with an 11" clutch, and 14" flywheel which all of the above-referenced applications used.

The only exceptions to the above during the 66-80 time period were:

67-69 with L-88/ZL-1 (used 10.4" clutch with diaphragm pressure plate)

69 L-71 with MA-6 (used 10.4" clutch with diaphragm pressure plate and dual discs with 10 splines)

70-72 ZR-1 (used 10.4" clutch with diaphragm pressure plate)

71 LS-6 and ZR-2 (used same set-up as 1969 with MA-6, except used 26 spline discs)

78-79 L-48 (used 10.4" clutch with diaphragm pressure plate; same set-up as used for all 1963-67 327, INCLUDING 10 spline disc; this was the ONLY Corvette use of a 10 spline clutch disc after 1970. Also, this set-up used a 14" flywheel drilled and tapped for the 10.4" pressure plate assembly, the ONLY Corvette use of such a combination)

I don't think that the 78-79 set-up (or the 63-67 small block set-up, for that matter, since they're the same) offers a significantly lighter clutch pedal feel.

[Duke W.]

Re: Chick clutch?

I agree - "bellville", "bent finger", and "diaphram" all refer to the same design, which it the only type of clutch used from '63 on.

Pedal effort is a function of the linkage mechanical ratio and the clutch clampiing force. Be sure your linkage is set at the "long travel" setting if you aare looking for minimum pedal effort.

Theoretically the larger the clutch diameter, the more torque it will hold for a given clmaping force, so the lowest clamping force 11" clutch you can find would give the least pedal effort, but it's tough to find published clamping force specs.

I recall that the OEM clamping force spec for the 10.5" clutch in my '63 is about 2350 pounds which is more than adequate for anything short of serious drag racing, but most aftermarket "heavy duty" clutches have more clamping force, which is going to increase pedal effort.

Duke

[G B.]

Diaphragm clutch design

I remember diaphragm pressure plates being called "Rockford" design clutches, but I've never heard/seen the term "Belleville" used for a clutch design except in the Chevrolet parts book. I don't know what design feature that word is describing. The "bent finger" term used with it in the book certainly makes it sound like a form of diaphragm pressure plate though.

All of the GM diaphragm pressure plates that I have installed have looked about the same. The throw-out bearing contact tips of the diaphragm fingers are straight cut and have a sharp edge. There is a different looking design of aftermarket diaphragm clutch that I thought might be a "Belleville" design. It has a rounded hump in the diaphragm fingers where they contact the throw-out bearing. I also noticed that the pressure plate is attached to the diaphragm at the outer edges with three spring steel straps. The GM diaphragm pressure plates I've inspected were attached to the diaphragm directly with three bolts that can be accessed through openings in the outer cover.

The pedal pressure of a diaphragm clutch is a function of the diaphragm spring steel design and the car's linkage geometry. Stiffer, wider, thicker, and shorter diaphragm fingers will increase the pedal effort to release the clutch. I know that different manufacturers use different diaphragms, but I'll be darned if I can find any meaningful specifications to help me choose a softer pedal clutch. Next week I'll get back on the phone to manufacturers and see if I can learn more specifics about their diaphragms. I'll also build a device to quantify the pedal pressure required on the cars I can sample.

Thanks for your research and input on this subject. I know it's stupifyingly boring to most of the board readers, so I won't post any more about it.

[G B.]

Clarification

I'm getting real sloppy with my terminology. Please let me clarify my description.

Let's say a diaphragm pressure plate includes three large pieces: the diaphragm, the plate, and a cover.

Using the above terms, the funky pressure plate (that I thought was a Belleville design) has three spring steel straps attaching the plate to the cover. Maybe these are an anti-chatter feature, but it looks like they might also counteract the diaphragm a little.

[Duke W.]

Re: Diaphragm clutch design

The term "belleville washer" refers to a conical shaped washer that acts as a spring along its axis. A "wavy washer" does essentially the same thing, but I don't think the name "belleville" applies to a wavy washer.

A diaphram clutch works on the same principal as a belleville washer, so I suppose that's why GM uses the term.

I don't know where the term derived from, but if anyone knows I'd like to here the story.

Both pictures are diaphram clutches though the design detail varies, and I wonder if the distance between the pressure plate and the end of the fingers is the same.

Duke

[Gerry Proctor]

Re: Clarification

The "belleville" refers to the spring type itself in a diaphram clutch pressure plate. While the belleville spring is used exclusively in the diaphram pressure plate, using that to describe the plate design is a little abstract, at least as much as it would be to call B&B, Long, or B&B/Long designs coil spring pressure plates. I can certainly see using this as a reference to a diaphram pressure plate would bring a lot of blank stares.