Clutch Overcentering Problem

[Joe C.]

I've been having a problem with my 65 L76, where the clutch "overcenters" with powershifts at 6500RPM. There is a slight delay, and then a "bang", and weak gradual engagement. Sorry to offend anyone hearing about this "rough" treatment, but I rarely do this, but when I do, I want positive clutch engagement.
The car has very low miles on a recent restoration. All parts are "correct" (bellhousing, flywheel, etc). Disc/pressure plate/t.o. bearing came as kit 90104 from Borg Warner ("Brute Force" brand). Disc/pressure plate are standard duty parts. Clutch is set to engage as per shop manual (about halfway thru pedal travel). If I simply "de-adjust" the clutch to engage lower, should this solve it, or does it sound like I should install a performance clutch?
I am looking at 2 assemblies. Any thoughts on them?

1. RAM clutch - 300M disc (standard), 401 pressure plate (super duty)
2. CENTERFORCE clutch - 383271 disc (standard), CFT361675 pressure plate (Centerforce II)

Thanks in advance.

Joe

[Michael H.]

Re: Clutch Overcentering Problem

Joe,

This is a very common problem with diaphragm clutches. Depending on the actual spring tension of the diaphragm, the point may change a bit but generally it starts around 6500, the exact RPM that you are at when your problem begins. If you adjust the clutch so that the travel is limited to just past the point of disengagement, it will probably solve your problem. If the diaphragm is pushed in beyond that point, it is then in a position where centrifugal force will hold it in until RPM drops off.

A common "fix" from the 60's was a one or two inch thick block attached to the bottom of the clutch pedal to limit it's travel. Even the hot dog aftermarket racing diaphragm clutches did the exact same thing. I even installed an L88 pressure plate in my new 68 Z28 but that only raised the RPM point of overcenter slightly. I eventually went to the block under the pedal, just like everyone else and it worked. Give it a try and let us know how it works.

Michael

[Joe C.]

Re: Clutch Overcentering Problem

Michael:

I have been looking on the Centerforce and Ram websites. I see that Bill Hays, of Hays Clutches, and founder of Centerforce, specifically designed the Centerforce line to eliminate the over center condition that you have so correctly described. I realize that much of this is advertising hype, and so, I wonder if anybody has some real life experience here, with either of the units that I have mentioned.....or any other. The Centerforce clutches use a system of weights attached to the diaphragm fingers, just outside the t.o. bearing annulus, which supposedly apply more dynamic clamping force. I am not convinced that this will alleviate the over center problem. In fact, it might exacerbate it. No manufacturer offers any design drawings of the OEM pressure plate vs. the performance units, so I am flying blind here.
I am willing to substitute a performance clutch, if necessary, if I can be convinced that it will cure the problem using normal clutch pedal adjustment settings. As a last resort, I will readjust the clutch with lots of slop in the pedal, or strap on the wood block if I get the urge to drive "spiritedly".
Again, thanks Mike for the great advice.

Joe

[Michael H.]

Re: Clutch Overcentering Problem

Joe,

I think the first thing I would try is the thing you mentioned last. Near zero work or $ involved and it takes just a few minutes to do.

I have no experience with the Centerforce clutch so it's quite possible that they've solved the overcenter problem. The only part that I have a question on is the fact that because the added weight add's force when the clutch is engaged at high RPM, would it also add force and make the problem worst if the diaphragm is over centered? I wouldprobably call them and see what they have to say about this.

After the last post, I happened to remember one of GM's recommended "fixes" for this problem. In one of my old GM documents, it states that if the over center problem exists, the pressure plate should be shimmed at each of the six mounting bolt locations. They recommended something like .015 to start with and add if necessary. I've always thought that sounded like a lot of work so I never tried it but that would definitely eliminate the problem. The diaphragm would not have to move nearly as far forward for disengagement. They went on to say that this was a problem that normally came with a brand new clutch disk but would eventually go away as the disk became thinner with wear. That's because with a new thick disk, the diaphragm has to travel further forward, towards over center, to release the disk.

During the 69 season, I switched to a coil spring clutch for one weekend's racing. It went in the garbage can on Monday and on Tuesday, the L88 diaphragm went back in the car. Once I had the adjustment and floor stop right, I had no more problem with it.

Good luck, hope it all works out.

Michael

[Joe C.]

Re: Clutch Overcentering Problem

Mike:

Your recommendations are very much appreciated, and thanks for staying with me. Obviously, nobody else here is familiar with this problem, or do not know how to address it. Any Corvette which is driven like Zora intended, whether it have a "Duntov", "30-30", LT1, or L72 camshaft, deserves to have a power transmission system which can stand up to engine output @ peak power.

I very much appreciate your input, and will likely 'DE-ADJUST the linkage to compensate, unless I find a better diaphragm system.

There are some here who refer to the "visceral" experience of our classic Corvettes. So, let us not change our engines' hearts. If your car originally had a 30-30, or Duntov cam, then changing that to an LT 1 would be a greater sacriledge than painting your car with clearcoat. If you intend to race your classic Corvette, then upgrade your entire drivetrain. If you intend to retain your car's originality, then do not give your engine a "heart transplant". All '64-65 Corvettes with L76/L84 should be restored with the proper 30-30 camshaft. If we agree to change this, then what the hell are we all about!

There are some who rail about the "benefits" of 70's era 350 camshafts, but, whether or not the benefits are true, the NCRS is all about originality.
I have a 30-30 cam in my 1965 L76, and have had some clutch problems at very high RPMs. If anybody was really interested in absolute power output, then somebody would have responded (other than Michael Hanson, whose input is very much appreciated). So, this tells me that I am the only one who has subjected his Corvette to such severe usage. So, then, why the hell would any of you be concerned about replacement camshafts. In the interestof this organization, just build your engine with the heart that it originally came with.

Joe

[Clem Z.]

clutch adj to prevent overcentering

the correct way to set the pressure plate is to jack up the back end of the corvette,put the trans in gear,and keep adjusting the clutch till you can just turn the rear wheels when the clutch is depressed all the way. start with a adjustment where you can not turn the rear wheels with the clutch depressed all the way and then keep adjusting till the wheels are free to turn. this will be the correct setting for the clutch to prevent "overcentering". of course you need 2 people to do this one to turn the rear wheels and one to depress the clutch. you can use this method even if you use a "clutch pedal stop" to get the clutch pedal higher from the floor

[Mike McKown]

Used to be a very common problem, Joe.

You can follow Michael's advice or Clem's advice, they will both work. Or, you can get yourself accustomed to just "tapping" the clutch pedal between shifts and that will work also. The trick is not to overcenter the pressure plate which I'm sure you know. If you want to get brave you can shift without using the clutch at high rpm. Just blip the throttle between gears and pull the shifter like you mean it. Don't recommend making a habit of it but you can do it. I think I remember I had this problem with the flat finger diaprhamn pressure plate but the bent finger pressure plates kinda' made this problem go away or lessen somewhat. I used to do it the way Clem suggested.

[Terry D.]

Re: Clutch Overcentering Problem

Joe

Just because only one person responded to your problem does not every mean he is the only one who cares. Many people are on vacation, some at the NCRS Convention, some choose not to respond when the correct answer is already given. Had you checked the archives you would have found this subject addressed before. There are many different Corvette clubs out there, I'm sure not everyone has the same views as to what they should be like, but try to remember this is the NCRS.

Terry

[Joe C.]

Re: Clutch Overcentering Problem

Terry:

Sorry I got your dander up. I do belong to some other Corvette clubs in addition to the NCRS, but none have the knowledge that is found here, IMHO. So, how do you deal with the overcentering problem?

Joe