65 Clutch adjusment problems

[Rich P.]

I need a littel help here, I am working on a 65 327 car that has a GM clutch pt# 381146. Here is the problem. To get any adjustment on the clutch I am at the last thread on the upper push rod. I have triple checked all parts on he car, clutch fork, ball stud, lower push rod, z-bar, upper push rod and pedal extension bracket and everything is correct all parts were cross refrenced with original and NOS parts.

The only difference I can find so far is that when measuring the diaphram height by laying the clutch and pressure plate on a flat surface along side another replacement clutch for the same application, the 381146 GM clutch measures 1/8" shorter from the tip of the diaphram to the what would be the flywheel surface.

Is this 1/8"difference enough when multiplied through the clutch fork/ball stud pivot and Z-bar leverage multiplcation to cause this problem?

The next part of the puzzle is this, in checking the problem out I contacted my Clutch supplier tech line and was told that GM had a low and high profile clutch available. The low profile clutch used a special throw out bearing that had a 1/2" spacer built into it to compensate. So another question would be: Is GM clutch pt# 381146 a low profile clutch? How do you identify a low profile clutch? I know the throw out bearing used was a normal bearing.

I did a search on the net and found this picture. Is this what all low profile clutch diaphrams look like, flat? I am trying to get a picture of another 381146 sent to me to compare and will post it when I get it.

Thanks in advance,
Rich.

[Rich P.]

Re: 65 Clutch adjusment problems

Here is a picture of the clutch in question. Note the bend on the end of the diaphram.

[Joel T.]

Re: 65 Clutch adjusment problems

Hi Rich;

I'm sure some other guys will chime in here... That second pressure plate looks correct to me.. I have no idea about the first one.

Are you sure that your bellcrank is the correct one. As I recall there were two different ones used in the C2's... but I do not recall what the usage was...

Was this car together and working properly before the clutch change?

Joel

[Joe L.]

Re: 65 Clutch adjusment problems

Here is a picture of the clutch in question. Note the bend on the end of the diaphram.

Rich-----



With respect to the 2 pictures of clutches you posted, the original clutch was NOT of the "low profile" design. Much later GM clutches for C4 and later did use a somewhat similar, "low profile" design, but this design was not used for any 63-81 Corvette clutch that I am aware of.

However, the diaphragm spring end design of the original GM clutch was not quite the same as the "high profile" clutch you pictured, either. This may or may not cause a problem.

With respect to the "381146" clutch part number you mention, this is not a GM clutch, per se. This is Delco #381-146. Basically, this is an aftermarket clutch manufactured by LUK. It is the correct replacement clutch for the 1965 small block application, though. Also, it's the closest to original that you're going to find for the application given that the last available GM pressure plate assembly for the application, GM #3841673, is long-since discontinued.

I do know that folks have used th Delco 381-146 clutch successfully for mid year small block applications. However, as I've said many times before, using any clutch besides an OEM GM clutch can be problematic in ANY 63-81 Corvette with respect to getting a proper and acceptable adjustment.

As far as I know, the Delco 381-146 is supplied with a release bearing. Since 381-146 is the correct part number for the application, I would assume the release bearing is also correct. In any event, it would be wise to check it. It should be 1.25" long. ANY dimension other than 1.22" to 1.25" WILL cause adjustment problems.

I don't know of ANY clutch release bearing for any 63-81 Corvette that had the appearance of having a "welded-on" washer.

[Rich P.]

Re: 65 Clutch adjusment problems

Joel,

thanks for the tip. I did compare it to a 66-up cross shaft to make sure it was correct when we began.

Joe,

Thanks for the confirmation.

So Now my one question still stands does 1/8" in diaphram height going to affect the pedal adjustment to the point of putting the rod on the last thread to obtain any kind of pedal?
Thanks, Rich.

[Joe L.]

Re: 65 Clutch adjusment problems

Joel,

thanks for the tip. I did compare it to a 66-up cross shaft to make sure it was correct when we began.

Joe,

Thanks for the confirmation.

So Now my one question still stands does 1/8" in diaphram height going to affect the pedal adjustment to the point of putting the rod on the last thread to obtain any kind of pedal?
Thanks, Rich.

Rich-----

The answer to your question is I don't know. However, I would not be, at all, surprised if the 1/8" difference would make a considerable difference in the adjustment position on the pedal push rod. What you describe sounds like a little more than I would expect, but I think it's possible.

Let's make sure, first, though, that you are using the correct other parts of the system:

1) You should be using FORK pushrod GM #3844209 which is 10-9/16" OL;

2) You should be using cross shaft GM #3832857. 63-81 Corvette cross shafts (there were 3 different ones used over the period) are NOT marked with a part number. However, the angularity between lever arms for this shaft is 135 degrees;

3) You should be using pedal pushrod GM #3819154 (original) or GM #3888213 (replacement for 1965). These rods are not marked, either. However, the original rod was 18" OL whereas the replacement was 19-3/16" OL (hint: and may provide extra range of adjustment).

If all the above is "right", then I'd say that the 1/8" difference in the original versus replacement clutch or some difference in the clutch disc is what's causing your adjustment problem. I don't know what else could be causing it.

[Rich P.]

Re: 65 Clutch adjusment problems

Joe,

every part has been triple checked and is correct.

Thanks.

[Bill C.]

Re: 65 Clutch adjusment problems

Be certain also you are using the correct lower push rod, the sb rod is longer than the BB and/or 67-up all. Its also not marked but is easily identified by the spring retainers being much farther apart on the sb rod.

If you have the chance, put your cross shaft down side to side with a known correct year cross shaft, the three available are pretty close by trying to "eyeball" one installed and one in hand.

Another potential culprit could be an incorrect pedal bracket, the one that the upper push rod mounts to, it bolts to the pedal and there is a different one for BB applications that if used on SB will give trouble. Be certain you are using the correct "style" of release you desire, the service manual describes how to adjust your linkage release for street/spirited driving by relocating the pedal bracket location and turning the upper pedal pushrod 180 degrees. Have you actually measured the upper push rod?
I have found lots of these over time that someone in past has either made longer or shorter, probably trying to fix the same problem you are having.

Last note, are you using a stock factory flywheel? Has it been resurfaced many times? Each time thats done the entire assembly moves closer to the engine.
Aftermarket flywheels are a crapshoot on this dimension and the source of plenty of geometry problems.

Good luck and keep us updated!

Bill Caldwell

[Rich P.]

Re: 65 Clutch adjusment problems

OK heres what happend.

the bracket that is welded onto the brake and clutch pedal housing was bent up allowing the pedal to travel back too far and adding an extra 2" of free pedal travel. I did not do the disassembly of the car if I had I would have noticed how much highre the clutch pedal was than the brake pedal in the resting position. Now with the bracket bent back to where it should be the clutch pedal sits just a tad higher than the brake pedal as it should and I am able to obtain the correct free pedal travel of 1-1/2".
Altough this clutch still has the adjustment almost on the limits of the adjustment I am able to have a few threads showing.

Thanks to all who helped with thier suggestions, Rich

[John H.]

Re: 65 Clutch adjusment problems

Altough this clutch still has the adjustment almost on the limits of the adjustment I am able to have a few threads showing.

Rich -

That'll work fine, as the clevis moves rearward on the rod when adjusting to compensate for clutch wear.

[Jim D.]

Re: 65 Clutch adjusment problems

Rich,

Your picture is exactly what mine looks like and it is an untouched original.

Jim

[Bill C.]

Re: 65 Clutch adjusment problems

Rich -

That'll work fine, as the clevis moves rearward on the rod when adjusting to compensate for clutch wear.

John,
I think you have it backwards, as the clutch wears the clevis must be moved forward towards the end of the rod to keep adjustment.
Bill

[Wayne M.]

Re: 65 Clutch adjusment problems

Rich -- too bad your picture doesn't (also) show the inboard bellcrank arm and clutch fork push rod.

Are you aware there is a minor controversy with respect to jam nut size and position on the upper clutch rod (either side of the swivel) ? '65 TIM&JG says both thin 1/4"; others say one side thin / other side thick. I've seen (perhaps) unmolested originals with your configuration (thin forward, thick towards firewall).

[Rich P.]

Re: 65 Clutch adjusment problems

Rich -- too bad your picture doesn't (also) show the inboard bellcrank arm and clutch fork push rod.

Are you aware there is a minor controversy with respect to jam nut size and position on the upper clutch rod (either side of the swivel) ? '65 TIM&JG says both thin 1/4"; others say one side thin / other side thick. I've seen (perhaps) unmolested originals with your configuration (thin forward, thick towards firewall).

Wayne,

here's the before shot. As you said I have seen it with all the different variations of thick/thin, thin/thin.

Also notice the adjustment on the upper push rod with the factory clutch!!!

Jim, thanks for the info!!

Rich

[John H.]

Re: 65 Clutch adjusment problems

John,
I think you have it backwards, as the clutch wears the clevis must be moved forward towards the end of the rod to keep adjustment.
Bill

Bill -

As a diaphragm clutch disc wears, the pressure plate moves forward (toward the flywheel), and the outer edge of the diaphragm ring follows it, which pivots the ends of the "fingers" rearward. That moves the throwout bearing rearward, taking the inboard end of the clutch fork with it, which pivots the outboard end of the fork forward.

That moves the lower fork pushrod forward at the cross shaft's lower lever, which pivots the cross shaft's upper lever rearward, pushing the clevis rearward on the pedal pushrod, which moves the clutch pedal arm rearward, decreasing the pedal free play. In order to restore the free play, the clevis on the pedal pushrod must be adjusted rearward on the threads, pulling the pedal arm forward, further from the rubber bumper.