The Stovebolt Site Forums The Shop Area Driveline 1941 Chevy clutch issue

Hey all,

Just got back from a 50 mile drive with my 1941 1 1/2 ton GI 235 w/stock non-syncro 4 speed. All went great except for a recurring clutch issue. As the truck and all its internal parts warm up, the clutch acts as if it comes out of adjustment, meaning that with the clutch pedal all the way to the floor, it is barely disengaging the clutch disc from rotating. Shifting becomes progressively more difficult as the truck is driven. When it is cold, all is fine. I have it adjusted to the 0.75 inch pedal free play.

Some background info: When putting this back together, the clutch adjustment "hook" has always needed to be adjusted nearly to the end of its adjustment. I removed the clutch and had it rebuilt by a local old time clutch/brake shop but it still has the exact same symptom.

Help me out here guys.

Paul

The only thing that comes to my mind is the throw-out bearing fork. Is it installed correctly? I know it has to snap in place to the bell housing. If it's not in there properly, it may cause a problem and it should be easy enough to drop the inspection cover and take a look.

Ed,

I checked the fork, still in the ball.

Paul

Through the years of say 1941-~1962, were there two part numbers for the clutch fork pivot ball stud? A quick "clutch not disengaging" search reveals some discussion of two different pivot ball studs; one that is 1.500 inches long, and one that is 1.800 inches long. This could have a lot to do with my clutch adjustment issue, but not so much for the change in performance when hot...

Something is expanding when it gets warm. It might be the throw out bearing. Pull the inspection cover. Measure as well as you can the clearance between the presser plate fingers and the throw out bearing. Long feeler will get you close. Drive it until it starts acting up, slip under and see if that has changed. I have seen the bolts on the presser plate strip and release the plate. Just a bit, when cold the plate is flat and straight to the fly wheel. When warm the plate is warped. The pivot ball might effect the adjustment? 0.3 inch is not a lot but maybe. It should not change as it gets warm, unless the pivot is loose, or the retainer spring is broken, that might do it. That is all i have at this time, maybe some one smarter will speak up.

Steve,
Thanks for your thoughts on this. I will try to determine an easy method to compare cold and hot distances on the throw out bearing. I will also check the torque on the pressure plate bolts, and look for any signs of warping. I have had someone else more knowledgeable than me state that they thought the pressure plate diaphragm "fingers" are the culprits.

I spoke with my clutch rebuilder, and he stated that it may be some debris caught up in the disk friction material. I will try to give her a shot of compressed air while running with the clutch engaged. Wouldn't that be a lucky fix!

I hope to get her in reliable running condition fast as we have an old timey festival this Saturday and I wanted to bring my old relic Chevy for her first public appearance.

Paul

I have the same situation with my 38 Master only not as severe. I had a spare new pressure plate and disc so when I pulled the transmission, I put the new set in. I had to go to the end of the adjustment to be able to work the clutch. When it is cold, there is no problem but after a drive it can sometimes grind a little going into first or reverse. I have the transmission out right now and I am going to put the original pressure plate back in again. I am not going to try and solve the mystery other than mentioning that the two pressure plates look identical sitting side by side.

I have seen this many times.
I believe in a lot of cases the difference between cold not grinding and hot grinding is the temp of the oil in the transmission.
Clearly the clutch is dragging.
In a lot of cases I have found that the problem is the clutch disc is not running true.
I just went through this on a 1941 PU and the new disc had close to 1/8" run-out.
It is always a good idea to slide the disc on the input shaft of the transmission and give it a spin to verify that it runs true.

Dave,

Great idea about checking the disk run out, but kind of late in my current situation. What can I do to remedy the situation in the short term? I had already planned to take out the transmission for a rebuild during the dark days of winter but wanted to enjoy the truck this fall...I have two events that I was much looking forward to being present. Can I visually check for disc run out if the truck is running with the clutch fully engaged? Can I put some thick 140 weight gear oil in the transmission to alleviate the symptoms?

It just runs and drives so darn good otherwise, this is killing me.

Paul

Adjusting the clutch to have minimal free play can help some.
Thicker oil will also help.
There is no ways to see run out with the clutch assembled.

Had a minute or two to look at my Chevy. Now the clutch disc will not release enough to engage into any gear even when dead cold. Transmission runs through gears just fine when not running. Seems like poor Audra is dead in her tracks.

I guess on the plus side is that she seems to be a trusty steed, she chose to tank right in her carport instead of out on the road.

What exactly happened?

Paul

There are a number of things that can fail.
Broken pressure plate diaphragm, broken disc springs jamming things up, or the lining coming of the disc, to name a few.

Any updates on this? Sparky, my '46 half ton Chevy project, has the same shifting problem.

"sparky",

I did figure out the problem, with the help from several other bolters specifically Dave in WA. Unfortunately most folks ask answers to problems, but few post results. With this issue, I am similarly guilty.

I narrowed down the symptoms to be the result of several issues, and in putting the assembly back together, I unfortunately didn't definitively isolate the absolute failed part as testing each was just too much work.

1) I checked all of the clutch disks I have and found that as Dave stated, some have excessive runout causing the clutch to drag. The disk I had in the truck at the time of failure had quite a bit of runout.

2)I installed 140 weight gear oil so there was slightly more friction allowing the transmission shaft to stop spinning a bit faster.

3) I checked the wear on the lugs of my "freshly rebuilt" pressure plate, and found them to be VERY worn and out of specification. This I think was the true point of failure as the wear point between the pressure plate cover and plate lugs allowed the two parts to bind "sometimes" and cause shifting problems. See images below.

[img]http://www.surfacezero.com/g503/data/3176/medium/IMG_7299.JPG[/img]
The pressure plate on the left is a new NORS, the "rebuilt" one on the right

I don't recall the manual specifications for this wear, but the defective plate was way out of specification at 0.040.

As for losing sufficient adjustment threads on the fork, I believe that is an artifact of years of use and indiscriminant machine shop surfacing's. Each time the flywheel or pressure plate is surfaced, the assembly gets shorter which will show up on that little "shepherds crook" looking adjuster. even a slight 0.020 here and a 0.010 there is magnified at the adjuster.

[img]http://www.surfacezero.com/g503/data/3176/medium/IMG_73001.JPG[/img]
Note the difference in the surface depth on the NORS pressure plate surface on the right and the "surfaced" one on the right.

Anyway, after checking disk runout, adding thicker oil, and getting another pressure plate (with no wear on the lugs) solved my shifting problem. Now the only problems the truck shifting are purely "operator error".

Hope this helps.

Paul

Thank you for the update.
I am expecting to get the cab back from the body shop in the next week or two. Prior to that I have been checking and re-checking to make sure everything that can be easier to remedy prior to cab installation is working as it should.
In checking the engine and tranny it was discovered the tranny would not shift into gear with the engine running. Both had been rebuilt by the PO in the early 90's. Long sad story, for some other time. Neither had been operated even for testing until several years ago. Even then only the engine was started for a short duration test in the shop only. In recent days a longer test, in the shop only, showed the engine check was good.
The tranny needed clutch adjustment, or so it seemed. I tried following the manual instructions without any joy. I removed the inspection cover. I can see the throw out bearing move as it should. But, it doesn't move forward enough to touch the diaphragm spring. I am now climbing the learning curve on all this. The shop manual is a big help, but it assumes the reader has more knowledge and understanding than this old coot. I am uncertain I have adjusted the clutch correctly.

It is nearly impossible to adjust to the correct clutch pedal free play with out the cab/toe board. Perhaps you just need to adjust in more get proper engagement? Might be a super easy repair.

Update
I readjusted the clutch and now can see the diaphragm spring move. I doubt the adjustment is final, based on the need to have the sheet metal in place.
Thanks for the help!