1970 M21 Clutch & Pedal Adjustment

[Mike F.]

Can someone explain the clutch & pedal adjustment for my M21? I'm either missing the obvious or some parts are out of whack.

I don't seem to have any trouble shifting while driving. The problem is when shifting into reverse. I'll be moving forward and come to a stop, clutch pressed to the floor, I'll bring the shifter out of 1st (or any forward gear) and put it in neutral, wait a few seconds and then slide it into reverse. When I go into reverse, I can feel the "reverse shaft" (not sure if that is the correct terminology) still spinning, so it will grind a bit to get into reverse. (Grind may be too harsh of a word, but a noise none the less) I've tried leaving the shifter in neutral for 10-15 seconds before going into reverse, same noise. You can feel (but not hear) something spinning.

I can start the car with the clutch pressed to the floor. While still stationary, I move the shifter out of reverse and into neutral, I'll then start releasing the clutch pedal, you can hear the clutch assy. engage and start whirring about half pedal travel. Push the clutch in and the whirring stops, move it into reverse and the reverse shaft is moving.

I also do not have any free play on the clutch pedal as described in the AIM.

Am I missing something in the adjustment or is the clutch/pressure plate/bearing out of whack so that no adjustment will work?

TIA,
Mike

[Patrick H.]

Re: 1970 M21 Clutch & Pedal Adjustment

Shift into second once you are stopped, then into reverse. It stops the shaft and your problem goes away.

Thanks to John "The Real Deal" Ballard for teaching me this one years ago.

[Edward J.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike, when you adjust the clutch pedal it should have about 3/4 " free play at the top. unless your pedal is releasing a few inchs off the floor there should not be a problem shifting any gears.sometimes its just a case of checking the shifter rods for proper alignment.

[Mike F.]

Re: 1970 M21 Clutch & Pedal Adjustment

Patrick - Tried your suggestion and it worked, but it doesn't address the lack of free play.

Edward - I know there should be some free play, 1.5 +/- .250 per the AIM, but how do I get it? I've run the adjustment up and down the rod, but don't ever get any free play.

[Paul O.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike

From your photo it looks to be you are close to running out of adjustable threads on the clutch rod. Over the past few years I have seen that problem now several time. If you have had the flywheel milled several times over it life and/or an aftermarket clutch assy. The problem is that we think that the small amount of milling or a small difference in a clutch assy overall dimensions would not cause a problem. But it does GM made a longer pivot ball stud for later bell housing that can solve this problem it is .300 longer but at the adjustment rod I have gained about 1.5" of further adjustment. Here is a photo of the pivot ball from GM.

Also as Patrick stated I learned that about 35-40 years ago I would always shift 1st the to 2nd then reverse. That stops the lower gear cluster to stop stopping the grinding going into reverse.

[Terry M.]

Re: 1970 M21 Clutch & Pedal Adjustment

You guys need to learn to drive with a transmission with NO syncro rings. THEN you will know how to get into reverse with no grind, and know when the clutch is properly adjusted.

[Michael G.]

Re: 1970 M21 Clutch & Pedal Adjustment

The wrong throw out bearing can and will cause adjustment issues.

[Edward J.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike the clutch pedal adjustment is really only where you may like the the pedal release point. if your car is releasing close to floor and cannot raise the pedal adjustment becuase the rod adjustment is at the end then as Paul says the flywheel turning over the years will only permit so much adustment.There are some choices to fix this. But generally its not a easy fix. They do make shims that can be installed behind the pressure plate to make up the difference that was removed from the flywheel during the machining process.and the clutch longer pivot. Also check all the clutch linkage at all the pivot points sometimes they wear enough to elongate the holes and you'll lose some adjustment.

Mike remember that if you clutch pedal is releasing just of the floor,It may cause engagment issues into Ist AND Reverse. and if it releasing to close to the top you will be have slipping problems and will short live the clutch.

[Monte M.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike,
I want to throw one more thing at you. You mention a sound you can hear. This will come into play with your other issues.
If the sound is anything like a gurgle, you have another issue to address.
The reason you are supposed to have free play in your pedal is so your throw-out bearing ( the bearing on the other end of the clutch fork that is on the in-put shaft bearing cover) does not wear out. If you have no play in your pedal, the throw-out bearing is always engaged. This will wear the throw-out bearing in a very short time.

The first sign a throw-out bearing is going is a gurgling sound. The second sign is your clutch adjustment may need a lot more attention than usual.
The easiest way to check your throw-out bearing is, once you get some play in the pedal, the gurgle should go away as soon as you put it in neutral and let the clutch out. Right now that does not apply because you have no end play in the pedal.

Next question:
How long have you had this problem?
Have you owned the car and the problem just started?
Have you owned the car for a while and it has gotten worse over time?
Have you owned the car and the problem has always been there?
You just bought the car and it was like this when you bought it.

A 69 I owned years ago had a similar problem. I lengthened the rod to make sure everything worked alright before taking everything apart. Once I realized lengthening the rod fixed the problem, I took it apart to find the wrong clutch in the car. I put a new pressure plate and disc in it and it worked he way it should without the longer rod.
This is just what happened in my case. I am not saying that is what is wrong with your car.
If you want to answer my questions, I would be glad to continue to give you my opinion.

Monte

[Mike F.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike,
I want to throw one more thing at you. You mention a sound you can hear. This will come into play with your other issues.
If the sound is anything like a gurgle, you have another issue to address.
The reason you are supposed to have free play in your pedal is so your throw-out bearing ( the bearing on the other end of the clutch fork that is on the in-put shaft bearing cover) does not wear out. If you have no play in your pedal, the throw-out bearing is always engaged. This will wear the throw-out bearing in a very short time.

The first sign a throw-out bearing is going is a gurgling sound. The second sign is your clutch adjustment may need a lot more attention than usual.
The easiest way to check your throw-out bearing is, once you get some play in the pedal, the gurgle should go away as soon as you put it in neutral and let the clutch out. Right now that does not apply because you have no end play in the pedal.

Next question:
How long have you had this problem?
Have you owned the car and the problem just started?
Have you owned the car for a while and it has gotten worse over time?
Have you owned the car and the problem has always been there?
You just bought the car and it was like this when you bought it.

A 69 I owned years ago had a similar problem. I lengthened the rod to make sure everything worked alright before taking everything apart. Once I realized lengthening the rod fixed the problem, I took it apart to find the wrong clutch in the car. I put a new pressure plate and disc in it and it worked he way it should without the longer rod.
This is just what happened in my case. I am not saying that is what is wrong with your car.
If you want to answer my questions, I would be glad to continue to give you my opinion.

Monte

I have owned the car a few years now and there was never any free play in the clutch pedal. (No issues in the past)

This past winter I pulled the engine for rebuild and figured I'd replace the clutch since everything was apart. After buying a new clutch assembly and comparing it to what was in the car I saw no differences or damage to the old clutch. I still went a head and installed the new clutch assembly.
After everything was back together, I decided I wanted the clutch to catch a little closer to the floor, so I adjusted it. After doing so and putting some miles on the car is when I noticed that going into reverse the shaft was still spinning (at times).

Last night I disassembled all of the clutch linkage/springs and started from scratch. I was able to get the recommended 1.5" of free play and did some test driving/shifting. My perception was that the clutch didn't engage until the pedal was too far out/up and that there may have been some slippage in 1st/2nd gear when I floored the accelerator pedal. I then adjusted the clutch to get about 3/4" of free play, which engages the cluth sooner and it doesn't seem to have the slippage (if it was there at all to begin with). Then I ran out of time. A little more test and tune, and hopefully good to go.


When the engine was rebuilt, the flywheel was "cleaned up" for lack of a better word. I would have to measure to find how much was taken off, but there wasn't really any damage to start with.


Thanks,
Mike

[Timothy B.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike,

The worst case if you had 1 1/2" free pedal play is that the clutch would not disengage completely when the pedal is pushed to the floor. The less play in the pedal 3/4" the higher off the floor the clutch engages.

The proper adjustment is under the hood at the bell crank to allow approx 1/8" between the adjustment nut and clevis with the bell crank pushed so the throw out bearing is against the pressure plate. After you properly adjust the clutch, jack the car up and if you can turn a tire while pushing the clutch pedal to the floor it's OK. OH, you need a helper for that, (wife works good). :-)

[Monte M.]

Re: 1970 M21 Clutch & Pedal Adjustment

Mike,
From what you are saying, it sounds like you worked everything out. You have some pedal play and the 1st/2nd slippage issue went away.
That is what you are looking for.

As far as the rod, check where it attaches to the pedal under the dash and make sure everything looks the way it should. If it looks good, measure the over-all length from the center of the fastener/pin that holds it to the pedal, out to the end of the threads and the over-all length as I just said.

I am sure someone has one out of a car, or I can measure the one in my car to see if there is a difference.
If that does not clear it up, we can start looking at the pedal itself or the bell crank.

I am just glad the car is mechanically sound for you to drive while we figure out the rest.

Keep me informed.

Monte

[Joe C.]

Re: 1970 M21 Clutch & Pedal Adjustment

All rules go out the window if the linkage is worn (ie: elongated hole(s), worn pin, etc). Any "slop" in the linkage effectively reduces the pedal travel. Another way of thinking about it is that it reduces the linkage ratio.

If the linkage is worn and the freeplay is set properly @ 1.5", then the clutch may not fully disengage with the pedal to the floorboards. Conversely, if the clutch is adjusted to provide clean disengagement, then there might not be any freeplay which guarantees early friction disc and/or release bearing failure. As stated, the most fail-safe method of checking freeplay is at the fork, where it should be at least 1/8".

If the input shaft does not turn freely within the pilot bushing (bearing if applicable) then a properly adjusted clutch will still grind reverse (non-synchro), will be harder to shift between all other gears, and will accelerate synchro and blocker ring wear. One very large contributor to this is misalignment between the bellhousing and the crankshaft. This applies to untouched components, but in situations where the crank journals have been align honed, the centerline is now relocated and offset bellhousing dowels should be used.