M20 not disengaging.....additional questions - Page 2 - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #16 of 32 (permalink) Old Nov 30th, 16, 11:34 PM Thread Starter
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Re: M20 not disengaging.....additional questions

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Originally Posted by Rich-L79 View Post
I've had engines sit for 10-20 years and while the flywheel may have gotten a little rusty, it was never enough to freeze the disc to the flywheel. The issue is either the clutch fork or the throwout bearing. Since the bearing came with the clutch it SHOULD be correct for that particular pressure plate but mistakes can happen. So that would seem to leave the clutch fork as the most likely suspect. I believe you could test this theory by putting in a slightly longer pushrod temporarily and see if that disengages the clutch. You can make a longer pushrod out of a bolt of the proper size and length with the head ground off.

I suppose it is also possible that the throwout bearing was installed incorrectly onto the fork. If it was put on backwards it might cause this situation.
Rich,
In my first long winded quote, I suggested temporarily fabbing a longer rod to add more push into the fingers and hopefully releasing the disc.

I really didn't want to yank the trans out which is why I'm picking everybody's experiences with this issue. If I have to, I'll pull the trans but was hoping to try everything anybody suggests before I do the deed.

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post #17 of 32 (permalink) Old Nov 30th, 16, 11:54 PM
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Re: M20 not disengaging.....additional questions

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Originally Posted by Antalamo View Post
Mike,
I replaced the pilot bushing with the new one supplied by Centerforce.



If the pilot bushing is too tight on the input shaft the symptoms you describe will be there. Did the trans slide in easily or was it pulled in using bolts...?

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post #18 of 32 (permalink) Old Dec 1st, 16, 12:08 AM Thread Starter
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Re: M20 not disengaging.....additional questions

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If the pilot bushing is too tight on the input shaft the symptoms you describe will be there. Did the trans slide in easily or was it pulled in using bolts...?
I got the trans to a point by hand then drew the last inch or so I with bolts.....it's been a while since the install but as part of my parts inspection, I believe I took the bushing and did a pre fit it on the shaft before I installed it on the crank. Any other way to check if the shaft is free wheeling inside the bushing....?

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post #19 of 32 (permalink) Old Dec 1st, 16, 10:22 AM
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Re: M20 not disengaging.....additional questions

Tony, often when you drive the pilot bushing in the shrink in size. This causes the input shaft to be tight on the bushing. Drawing in the trans with bolts aggravates the problem.

I would pull the trans. and check the clearance between the input and the bushing and replace the bushing. Make sure it is a non metallic one (magnet check) and there is the .003-.006" clearance between the input and bushing. This spec is one I found on the Site somewhere.

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post #20 of 32 (permalink) Old Dec 1st, 16, 11:35 AM
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Re: M20 not disengaging.....additional questions

I had a pilot bushing once that fit on the test fit on the input shaft but it was a bear to drive into the crank. When I got it in, it fit waaay too snug and I had to cut it out and replace it.

Given all the oddities going on, possible wrong TO bearing, possible weird flywheel/pressure plate combo, possible pilot bushing issues it might be best to simply pull the transmission and give things the once over.
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post #21 of 32 (permalink) Old Dec 1st, 16, 12:21 PM
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Re: M20 not disengaging.....additional questions

When I installed my M20 a few weeks ago, I checked the bushing to input shaft fit before and after installing the bushing in the crank. When I installed the transmission, there was no resistance, I could practically push the transmission in with one finger. You shouldn't have to pull it in with the bolts...
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post #22 of 32 (permalink) Old Dec 1st, 16, 8:35 PM
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Re: M20 not disengaging.....additional questions

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You shouldn't have to pull it in with the bolts...
X2. If you do the.pilot bushing has compressed too much. The pilot bushing should only be 1.5 to 2 thousandths larger than the bore of the crankshaft. Last one I installed it was 7 thousandths larger than the bore. Had to have it machined down. Some guys sand/grin them down themselves.
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post #23 of 32 (permalink) Old Dec 2nd, 16, 10:07 AM
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Re: M20 not disengaging.....additional questions

Are you using the correct length fork ball?
Brass pilot bushing?






If you donít push the limits how are you going to find the weak spots?
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post #24 of 32 (permalink) Old Dec 2nd, 16, 10:13 AM
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Re: M20 not disengaging.....additional questions

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Originally Posted by blm View Post
X2. If you do the.pilot bushing has compressed too much. The pilot bushing should only be 1.5 to 2 thousandths larger than the bore of the crankshaft. Last one I installed it was 7 thousandths larger than the bore. Had to have it machined down. Some guys sand/grin them down themselves.

A dremel with these sanding wheels works well.


If you donít push the limits how are you going to find the weak spots?
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post #25 of 32 (permalink) Old Dec 2nd, 16, 3:14 PM
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Re: M20 not disengaging.....additional questions

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Originally Posted by 6T9 Chevelle View Post
When I installed my M20 a few weeks ago, I checked the bushing to input shaft fit before and after installing the bushing in the crank. When I installed the transmission, there was no resistance, I could practically push the transmission in with one finger. You shouldn't have to pull it in with the bolts...


this is true assuming the clutch alignment was /is perfect . When new parts are installed , its difficult to get it perfect. An old input shaft works better than the plastic tool provided. An assistant to depress the clutch pedal once you are within the last inch sometimes will help as a last resort.
Loosen the trannie bolts and see if will slide back easily... if so, not the bushing, but it may need to come out anyway to see what is really going on

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post #26 of 32 (permalink) Old Dec 3rd, 16, 9:48 PM Thread Starter
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Re: M20 not disengaging.....additional questions

Thanks to all reply's.....the trans has to come out. Starting thar chore on Sunday. I'll know more by then. I'll get back to this thread with the final diagnosis to hopefully help someone else going through what I experienced. Thanks

'64 SS Coupe, 4 spd Muncie M-20, 12 bolt 3:42, 350sb 9.75:1, Edelbrock RPM top kit with roller cam, EB 600cfm carb.
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post #27 of 32 (permalink) Old Dec 4th, 16, 10:54 PM Thread Starter
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Re: M20 not disengaging.....additional questions

For those of you still interested. I wanted to try one more thing before pulling trans. I jacked up the rear wheels and started the car in gear. With engine running and tires rolling I pushed the clutch in and hit the brakes. By the 3 rd time hitting the brakes with clutch in, whatever was holding up the clutch broke free. The clutch now free wheels an disengages correctly. I shifted into every gear easily and clutch disengages nicely. No noises, sounds or any thing that would indicate something wasn't right to begin with.

So I can only assume I either had a clutch disc stuck to the flywheel or an input shaft stuck inside the pilot bearing. It appears things are OK for now but if that pilot bearing was a little tight I may need to keep an eye on it for now.

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post #28 of 32 (permalink) Old Dec 5th, 16, 1:48 AM
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Re: M20 not disengaging.....additional questions

Glad to hear. Good thing you didn't have to pull the trans.
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post #29 of 32 (permalink) Old Dec 5th, 16, 8:14 AM
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Re: M20 not disengaging.....additional questions

Well Tony something to think about for next time at least from my experience. Those plastic clutch alignment tools are junk. An old input shaft is the way to go. Measurement of the crank bore and the outer diameter of the pilot bushing is imperative. Not sure you just didn't use enough force to distort the pilot bushing. Here is to hoping it was just the clutch disc stuck to the flywheel
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post #30 of 32 (permalink) Old Dec 5th, 16, 12:13 PM
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Re: M20 not disengaging.....additional questions

I'll bet it was the pilot bushing. At least you got it worked out. Keep an ear open for the pilot bushing squealing in the event you dinged it up a bit.

I have a very old wooden alignment tool that always works perfectly. It is even a good judge for an improperly sized pilot bushing. I'll never let that tool go! Not sure why they can't make a good alignment tool anymore. They MUST fit snug in the pilot bushing (which they don't) or they can let the clutch disc sag and getting the input splines into the disc will never work.

I wonder if someone good at a wood lathe could tool up a few good ones out of a good solid hardwood?

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