M-21 rebuild tips

[Chuck R.]

I'm at the point of throwing...well Ok, placing the transmission on the bench and starting it's clean up and overhaul if needed.

As I never had the chance to put the driveline through it's paces, I have no idea as to possible internal tranny issues.

Other than the obvious planetary shaft wiggle or seal leaks, is there any other up front give aways that can help me to isolate any syncro or gear mesh issues?

Any reccomended reading materials?

Thanks,

Chuck

[Steve D.]

Re: M-21 rebuild tips

Chuck

I found this video extremely helpful. It runs over an hour, and goes step by step for disassembly and assembly, with lots of pointers along the way. It's not a slick production; just a video camera pointed at a guy at the work bench. If you have questions, the guy in the video answers the phone.

Steve

http://www.5speeds.com/video.html

[William C.]

Re: M-21 rebuild tips

If you have a chevy service manual from the era, Corvette or other, they provide an easy way of measuring the wear on the brass synchro rings, but my recommendation would be if you have the trans apart, just replace them all, they are the least expensive part of the rebuild.

[Terry F.]

Re: M-21 rebuild tips

I used the overhaul manual for my year car and it worked fine. You will need a press to disassemble the cluster shaft and reassemble it. Inspect the gears (very typical to find a reverse gear chipped or worn), replace the bearings with quality bearings, wash the cases out real good and look for gear chips, replace the gaskets and seals, replace the tailshaft bushing, inspect the shift forks (new or good used ones can improve the feel of the shifting), replace the detent keepers (not sure what they are really called) on the clutch packs.

I would recommend that you mark the clutch packs and put them back together the exact way they came apart. Clutch pack = the part that has the shifter fork collar and the brass synchros. I am not sure if that is the technical name for them or not. I heard they were individually matched during manufacturing. Use heavy grease to hold all the neadle bears in place inside the cluster gear and input shaft gear.

Always make sure you have plenty of grease or transmission fluid in it before big burn-outs. I think one of the big problems of failure with these transmissions is low fluid/grease levels and not warming them up enough. If your tail shaft seal leaks or the counter shaft hole leaks, that is not good for them because it lets the fluid level drop. I believe they blow up when 2nd and 3rd gear gauld themselves onto the cluster shaft during hard excelleration. It will make sense when you take it apart. When they lock up, they put the cabosh on the counter gear. They also blow up from too much torque. I am not an expert, just my opinion from talking to people.

Inspect your drive shaft yoke for wear. If badly groved, replace or rebuild.

Buy a good quality kit and all the parts you need will come in it. Ask around on the board and someone can probably recommend a source. It is a very fun project and easy to do if you have a press.

Hope this helps, Terry

[Werner R.]

Re: M-21 rebuild tips

after that excellent and detailed discussion, i hesitate to offer my two cents worth [probably less than 2 cents], but i tried the heaviest grease i could find and i was still not able to hold the needle bearings in place while i inserted the counter shaft. therefore, i inserted a wooden dowel [as i recall from 5 years ago it was a nominal one inch dowel]first, ahead of the countershaft, and then i had no trouble holding the needles in place as i inserted the true countershaft using the counter to push out the dowel.
good luck
werner

[Terry F.]

I forgot something.

Most kits will come with thrust washers that are placed at each end of the counter gear. It is important that they have the correct clearance to minimize the movement of the counter gear longitudinally with the shaft. The chevrolet overhaul manual will give clearances for it and chevrolet should sell shims as need, I think. This will improve the feel of tightness in the transmission if they were badly worn.

I would avoid replacing the shift collars. I don't think anyone makes them any more any ways. I believe they were matched during assembly.

I recall there are shims on the reverse gear also, they are less critical.

Inspect the inside of the input shaft for flaking and wear. Trans missions that have been sitting or had water in them will wear in there.

You will need a special tool to insert the tail shaft bushing. Just take it to a transmission shop and they will install it for you.

If the transmission wasn't giving you too much problems to begin with, then you should have excellent results. If the case was leaking badly at the front of the counter shaft, then you should have a reliable transmission shop repair that. Means your case is possibly stretched.

I see overhaul manuals on ebay cheap. There are also a few muncie parts on there most of the time.

Terry

[Chuck R.]

Re: Sounds like no real surprises here

Just have to open the can up and carefully scrutinize components for excessive wear.

Terry, as my cross flagged carcass was trailered to my garage in pieces, I didn't have the luxury of test drives or operational assessements.

I'll share how I fare unless of course I get my shorts in a bundle over an issue I can't resolve, then your gonna hear from me sooner!

I wonder if my wife will get too too upset if the 1" dowel in the hallway closet disappears

Thanks for the insightful direction.

Chuck

[Patrick H.]

Re: M-21 rebuild tips

Highly agreed.

I had two shop manuals for reference, but the video told me most of what I wanted to know.

The only time I "needed" a press was 1) removing the speedo gear and 2) installing the speedo gear. The only other special tool was the wrench for the front of the case. I purchased the video as well as the rebuild kit and wrench from the web site cited, and it was all a LOT easier than I thought it would be.

Regular chassis grease did well for me; nothing super heavy needed.
I just drove the car today, and it's doing fine.

Patrick

[Terry F.]

Re: M-21 rebuild tips

Inspect the bore of the counter gear for flaking also. I am not sure who to buy gears from anymore. I heard original gears were high in nickle and it made them longer lasting. I am not sure if the new stuff is the same way. I wish someone would comment on this.

If something does not feel right or look right, ask questions and probably replace it.

You will be able to do the job. Sorry for so much rambling on. Enjoy, Terry

[Brian Monticello]

Re: M-21 rebuild tips

How would you guys rate a muncie rebuild vs a complete engine rebuild? I recently completed a complete tear down and reassembly. For some reason, the muncie has me nervous. The part that has me worried is the judgement involved in deciding whether or not parts are usable and shimming up everything for proper end play.

Brian

[Terry F.]

Re: M-21 rebuild tips

I think it is a good question. For the most part, if your transmission has no problems I would tend to leave it alone. I would replace the tail shaft bushing and the tail shaft seal and the little O-ring seals on the side cover. I would wash it out real good. If you are taking it apart to inspect and replace as needed, I guess that is another thing.

I have no problem puting new bearings in a transmission that has a moderate amount of wear on it. But I question what will happen to a transmission if you start putting good used or new gears in with old gears. If the car is just going to be driven normally, I would probably try to find good used gears. I would only replace gears if they were flaking, rusted or pitted or broken.

Used gears are broken in to the transmission they came from. They will probably have to establish a new wear pattern on the transmission you put them in. New gears will break in and probably establish a new wear pattern on the old gears they are meshed with. I don't know what effect that will have on gears that may already be fairly tired. It is all probably just splitting hairs.

I don't know who makes good muncie gears. NOS is good when you can find them. I don't know about the stuff comming out of Italy. I heard nickle content is important.

There are kits that sell a complete set of gears.

To answere to your question. Transmission (muncie) tear down regardless is easier than engine rebuild. My opinion, Terry

[Chuck R.]

Re: Well Brian

Considering the multitude of critical moving parts in a power plant vs. a tranny, I'd have to give the big "Worry Wart" award to the power plant hands down!

The way I see this going down, is if tollerances are within the acceptable range and there are no ground up syncros, missing teeth or contamination, I'm going to slap in some fresh seals and gaskets and let it fly.

If I find shrapnel, THEN I'll get all anal over it.

I will probably lean towards looking for good used NOS components as opposed to fresh aftermarket if required.

I guess I'll see where this road leads when I break the case open this weekend.

Thanks for the great information Gents,

Chuck

[Patrick H.]

Re: Well Brian

Chuck,

(Virtually) every part of a Muncie is now available in reproduction. Many of the parts available are as good or better than the originals, and I would not hesitate to use them over any used part. I now have replacement reverse gear, reverse idler and speedo drive gear in my transmission and they work very well.

It was only after hearing several times how easy the transmission rebuild is from Joe Lucia, and talking with him again in person in March, that I decided to try it myself.

As noted above, the video and rebuild kit were VERY helpful. The quality of the video is somewhat amateurish, but I learn by seeing much more than reading, so I enjoyed it. The rebuild kit contains ALL the parts you will need, and high quality ones at that. If you're taking it apart anyway, I couldn't imagine not replacing the synchros, needle bearings, etc. while you're in the neighborhood. I decided that it wasn't a job I wanted to do twice (i.e. the hassle of removing and reinstalling, not the rebuild itself) so I made sure that everything was perfect before reinstallation.

I've done a few engines, and I'd rebuild a transmission in a second before doing an engine.

In addition to www.5speeds.com or www.4speeds.com (same company, they sell the video and rebuild kits as well as the wrench) I used www.dandltransmission.com (parts and advice) and www.autogear.net (they make the gears).

Good luck!

Patrick

[Dick W.]

Re: Well Brian

I would like to add one comment. If you do disassemble the transmission, take a good look at at the bearings and chunk them in the scrap pile. My experience with reusing bearings, no matter how good they look, has been very poor. Penny wise and dollar foolish to reuse in my opinion.

[Patrick H.]

Re: Well Brian

New bearings come in the rebuild kit from the company I mentioned. Again, as long as you're in there.....

Patrick