Converting M21 to M20?

Discussion in 'U-shift em' started by knucklebusted, Sep 3, 2017.

  1. knucklebusted

    knucklebusted Well-Known Member

    So, I've been looking for a later model (fine spline input, large output) M20 to swap into my 92,000 mile 71 GS 350 4speed with M21 & 3.08 rear. I'm not ready to pay $1500 for one. If I was going to spend that much I'd go ahead and do a 5spd for a bit more.

    I've been reading some threads and see that the parts to make my M21 into an M22 wide ratio are only about $615. I know it will take a kit with gaskets and bearings at a minimum so I'm probably looking at $750 just in materials.

    What is the mechanical requirement? I've only had one manual apart and that was the Mopar NP 4spd in my 70 340 Cuda way back in 1984 when little brother cracked 2nd gear synchro. It worked but nothing had to be pressed on best I remember. Just some snap rings and a crap load of roller bearings that went everywhere.

    I have built a few motors that haven't flown apart yet. I have installed automatic shift kits, swapped motors, trans, front ends and rear ends. I have a garage and plenty of jacks and jack stands. I'm no pro but I'm not really afraid, more like dread getting into something I'm not equipped to handle. Pressing things on and off would be something I'll have to take to a machine shop but not outside my tolerance.

    If my ability and tools are lacking, what's an independent shop likely to charge me to swap it? How many hours?

    Anything else I should look for or do while I'm in there? Is the iron mid plate something that would be needed only if I swap in a 455 later?

    STAGE III Lost Experimental Block

    Harbor Freight runs a special ocassionaly on a 12 ton press for $99.
    Did great on rebuilding my L-69 SuperCharger & A-arm bushings so far. I hate farming stuff out and being at someone else's mercy & all.
    Gonna follow your thread, good topic: )
  3. buick64203

    buick64203 Buick Hunter Staff Member

    Check out Paul Cangaliosi's "Gearbox videos" on You Tube. He literally wrote the book on Muncies and has quite few very informative videos. How to rebuild, differences in early to late synchros and hubs, differences in original and aftermarket parts, How to load the needle bearings in the countershaft gear, etc. The guy really knows his stuff.
    Brett Slater likes this.
  4. 455 Powered

    455 Powered Well-Known Member

    They really aren't bad to do. Keep your parts in order. A good shop manual will help a lot
  5. NZ GS 400

    NZ GS 400 Gold Level Contributor

    I read Paul Cangaliosi's book, watched his videos on youtube and asked him a couple questions re: some parts and rebuilt my m20. You need a hydraulic press and some snap ring pliers (and of course the rebuild parts kit). A propane torch is also needed for reinstalling the speedo gear. I found the re-insertion of the countershaft to be the trickiest part. Real sticky assembly lube helps keep the needle bearings in place. I highly recommend Assemblee Goo. Feel free to PM me if you have any questions. I really enjoyed the project and would never dream of paying someone to do the job now that I know what I know and have had the experience.

    A cheap Harbor Freight press will not set you back much more than having a shop do the pressing for you. Then you have complete control over how things are done and can insure that nothing gets damaged.
    Last edited: Sep 28, 2017
  6. knucklebusted

    knucklebusted Well-Known Member

    I may look into it. The press is my only hold up. Not needing a new tool in the shop just now but think I have a friend with one I can visit and get it done.
  7. NZ GS 400

    NZ GS 400 Gold Level Contributor

    Hi Greg,

    I don't know if you have done anything with this project yet. Sorry I have been distracted with other things and haven't logged on in a while. I thought of another tip for you. Another thing I learned by trial and error is NOT to lube the synchro keys and snap rings that hold them in. I did at first it makes the keys pop out too easily during assembly. I cleaned all the lube off of those parts and it stayed together as necessary.
  8. knucklebusted

    knucklebusted Well-Known Member

    Good to know. Some things learned the hard way are priceless.

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