4 speed Conversion help/advice

Discussion in 'U-shift em' started by apachepatmanfx, Sep 22, 2015.

  1. apachepatmanfx

    apachepatmanfx Well-Known Member

    Hello, I've been a reader for a while, but this is my first post. I am converting my 72 Skylark from a Th350 to a muncie 4 speed. I can't decide between and M20 or M21. I originally wanted the fine spline 71-74 M21, but later learned it would not be a direct fit with the Th350 driveshaft length, only Th400.

    Now that I'm back to square one, I can't decide if I want the 2.20 or 2.52 gearing. I do Street/Highway driving, and don't have any track plans in the near future. I have work done to the motor.. probably has 350-400hp on a Buick Small Block 350.

    So my questions are

    -Do I go with the M20 or M21? and which years in specific for a 72 Skylark?
    -Depending on the M20 or M21, what should I run in the differential if I go posi in the future?
    -Also what do I need to know about the reverse lockout situation specific to the 70-72 Larks?
    -Will I need a new speedometer or cables?

    I went through the stickies already, and apologize if I missed anything.
  2. buick64203

    buick64203 Supercar owner Staff Member

    Well, it really depends on your rear gears. If you have a dead rear like a 3.08, you'd probably be better off with he M20's 2.52 first gear and gear ratio spread. Anything from 3.42 and numerically higher, Buick put an M-21 in their cars

    The rod from the column and the bellcrank from the trans to the frame is the same as your T-350. Repro reverse lockout linkage is readily available. You'll need to replace your neutral safety switch to a back up light switch and get a clutch safety switch and harness. All available on sites such as Ebay

    You'll need to remove the PRNDL indicator off the bottom of the speedo. You can remove the speedo, take the lens off and carefully remove the PRNDL lettering with some Windex.
  3. apachepatmanfx

    apachepatmanfx Well-Known Member

    Thanks, I have a feeling its the 2.56 carrier.. car is pretty basic. Once I'm back at my shop I can confirm. I plan on changing the gears after this tranny swap once I switch to posi and get a 12 bolt.

    Is it true that I will need a 1970 or earlier tranny is I want to avoid switching the driveshaft, due to my th350 output shaft? Its unfortuante, I was hoping to get a 71 or later tranny and take advantage of the 26 spline shaft as opposed to the 10.
  4. buick64203

    buick64203 Supercar owner Staff Member

    In the grand scheme of things, isn't the driveshaft is the least of your worries? Probably the most inexpensive items on the list

    An M21 and an M20 is the same length. To the best of my knowledge, the only thing different between the later 71 and up Muncies is the yoke. Cant you just swap it out?

    Pretty sure the speedo cable on a T-350 is on the drivers side? The speedo on a Buick 4 speed is on the right, so the cable will need to be swapped out to a 70" length
  5. gsgnnut

    gsgnnut Well-Known Member

    Id go with the m21 and get a 400 yoke. its a stronger trans. the others are right, the driveshaft should not be the primary driving factor in deciding which one to go wth. Car use, rear ratio, engine size, trans avalability ecetera are more important considerations. Many times when you buy a trans you can score all the smalls with it. I just did a M21 swap into a former 350 car and got everything from the donor car including the speedo , column, driveshaft, x member and rear axle. Only extra to buy was 455 flywheel, PP an clutch.
  6. apachepatmanfx

    apachepatmanfx Well-Known Member

    Good point, Am I purchasing the yoke and the driveshaft or just the yoke? Also why are some engine blocks drilled for the ball stud and some not? Thanks
  7. cstanley-gs

    cstanley-gs Silver Mist

    most everything has been said.
    You will need a longer drive shaft
    And a TH400 yoke.
    A drive shaft can be made for a couple hundred - thats nothing to the other costs.

    A few things I found
    - The floor hump - hard to find an original that anyone is willing to part with. An alternative is to buy a repro for a 67 chevelle WITH console, its rounded similar to the Buick hump. DONT GET the one for without a console, its very square.

    - I assume you're not going to use a console, I didn't due to the cost. Therefore you need a floor shifter boot and ring. I used one from a 69 Olds Cutlass, its as similar to the Buick setup as you can get without going NOS.

    - Buick pedals are unique so you'll need to source those too. But I found that the bracket is the same on auto/manual pedal assembly. so you only need the clutch pedal and a longer bolt to put it on the bracket with the brake pedal

    - clutch start switch - another unique piece to Buick but I found Pontiac used the same one and its available as a repro - worked great.

    - I used an 80" speedo, it tucked up nicely, though as mentioned 70 may be all you need.

    - The auto shifter linkage from the column to the TH400 trans to the frame worked perfectly on the manual trans. The repro reverse lockout linkage works perfectly too. My car has to be in reverse before the key can be removed. And my reverse lights work too.

    - my flywheel came from TA - $400+. Or you'll have to find an original and have it re-surfaced

    - I used a stock clutch kit from Autozone $120

    - I used a fine spline M22 from a 72 Corvette ($600 on Craigslist, nothing wrong with it, the guy removed it to do a modern 6 speed swap)

    - I have 3.42 gears. I find Im not in first very long before going to second. I believe the TH350, TH400, M21, M22 etc. all have a final drive of 1-1 so highway RPMs wont change in that respect

    Some things to consider
    - Some sources say you will need a manual or big block cross member - not sure which you have - My car is a 455 car so I was ok there

    - E-brake cables are supposedly different, they route though the big block cross member - I never changed them and the ebrake works fine.

    - Ive read some cranks dont have a place for the pilot bushing or a place for the pivot ball - I never had this problem

    - the Zbar on my car came from The Parts Place
  8. buick64203

    buick64203 Supercar owner Staff Member

    Some 350's aren't drilled for the ball stud. I'll guess that if it wasn't going to be a 4 speed, they didn't drill them to save on machining costs. In any case, if your engine isn't drilled, you can drill and tap a hole. The extra material is still cast into the block regardless. I've drilled a couple. Measure twice, drill once! I think its 1/2-13 thread but double check that.

    Check the hole in the back of the crank. Some 350's aren't machined all the way to the back of the pilot bushing hole. Ive seen a couple that had a slight "step down" in diameter towards the back of the hole. Especially on the ones that weren't drilled. You'll have to measure the diameter of the pilot bushing hole anyway with a digital caliper and make sure the pilot bushing is made to the correct diameter. If there is an inconsistency in the diameter of the crank, just have a machinist cut the bushing a little shallower to compensate for the neck down in the back of the hole (if there is even one there). Clear as mud?
  9. apachepatmanfx

    apachepatmanfx Well-Known Member

    Thanks for all the info guys.. starting to think It may be easier to go with a tremec tko elite package. I'll have more gears and less headache.
  10. flynbuick

    flynbuick Guest

    You will have more gears but the headaches will still be there.
  11. tsgp51

    tsgp51 Well-Known Member

    Good day here's a few things that might help a 26 front spline muncie will have a big 32 spine output shaft you could get a m 20 21 and 22 this way . all you will have to change on the driveshaft is the yoke. Muncie and th 350s are the same length so the cross member parking brake etc all work if you use the 10 spine input /27 spline output you won't have to change the driveshaft at all. There is a websight 5 speeds .com they have a muncie page which shows how to ID muncies (very helpful) .with a 350 a m-20 works great but if you go big block it cant handle the torque. M 21 and 22's need lower rear gears to work well.Auto gear makes m 21 and m22 gear sets in wide ratio. 2:56 first gear. I put a m 22 set in my gsx works great with 3:42gears and the gear whine is bad ass. I hope this helps. Tim
  12. buick64203

    buick64203 Supercar owner Staff Member

    I would imagine putting a factory available transmission in the car would be a lot easier. A Tremec TKO is going to need some extensive floor/ tunnel mods
  13. sriley531

    sriley531 M.M.O.G.

    More gears: yes. Less headache: no.

    I put in a Muncie when I did my car (originally a 350/350 turbo car, now a 455/4 speed) and looked hard at the tremecs. There's some considerable reshaping of the trans tunnel that's needed to make it fit, also the linkeage and shifter location takes more consideration. I had bad luck with clutch linkeage and went to a hydraulic setup, but otherwise the swap was pretty straightforward. In my opinion, if you want a rear higher (numerically) than 3.42, you may want to consider an od transmission for highway driveability. If you're OK with a 3.42 or (numerically) lower, go with a 4 speed for ease of installation. For reference purposes, I have a 3.42 and a 255-60 rear tire and run about 2850 rpms at 65. It's an M21 and first gear is quick, an m20 would be even lower (I sometimes take off in second). But 1:1 4th either way.
  14. apachepatmanfx

    apachepatmanfx Well-Known Member

    Do you recommend the m20 or 21 in my case? i have a 350 with work done coupled with a th350
  15. sriley531

    sriley531 M.M.O.G.

    Power-wise I think you'd be OK either way. In my opinion, it more so comes down to your rear ratio. Lower (numerically) than 3.42 go with a 20 for that low first gear. 3.42 or (numerically) higher, 21 or 22 or youre gonna be shifting into second real quick! I love my 21/3.42 combo, its low enough I can easily take off in second if I want, but not so low that first feels like a granny gear.

    But it'll waste the tires from a roll in first or second, so it all good! :cool:

    By the way, an SBB with a four speed is BAD#SS, I love unique tree-fiddy cars! If you do the swap, be sure to put up a build thread, we loves us some pictures!
  16. moleary

    moleary GOD Bless America

    BTW, I have a OEM restored clutch fork and Bell Crank (Z bar) for a 350 car for sale $300 shipped. Not repop crap, factory OEM parts! If interested, PM me.

    I have done a few conversions also if you have some questions, let me know, I am happy to share my experiences with you.

    I also have a template I would loan you to cut the floor for the floor hump you will need to weld on...
  17. crash

    crash Well-Known Member

    The 1971-74 Muncies used tail housing casting #764 which is 3/4" longer than any of the earlier tail housings...

    The combined length of an early Muncie and bell housing is the same length as a th350...

    The combined length of a late Muncie and bell housing is the same length as a th400...

    Hope this helps... Crash
  18. cstanley-gs

    cstanley-gs Silver Mist

    I ditto that, my TH400 swap to a 72 Muncie used the same driveshaft/yoke with no mods.
  19. buick64203

    buick64203 Supercar owner Staff Member

    Your 4 speed knowledge and ability to identify parts always impresses me Crash. Good info! I didn't know that. So he would just need to source the correct used 71-72 small block 4 speed driveshaft.
  20. 87GN_70GS

    87GN_70GS Well-Known Member

    If you want to avoid trans tunnel mods and want extra gears, a Legend LGT-700 or Mcleod Muscle car 5 may be for you

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