V8 swap to stock SM 420 4-speed

Discussion in '1947-1954' started by Chiro, Feb 16, 2017.

  1. Chiro

    Chiro Member

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    A New York Yankee living in Virginia
    I am considering the above. I have a 235 now, SM 420 4-speed with open drive and a '72 Nova rear with 3.08:1 gears. Cruises down the road really well and 65-70 MPH is not a problem. Just wondering if swapping in a SBC is possible while retaining the rest of the driveline I have and what all is involved. I'm wondering if I can retain the stock trans cross member and just drop the SBC in with new motor mounts, a 265 left hand exhaust manifold (to clear stock steering box location). I need to know about flywheel/clutch compatibility, etc. Anybody out there have any idea...like maybe Evan???;)

    Can this be done? I usually see everybody swapping out the trans and going to an auto with a new cross member, etc. It is my understanding that the SM 420 4-speed is a pretty bullet proof trans and will hold up to a SBC very well.

    School me please and thanks,

    Andy
     
  2. coilover

    coilover Member

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    Andy.
    We have done several of theses "backward" swaps; auto to standard rather than the other way around. Last one was on a 40 International with a 3.08 rear end. The owner likes it and now uses 1st or granny gear every time. He said he had to learn to come to a complete stop as 1st is not synchronized. IIRC the front bearing retainer had to have a bit, maybe 0.100 turned off the circumference to fit a car bell housing but nothing major was required. Oops, wait a minute, the 235 had a different trans to bell housing bolt pattern so this might be a problem. I got on craiglist and found one for a 65 GMC for $50. Anything 63 or newer has the the right bolt pattern. Weather permitting I have an old granny trans and an aluminum bell housing to check the fit. Heck of a lot easier to carry the bell housing to the trans rather than visa versa.
     
  3. Joseph Martinez

    Joseph Martinez Member

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    People always treat a 4 speed granny gear transmission as a regular 4 speed. The Granny Gear should only be used for when pulling power is needed. Otherwise, the transmission is used as a standard 3 speed, meaning that 2nd gear will act as 1st gear, 3rd is 2nd, and 4th is 3rd.
     
  4. coilover

    coilover Member

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    Agree if you have the standard 4.11 or 3.90 rear. With a 3.08 rear if you didn't use granny you would be buying clutches by the dozen.
     
  5. Joseph Martinez

    Joseph Martinez Member

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    But that's what happens. The granny gear is designed for a certain rear ratio. Regardless, the engine, transmission and rear gear ratio, even the size of the tires should match so it all works properly, in unison.
     
  6. Bill Hanlon

    Bill Hanlon Member

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    I start my 3.55:1 with tall 31" tires in 2nd unless starting uphill, plowing or pulling stumps. Neck snapping acceleration with all 90 horses pulling strong.
     
  7. Zig

    Zig Member

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    Same here Bill~ 373 Colorado rear end.
    Just got through rolling around town in it, as a matter of fact.
    Andy~ I look forward to hearing more about this iffin you end up doing this. I remember seeing a 283 bolted to a 4 speed. It was an old truck like ours. Sadly, the junk yard over in Goodman Missouri closed so I was not able to go back and check further on it.
     
  8. sleepindawg

    sleepindawg Member

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    I have a 283 and what I assumed was a 420 w/a hand brake (both tranny and bell housing out of a '60 or '61 3/4 ton) in my '50 3600 along with a 3:90 axle out of a 58 1/2 ton. I did have 31x10.50 on the rear and a second gear start was fine. I haven't drove it since I put the 235-75's on it to see if it's too little a tire for a second gear start.

    I believe the bell housing on the 235 has a different block bolt pattern compared to the first gen small block chevys. The second gen inline 6's have the same bell housing bolt pattern as the small blocks. I do know that in '54 the cross member for the bell housing was changed to angled rubber mounts.
     

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