V8 swap to stock SM 420 4-speed

[Chiro]

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

 

[coilover]

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.

 

[Joseph Martinez]

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.

 

[coilover]

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.

 

[Joseph Martinez]

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.

 

[Bill Hanlon]

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.

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.

 

[Zig]

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.

 

[sleepindawg]

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.