The Stovebolt Site Forums The Shop Area Engine Shop 3 speed to SM420 conversion

I recently found a complete SM420 in a pile of scrap metal at the local dump. What luck! I had been looking for one to replace the 3 speed in my 1952 3100 1/2 ton and literally tripped over this SM420, shifter and all.

I'll be converting the truck to a open driveline and already have a good rear end to put in as well. I have read a number of archived posts here about the SM420, but I still have a few unanswered questions:

I read that the SM420 will bolt up to my 235 bellhousing - but will I need to change the clutch?

Is there a transmission tunnel for the cab that was made for this trans?

And lastly, do I need a transmission crossmember to support the weight of the trans? Was there one in 1952?

If anyone who has rebuilt this trans and/or done this swap has any tips, I'd love to hear them as well. Thanks in advance for your help!

Keahi

Go to this link.

http://www.6066gmctrucks.org/SM420.htm

this link is for upgrading.
http://www.6066gmctrucks.org/3-to-4-speed.htm

Thanks! I don't know how I missed that page. How rare are the transmission covers?

That site shows the conversion on a later model Suburban. Some of the details will be different on your 52. I think that the clutch will be different from the 3 speed setup. My clutch on my 1 Ton is the larger 10 3/4" clutch.

There is no special tunnel on the 52. I think that the cab should fit fine. The transmission cover will be different.

MOst of the major venders have the 4spd tranny cover (made out of fiberglass) for 40- 50 bucks.

The clutch can be the same, but most trucks equipped with an SM420 had a larger clutch. My 216 equipped factory SM420 '50 truck had a standard 9" clutch.

Look closely at the output end of your 420...is it set-up for open driveshaft, or does the u-joint sit in a cast-iron dish with four ears & bolt-holes, to mate with the torque-ball on the driveshaft?

All '47-'54 Chevy trucks have flat floors (no tranny tunnels), and the tranny-to-bell housing bolt pattern is the same for all trucks from '47 - '66...

You will need to get the floor-shift center floor cover.

Also, the speedo might read incorrectly with the four-speed...the 420 was available in ALL sizes of Chevy trucks, from 1/2 ton thru 2 ton, which covered a range of rear end ratios of 4.1 for the half-ton through 6-something for the 5400 & 6400 series trucks...

Good luck with it !

Thanks to everyone! It's an open driveline trans - off a 1961 if I decoded the casting numbers correctly. However, I wouldn't mind getting the necessary parts to connect it to the torque tube so that I can just bolt the trans up now and then change the rear end and driveshaft in a future weekend project. Maybe I'll put a request in the swap meet section.

Aloha,
Keahi