The Stovebolt Site Forums The Shop Area Driveline Transmission identification and swap question.

Hi,

I have been searching the forum but haven't seen a reference to a transmission identification guide. My transmission cast numbers are 2 GM 59047 and stamped numbers GM 52408. The cover has cast numbers 590511 with a bow tie and a number 9 and what I believe is a date code of F177. I believe this is a 4-speed non syncro. My guess is that it is not original to my 1940 1/2 ton. I am wondering if a swap to an SM420 is doable and if so what issues will have to be overcome.

Thanks

SD,

Check your casting number again, your one number short. I believe you have 590474 (p/n 590524) which is a Second Half of 1931 thru 47 truck 4-speed transmission.

The date code (F177) could be June 17 of 1937 or 1947, but with the Bow Tie I'm betting it's '37 (I don't think they put the Bow Tie on castings in the late 40's, but I could be wrong).

Someone else will be along to advise you on the swap.

Mike B

When considering a 420 swap, measure input shaft stick out, OD and count of splines, retainer dia., mounting bolt pattern, they all need to match up, whether they do now or when you modify a 420, it doesn't matter. A good idea to do.

Ed

Thanks Mike,

You are correct, the casting number is 590474

Thanks Ed,

The 420 I found on Craigslist doesn't look to have any tappings for the clutch, brake and master cylinder mounting as my transmission does. Perhaps I would be better off learning to double clutch.

There were a few different SM-420 configurations, so pay attention to which one fits your application.

Open or Closed Drive Shaft.
With or Without Drive Shaft Parking Brake.
Drive Shaft Parking Brake with External Band or Dual Shoe.

Mike B

It is not so bad once you get the hang of it...fun really.

Doesn't the non-synchro transmission have a different bellhousing bolt pattern than the 420? I believe somebody came up with a way to adapt a later model bellhousing to accommodate the pedal pivots, and there might be a tech tip on that procedure.
Jerry

These are the issues that I know of when converting from a crash box to a sm420 four speed:

1. bellhousing bolt pattern is different and input shaft bearing retainer diameter is bigger on the sm420. You will need a bellhousing from 48 through the early 50's
2. the mounting holes for the brake/master cylinder are on the bell housing and your replacement bellhousing will not have any. There have been threads in the past about how to make an adapter.
3. The sm420 is about 2" longer than the crash box. This means you will need to shorten the closed torque tube drive shaft or convert to open custom drive shaft. SM420s were made for both open and closed drive shafts.

Good luck, can be done with mostly off the shelf chevy parts but not a weekend project.

As I recall, I cut off a piece of early bellhousing with brake and clutch mounts, cut off part of later bellhousing and welded in place using Ni rod, pre and post heat, works great.

Bellhousing can be bored out to fit 420 retainer., not on a drill press.

Seems like early trans did not have bottom right mounting hole ofset to right, easy fix, mount trans with others, mark hole from bellhousing in trans, drill and tap early mounting hole on late bellhousing.

Ed

I have same transmission in my 46. 590474. does any one have any good diagrams of this transmission. trying to locate some parts. mainly back end that houses speedometer.

Thank you all! You guys are a fantastic wealth of knowledge and you have helped me decide to keep it simple and stick with my current transmission. I read in another post that double clutching is a form of theft deterrent, I like that.