The SM420 is the old Chevy heavy duty granny four speed. Of all the
heavy duty granny transmissions, it has the lowest first gear and a
particularly short overall length (the Jeep T-18 is the shortest when
you add transmission to transfer case adapter.) Another big adantage
of this transmission is it's input bearing retainer size and input
shaft length is compatible with most GM engines. The only downside to
the SM420 is it has been out of production so long, parts have become
scarce. Most of the standard wear items like synchros and bearings are
still available though and spare transmissions can be purchased
cheaply for other parts.
The SM420 was only used in 2WD applications and 4WD applications
with divorce mounted transfer cases. Unless you plan on using a
divorce mounted transfer case, you will need an adapter regardless of
what transfer case you have.
SM420 Side view.
Photo by David Moulton.
Swapping a SM420 into a YJ: David Moulton puts an SM420 in his
Wrangler with a 4 cylinder engine. This swap is a relatively
inexpensive swap for a 4 cylinder YJ.
SM420 swap story links here.
The SM420 has been used in various GM trucks from the 40s until the
late 60s. The following articles cover some things to look for to find
the SM420 you need.
GM SM420: Chevy used the SM420 front the 40s until the late 60s.