Jeep Transmissions Conversions: SM420
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 Swap Stories
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.
GM used the SM420 in trucks rated from 1/2 ton to 2 1/2 ton from
the 40s until 1967.
Last modified Wednesday, 01-Dec-2010 08:12:52 MST
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