The SM420 is the older heavy duty, granny, 4 speed transmission
used in Chevy and GM trucks. The large cast iron case with large bulge
on the passenger sidemakes it easy to identify.
It is an excellent 4 speed to swap into a Jeep with a Chevy or
Buick engine. It will work well with any GM engine because it uses a
standard GM size input bearing retainer, so getting a bell housing for
it should be easy.
Another advantage of the SM420 is it's length is the shortest of
all the heavy duty 4 speeds. When you add the adapter, the Jeep T-18
is shorter, but the SM420 is shorter than other adapted granny
trannies.
SM420 casting numbers.
Photo by David Moulton.
The SM420 only had a backup light switch in 1967. Earlier versions
of the SM420 did not have this feature which many would take for
granted today. The ribbing was only available on late cases and some
grinding of the ribbing is required if it is to be used with a Dana 20
or Dana 18. The same is probably true if the ribbed case is used with
a Dana 300. The ribbed SM420 case should not be confused with the
later SM465 transmission.
All SM420s have a casting on the drivers side. The numbers seem to
vary from transmission to transmission, but every one I've seen has
"GM" and the large number starts with a "5". Most likely the "E43" in
this casting is a date code. The screw heads are part of the casting.
| Transmission |
1st |
2nd |
3rd |
4th |
5th |
Rev |
| SM420 |
7.05 |
3.58 |
1.71 |
1.00 |
|
7.05 |
SM420 front. Photo by David Moulton.
SM420 drivers side. Photo by David Moulton.
SM420 back. Photo by David Moulton.
SM420 passenger side. Photo by David Moulton.
SM420 late version with ribbing. Early cases do not
have this ribbing. Photo by Paul Weitlauf.
SM420 late version with ribbing with Dana 18 and Warn
OD. A Picture from the back with a Warn overdrive and Dana 18 transfer
case attached. You can just make out the backup light switch to the
right and back of the transmission shift lever. The switch is
partially obscured by the transfer case shift levers. Earlier SM420s
do not have a backup switch. Photo by Paul Weitlauf.
SM420 drivers side.
Photo by Terry Howe.
SM420 front.
Photo by Terry Howe.
SM420 passenger side.
Photo by Terry Howe.
