SM420 is the older heavy duty, granny, 4 speed transmission used in Chevy and GM trucks

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.