GM SM420

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.

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.

	Note the ribbed side of the case. Early cases do not have this ribbing.
	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.

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.

Contributors

Thanks to David Moulton for the good clean SM420 pictures at the top of the page. I took the blurry pictures of the dirty SM420. Thanks to Evan Chapman for the 420 gear ratios. Thanks to Paul Weitlauf for the pictures of the 1967 SM420.

Transmission	1st	2nd	3rd	4th	5th	Rev
SM420	7.05	3.58	1.71	1.00		7.05