The Stovebolt Site Forums The Shop Area Driveline SM420 compatibility

I’ve been (very) casually looking for a replacement SM420 for my ‘51 3100. My transmission still works, but the shifter is all kinds of sloppy (I know there’s a fix for that), synchros don’t function perfectly, gears whine pretty loudly, etc. It also leaks from every possible orifice.

These are still common enough that a replacement is probably more cost effective than rebuilding.

My truck was converted to an open driveline by the previous owner some decades ago. So, is there any SM420 (other than the torque tube variant) that will NOT work as a replacement for me? A lot do what’s available on FB and such are units from bigger trucks with driveshaft brake drums and/or some remnants of a PTO. Can all that stuff be simply removed and I just use my existing output yoke?

The 55 to 67 trucks used a SM420 trans with an open drive shaft. The ones from a light duty 1/2-3/4 ton ought to fit.

George

I am not sure if this is factory, probably not. My 59 Panel came with what
appears the original transmission. It looks like an extension was added to the drive shaft.

The pics are of an original equipment driveshaft and slip yoke arrangement for a trans without a driveline parking brake. The only difference in the SM 420 for a torque tube driveline and an open shaft is the rear bearing retainer. Swapping the bearing retainer, output yoke, U joint, and slip yoke assembly is all that's needed to adapt the transmission to different applications. The only sticking point might be the speedometer drive gear, which must match the rear end ratio.
Jerry

I used a newer sm420 on mine and just had to swap the cover from open to closed so even the older sm420 should work just have to swap the closed ball cover for the open cover that you already have .the newer ones do have a seal at the front flange which keeps the oil in when parked facing downhill.Also keep the yoke and speedometer bullet to match and keep your speedometer the same

I bought an overhaul kit from the rock-crawler/jeep guys in Logan UT N...... Can't remember just now the name. Any how it had everything you need to overhaul it except one syncro, third gear, I think. There are two versions early and late and it it the early syncro that is NLA. Anyhow the price wasn't too high. If you start swapping gears from various transmissions the gears that weren't originally mated will be noisy. So keep the whole gear set together.

Even in the best of condition these transmissions were not quiet but with good bearings they are MUCH quieter.

just looked at my trans it has the brake on the rear, the cover is much different than yours It has all the mounting for the drum brake. but the bolt holes are the same. Try novak trans web site lots of info and parts. good luck

Sounds right. All I know is the drive shaft for the 420 in the 59 panel is longer
than the shaft for the 58 panel with a 3 speed column shift.
When I replaced the trans from a 420 to a M-20 the old shaft would not work.
Might not apply, just what I ran into.

Edit>>> the wheelbase on these panels is different than a step or fleet side same year.