The Stovebolt Site Forums The Parking Lot Big Bolts SM420 interchange question

posted this over in driveline, but did not get much response so, posting in here. The SM420 in my 58 2-ton is slipping out of 2nd. Has anyone taken a 420 out of a small bolt and put it in a big bolt? I am wondering if the park brake stuff from a big bolt will bolt onto the 420 out of a 1/2 ton. I am not sure on the tail shaft, rear bearing cover (looks like it is just a bracket that bolts on over the cover though), and pivot for the brake lever.

The SM420 from different sized trucks should interchange.

The rear covers differ for open/closed drive shafts (parts can be swapped).

The front cover is different from 54 and earlier, but they can also be swapped (I think).

Parking brake and/or drive-shat brake parts can be bolt-on swapped.

If any of this wrong, we will see quick corrections.

http://www.novak-adapt.com/knowledge/transmissions/manual/sm420

thanks, I was reading that Novak page earlier, they mentioned the following that had me wondering:

The SM420 most commonly has a large, 10-spline output mainshaft for both 2wd and 4wd versions. Some fairly rare 2wd models (likely the 2-ton versions) featured a 35-spline and a male threaded output. These versions also have a rare, 22-spline first gear journal, instead of the usual 11-spline "skip-tooth" journal.

that is why I was wondering about bolting on the yoke and brake drum.

About 1978 I had a F 1 that would slip out of second under a heavy load. Such as giving it too much gas climbing a steep grade.

Back then people said it was the gears that were worn and would cause the transmission to slip out of gear. Others said then bearings were to worn and that allowed to much play in the shafts when under load and the transmission would slip out of gear. Take your pick.

My dad I were able to buy all new gears and bearings and rebuilt the whole transmission. Lot easier then trying to remove the bronze bushings in the main cluster gear and find new bushings to be pressed when you could buy a brand new cluster gear.

So if the parts are available try to rebuild it.
Jeffrey

you can get a rebuild kit for the SM420 but I don't believe the gears etc are made anymore. I can find 420's here locally for 100-150 but all for the small bolts, that is why I was asking. If it was just bearings, I'd pull it apart and rebuild it in a heartbeat, but I don't buy jut bearings causing it to pop out (done a few trannys, never had bearings do that although I could be wrong, once again!)

well, I got the tranny out of my 58. It is a 10 spline output, but the gears inside are worn (1st gear) so that is why it was slipping out of 1st. Also, the front of the bearing retainer was broken off, that is why I was having clutch problems!

Sounds like you have found reasons for your serviceability issues, that's always good.

So does all of the tail shaft stuff interchange? Inquiring minds want to know.

Paul

I believe so, will find out exactly once I get the replacement home and everything figured out.

The tranny out of the 1963 1/2 ton that I got has the same output shaft that the one in my 58 2-ton does. (10 spline, threaded hole in the end). The tailshaft housing for the 2-ton has the park brake stuff cast into it (except for the bottom adjustment bracket that bolts on). The complete tailshaft housing is a direct swap. The other difference is the top cover plate has a pivot pin in it (Passenger side) for the 2-ton for the brake lever. It looks like it is held in by a roll pin, I did not remove it since I am just swapping top covers (drilling for a backup light switch as well).

I have a '50 model 4100 that sat up for almost 40 years. Long enough for ALL the oil in the SM420 to leak out through a leaky PTO shaft. Then it proceeded to rust and actually got stuck. First manual transmission I ever saw get stuck. I was able to free it up, flush it out and put new oil in it and it still works fine. lol It's not even noisy.

Mexhanic58. that is a GREAT story!

Update: I ended up swapping the top plate back onto the tranny that came off of it. I drilled it out for the park brake handle pivot pin. THe tranny would stick in 3rd BAD when driving! a quick google search revealed that is a common problem if you swap the top with a different year. I guess they made a 1st series (47-61) and 2nd series (62-67) 3-4 shift fork! I put the top from my 58 on the 63 tranny... DOH! Supposedly, there is a 3/16" difference in length between the two, not enough for it to grind but enough to throw it too deep into 3rd so it won't come out. I swapped the top out and it works fine again!