I got my spicer 18 adapter, now I just need to bolt my SM420 up to it. The SM420 that I bought has an unknown history (to me). I'd like to try running it without a rebuild if possible. Is there anything I can inspect in this tranny to give me an idea of its condition?

:idea: How about if I hook up a drill chuck to the input shaft and spin it in each gear, is this helpful in any way?


As always, thanks for the help and advise.

how about you pull the cover and see what it's history REALLY is!!! :laughing:

You can spin it by hand. Look at the Main gears, and what synchros you can see. You wont be able to see the synchro between first and second.

ITs best to take it apart and inspect it. Or examine the main gears and go

I've been down this road, I bought a used SM420 from behind my buddies trans shop, it had been sitting there since he bought the place several years ago. I tore it down to swap out the main shaft for my new Novak one and found that the large gear on the counter shaft had some hefty chunks missing on on side of the teeth from aparent PTO use/abuse the damage didnt' seem to effect the operation of the trans, so I cleaned every thing up and put it together with new brass syncro's and new bearings. Well it turns out I have a three speed SM420, creeper, 3rd, and 4th, no 2nd. The 2nd gear syncro is the biggest problem with these trannies, make sure you pop it apart and check it out. I've since picked up three other trannies that were all cracked and I ended up using the counter and 1st gear/2nd gear syncro assembly out of one of them, it's not all back together yet so I don't know if it's going to work or not but defineately tear it down and check it out, they are super easy to work on.

Pick it up, hold it by your ear and shake it.
if it rattles, that's bad.
:flipoff2:

If you can hold an SM420 up by your hear and shake it I don't want to piss you off.

Originally posted by WheelingPiazza

ITs best to take it apart and inspect it. Or examine the main gears and go


By 'take it apart' do you mean take the top off or disassemble it?


I will try all the things mentions so far (except shacking it and listening for rattles).

I've always heard that these trannys have a will to live nd they seemingly never need rebuilt. Hopefully mine is the same.


:cool:

Keep in mind that an SM420 of unknown origin could have come from anything from a 2wd 1/2 ton pickup to a 2-1/2 ton dump truck and anything inbetween, including wreckers and other PTO using applications, many of these trannies have lead long had lives and the newest ones available are still 35 years old, nothing lasts forever. Clean up the outside of the case real well and inspect it for cracks, if you're in a cold climate and that tranny has ever had water in it it may be cracked, I suspect that is what happened to all of my parts trannies, confined water + Michigan winter = case on crack......er.....crack on case.

Originally posted by liveaxle



By 'take it apart' do you mean take the top off or disassemble it?


I will try all the things mentions so far (except shacking it and listening for rattles).

I've always heard that these trannys have a will to live nd they seemingly never need rebuilt. Hopefully mine is the same.


:cool:

Take the PTO cover off too and look at the magnet on the bottom of the case. If you see a bunch of chunks or flakes of metal as opposed to fine powder, it might be worth tearing it down for a closer look.

I would also recommend replacing the 3rd and 4th gear synchro rings at a minimum. They are cheap ($10-15 each) and a complete rebuild kit is about $125. it is not hard to do and no special tools are required.

Hey Pin Head, my 420 feels like it doesn't want to fully get into third, and when it is there it pops into neutral as soon as I get out of the gas. Syncro problem, or rebuild time:confused:

Originally posted by Mustard Dog
Hey Pin Head, it's 420 and I'm a pig,feels like I can't fully get to third with the old lady, and when it is there it pops into neutral as soon as I get out of the gas. Syncro problem, or rebuild time:confused:

Damn, MD...:eek:

You might condsider some of the products made by the Swiss.:confused:

Originally posted by fatkid


You might condsider some of the products made by the Swiss.:confused:

That's not my bag baby:flipoff2: :flipoff2:

One reciept for a Sweedish made Penis pump made out to Mustard Dog.:eek:

Hey Pin Head, my 420 feels like it doesn't want to fully get into third, and when it is there it pops into neutral as soon as I get out of the gas. Syncro problem, or rebuild time

465 has similar issues.
difficulty into gear screams syncro rings.
the popping issue sounds like a thrust bearing.
in either case, you gotta rip it all down

If you can hold an SM420 up by your hear and shake it I don't want to piss you off.

what? you can't hold up a 200lb tranny?!?
:D
( I think I could, but I'd be on medication for the next month afterwards. and I'd have to find my left nut after it shot out the leg of my pants)

Originally posted by Mustard Dog
Hey Pin Head, my 420 feels like it doesn't want to fully get into third, and when it is there it pops into neutral as soon as I get out of the gas. Syncro problem, or rebuild time:confused:

Hey Eric,
Yeah it is a synchro problem, but not one that you can fix by just replacing the rings. Check that the shift fork isn't worn or grooved out first. If this tranny has been apart before this started happening, it is possible that the sychro clutch gear hub was installed backwards. If it is a problem with wear on the hub or the 3rd gear dog teeth or its bushing, it is probably easier to just find another good 420 at the pick n pull in Wilmington. "Stick Only" transmission part up in Sun Valley is the goto place for parts.

As for the problem that F-K identified, there is always Viagra. It is a Pfizer product. Just ask Nolen about it; he is a Pfizer rep. :D

420s only weigh 130. As far as holding it up and shaking it I am pretty sure I could accomplish that but I am also pretty sure that it wouldn't be the most graceful of manuvers!! You should do like I did. I had my 420 completely rebuilt, new seals, bearings, and gears!!! all for 350 and done by this old guy who has been working on granny 4 speeds for more than 50 years. He also happened to show me why they usually slip out of gear but I cant rmember what he said. It is wear on some part in there but he told me along time ago and I cant remember what he said. He also drilled some extra holes in the case and made a few slight modifications that he said would make the synchros last longer due to better oiling. Not sure what he did but I might stop by and ask him so I can post it here. This guy has worked on all the G4spds in our club and they all work great.