The Stovebolt Site Forums The Shop Area Driveline Rebuilding a sm420 transmission

I need some advice on rebuilding a sm420 4 speed. I think the pilot bearing may be worn and causing a little “lurching” when driving downhill without using the accelerator. I was told I should rebuild the transmission. I haven’t rebuilt a transmission before and was wondering how hard it will be. Is there anything I should be aware of before/ during the build? What kit should I be looking at?

Very easy transmission to rebuild, get a manual or look on line for free, tells you all you need to know. The bad news some parts are really tough to find. Bearing kit easy synco"s are a bear to find, gears are a tough to find . Input shafts can be found, Also gaskets and seals can be found. Have fun .Pilot bushing can be found easy. Spare parts look for complete transmissions on the bay. That's where I found most of my parts.

https://www.ebay.com/itm/Transmission-Rebuild-Overhaul-Kit-GM-Chevy-SM420-Rockcrawler-BK108/201172915184?epid=686087659&hash=item2ed6d713f0:g:6esAAOSwQItUGN7X

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

Novak supplies kits and parts.

Tim Tygarts is no longer in business. Apparently now doing business as "Chevs of the 40's"

I am unsure why you need to rebuild the transmission for a pilot bearing issue? Is it just because you will pull it out to replace the pilot? SM420’s last for years with just normal care.

Truth of the matter is sm420s last fot years with "no" care. They last forever with "normal care".

I wouldn't waste my time rebuilding a SM420. There are so many good used ones around and most of them sell for practically nothing. That is the reason parts are difficult to find. We ran quite a few SM420s back in the '50s and 60's. The only one I remember failing was a PTO leaked so much oil that it ran dry. We went out and bought a used one like everyone else did back then. I'm not sure what you mean by a little lurching down hill and whatever it is probably wouldn't be fixed by a complete transmission rebuild. What kind of experience does the guy that suggested going through the transmission have?

[quote=Snowman51]I need some advice on rebuilding a sm420 4 speed. I think the pilot bearing may be worn and causing a little “lurching” when driving downhill without using the accelerator. /quote]

So going downhill, foot off the accelerator, is the clutch engaged or released?

Clutch is engaged. Vibration/noise seems to stop if I put in the clutch.

Foot off accelerator.

I'd taker a very close look at the universal joints and the rear end pinion bearings before digging into the transmission. Coasting with the clutch disengaged takes the load off the entire drivetrain, including the rear end and driveshaft. An SM-420 is about as bulletproof a transmission as there has ever been. The rock crawler crowd loves 'em because of that super-low ratio granny gear, so the prices are getting a little steep on good condition used transmissions.
Jerry

Thanks for the advice. It has new universal joints and drive shaft. Sounds like I need to check the rear end first. It’s an open driveshaft 3:90 from a ‘57 HO32. The PO put it in and it looks like some farmer welding for the attachment. Could the problem be pinion angle at the rear end?