The Stovebolt Site Forums The Parking Lot Big Bolts rebuilding sm420

hi guys, well my tranny was mighty tired and I decided to have it rebuilt. I contacted 2 truck restorers who were recomended. Both said it was easy and they would NOT do it :-( Really surprised me. Contacted 2 other places and they said about 800 to 1,000 bucks!
Called around some more and had a bunch more folks tell me its easy - so I bought a rebuild kit from Novacs - spelling, my instructions are still in the garage,and I am giving it a go. This is the heavyest stuff I have ever worked on. So far though its going ok, the kit was almost 200 bucks - I am ok with that, its seems ok and the instructions are fairly good, but could be better for some one on their first try.
Funny things so far - most of my tools are not big enough, usually working on motorcycles - not 2 ton truck parts. I needed bigger hammers,pullers, presses, punches - my standard tools looked like toys when trying to work on this stuff.I bought a book when I first got the truck - how to restore your Chevy truck. I turn to transmission section and it says its to complicated for the normal shade tree mechanic :-( funny.
Then today was really something, I am abit of a worry wort and really trying to do it correct and not screw up so that I can get my truck back up. Its my newest muti wheeled vehichle. Well I had a couple questions and just wanted to check with someone who knows. So as per the Novac instructions I called their "techs" ask them some of the things I was wondering about - the guy says , I dont know, we dont rebuild them here and I have never rebuilt a tranny!!! I was just flabergasted, the guy was nice but seemed to know less than me. He told me they have had trouble with people not knowing how to rebuild them and maybe they should not even sell the kits for the sm420s! After my shock I thought it was quite funny.
I bought a digetal camera and am taking pictures as I go and I think I am ok - plus I know some folks who could assist if needed, much more experience and skill. But thought you would get a kick out of my experience so far. Anyways cross your fingers for me and I will keep you posted, Tom.

I'm wandering into the Big bolt section, couldn't resist when I saw the thread on the sm420 since I have one in my GMC 3/4 ton.

Check out this site if you haven't already. Several pictures and commentary of sm420 overhaul and 9 page overhaul manual download available under the first picture.

http://members.cox.net/cglabe1/SM420/sm420.html

Good luck on your project. I'm sure there are others here who have actually worked on them and will be glad to help.

When I decided to restore my truck I invested in repro shop manual and parts catalog. They are reasonably priced especially after they save you a lot of headaches. The shop manual has a chapter on the transmissions where I rebuilt my SM420 and it works great, considering it still feels "industrial". My truck is done and both books are dog eared.

The best thing to do is have a lot of space and lay everything out the way it goes, as it comes out. Its pretty simple and youll love it when you put it back in and it works! Just make sure everything is clean, clean, clean!

Love your coe by the way! Your carb adapter is pretty freakin cool!

Jeff

sm420 is just like any standard xmission, not a big deal. The one thing thats gonna puzzle you though is when you pull the input shaft, all the little pocket needle bearings will fall out. thats normal. to put it back together if i remember right there are 14 of them? just pack them in the hole with grease and it will go right in... The Input shaft also has a cut on the bottom of it that has to line up just right and cocked a tad to get it out over the countershaft. Otherwise just a transmission, none of the bearings are even pressed that hard...can rebuild it with just standard tools and a small persuader... hehehe.

Tom,

Go to our Links page. Look in "Parts" under "Transmissions" There's a link there for an article on rebuilding your SM420.

I followed it when I rebuilt mine. I also got a kit from "Stick Only" in California -- They were great and helped me get mine back together (As the guys will tell you, I'm no mechanic! But with their help, even I can rebuild an SM420!)

Stick Only Transmission Parts
9900 Glenoaks Blvd. Suite 2
Sun Valley, CA 91352
(818) 768-4747
(800) 225-8824

I highly recommend these guys.

Have fun!
John

Way to go guys, thanks alot - man I like this site. Great articles thanks and also for the phone numbers, comments and advice,I appreciate it for certain. Jeff, I like your coe too, been looking at your photo's. You should see the looks I get when I show folks my carb adaptor - really funny! :-) and John - I am no mechanic either, but if I keep playing with this stuff, maybe someday. Thanks all, Tom.

Tom
Great looking COE, I too would love too have one like that. It would look great hauling my little Mitsubishi BD-2 dozer.
I am sure you will do fine with your transmission rebuild. I have one that needs both first gears and a rebuild from my 64 sitting on the shop floor now. I will be earmarking all this info for future use. I am no stranger to gearbox rebuilding but never anything with synchros, mostly industrial pump drive boxes.