SM420 Transmission Rebuild Walk through

This is a walk through of a disassembled SM420 transmission. I am not a pro, and this is my first transmission rebuild. I saw posts from others asking for SM420 videos so I wanted to help folks by documenting how it went. I like to see how something is done when I'm learning. The video doesn't cover the removal of transmission. You should have wrenches, snap ring pliers, gear pullers, bearing pullers, a pin spanner wrench, and a shop press or arbor press. If you have these tools, it won't be too hard. I cover a couple of the issues I ran into as well.
I have other videos in my channel that detail specific parts of the transmission, as well as other sub assemblies found on 47-54 Advanced Design Chevy/GMC trucks.

[@jamessellards2900]

Thank you for taking the time to do this video.

[@paulstanley31]

This is a big help. I was cleaning up my transmission and found a broken synchro ring for 1st to 2nd. I have a donor trany and got the clutch gear out of the donor and heard the tinkle of the roller bearings going into the bottom of the case. Kind of a surprise.

[@theworkshop3706]

Hi Paul, glad it helped! Yes - that's a scary noise when you hear all those needle bearings fall. It's like "Uh-oh, I'm committed to rebuilding this thing now!"

[@douglasferguson4711]

Great video. I rebuilt mine a year ago and couldn’t find anything either. Hopefully this video shows someone on the fence that it really isn’t that hard.

[@NipkowDisk]

The SM420 was one of the very best transmissions ever built. Period. Too bad they haven't been made in 53 years... It would be great to see more aftermarket replacement parts such as cluster gears, etc. but I'm just dreaming out loud :)

[@theworkshop3706]

Agreed - the thing is so simple and solid. It would be cool if someone made aftermarket gears and stuff, but it's probably expensive and not profitable.

[@NipkowDisk]

@@theworkshop3706 Yeah, it's really too bad. Especially with modern metallurgy, CNC machining and so forth- these could last indefinitely.

[@loganshotrod4x464]

I rebuilt an SM465 & they’re cool... but now I got the itch to find an SM420 just for fun

[@jeepcj3b436]

Nice job. Thanks

[@theworkshop3706]

Thanks, hope it was helpful! I have more shorter videos showing specific sub assemblies too.

[@joelgauthier8792]

Hey, going through a SM420 now. Are the replacement bearings for the input shaft 6209N and is ti same for output? Great video!

[@theworkshop3706]

Thanks! I'm sorry but I forgot all the part numbers. I got all the parts in a kit from Novak Adapt

[@Schetmesa]

Thank you

[@theworkshop3706]

No problem, thanks for the comment! I have shorter videos showing specific sub assemblies too.

[@christopherday7346]

what manual are you useing, ive been looking for one, awesome video

[@theworkshop3706]

Hey, I'm using this manual: www.oldchevytrucks.com/cart/product.asp?prodid=LI111&i=19502||||| Most of the classic truck sites sell them. I'm glad you liked my video, thanks!

[@daveinidaho7970]

Good video. What is the part number of the kit you refer to?

[@theworkshop3706]

Thanks! I got my kit here: www.novak-adapt.com/catalog/transmission/transmission-parts/sm420-parts/
I have the '47-53 version which is part# 420MR-1

[@daveinidaho7970]

@@theworkshop3706 thanks. I have one in 59 3200!

[@josephg41]

I have a question on the shifter itself. What is that rod for that comes out of the right side of the shifter? I have an SM420 out of a 1954 Cab over engine truck and I'm putting it into a 1956 3600 and I'm wondering what that's for. Thanks.

[@theworkshop3706]

On mine, it's the shaft for the parking brake lever, if we're talking about the same thing. Some trucks use a floor pedal to the left of the clutch for the parking brake.

[@josephg41]

@@theworkshop3706 OK thanks for the info, that's what I suspected it was.

[@russellmiller2801]

165 pounds assembled