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SM420 question
#1254472 Sat Feb 10 2018 11:28 PM
Joined: Jan 2013
Posts: 1,662
S
sstock Offline OP
Shop Shark
Gentlemen, I just finished rebuilding my 3sp in my 53, it was the original tranny. I put new bearings and synchros in it and a fresh first gear. The trans really performs well with no issues and it wasn't at all difficult to do but this isn't the first trans I have been in either but it had been many years since I was in a trans. Now the question, I picked up a really nice clean looking sm420 at a swap meet and got it for $25. The owner said it came out of his 54 and worked great. I figured I would put new bearings and sychos in her and seal it up. I pulled the top and it looks really nice inside, all the gears look good, no wear at all that I can see, then the pto cover came off and the counter gear and reverse idler look great too. Decided to take the rear bearing cover off and tipped the trans back to remove a little 90 weight left in her and noticed the main rear bearing slid out and the counter gear rear bearing did the same dumping out the roller bearing out of the input shaft. No biggy on that but I'm puzzled that it slid a part this easy. Is this odd? IBTW this trans has seen only 1/2 ton duty. I am amazed at how stout this sm420 is, it looks completely indestructible.
Thanks in advance,
Steve

Last edited by sstock; Sat Feb 10 2018 11:29 PM.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: SM420 question
sstock #1254892 Tue Feb 13 2018 09:46 PM
Joined: Apr 2010
Posts: 48
R
Wrench Fetcher
Yep, I did the same thing. I took the back cover off and was turning the shaft and looking at stuff and out it slid. No big deal to put it back together. I got every part I needed except for the 3-4 synchronizer at American Powertrain Warehouse. The gaskets that come with the kit aren't that great so I used the Fel-Pro version. I'm not even a transmission guy but I found the SM420 to be easy to rebuild.

Last edited by Redline; Tue Feb 13 2018 10:08 PM.

Don't pay no mind to that whirrin' sound, she uses a little oil but outsida' that she's cherry.
Re: SM420 question
sstock #1256376 Sun Feb 25 2018 04:52 PM
Joined: Jan 2013
Posts: 1,662
S
sstock Offline OP
Shop Shark
Thanks Redline. Just buttoned up my rebuild too and found it pretty straight forward. It even miked out to .012 between the thrust washer and 2nd gear after it was assembled. I'll be dropping it in soon for the road test, it bench tested nicely. A buddy of mine that is ready to go through his M22 cant believe the size of the internal components. I explained to him that the same trans could be found in the 1/2 tons all the way up to the 2 top cabovers. Would recommend any one doing one of these to have some lock ring pliers and a press.


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: SM420 question
sstock #1256396 Sun Feb 25 2018 07:42 PM
Joined: Mar 2004
Posts: 4,628
M
Shop Shark
The SM420's we're put in the 2-1/2 ton trucks as well!

Mike B smile


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