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Rebuilding a sm420 transmission
#1351338 Mon Mar 23 2020 09:37 PM
Joined: Mar 2018
Posts: 141
S
Shop Shark
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?

Re: Rebuilding a sm420 transmission
Snowman51 #1351345 Mon Mar 23 2020 10:16 PM
Joined: May 2017
Posts: 94
S
Shop Shark
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

Last edited by sack; Mon Mar 23 2020 10:31 PM. Reason: parts location
Re: Rebuilding a sm420 transmission
Snowman51 #1351349 Mon Mar 23 2020 10:39 PM
Joined: Nov 2000
Posts: 2,017
F
Shop Shark


Professional Novice
Re: Rebuilding a sm420 transmission
Snowman51 #1382038 Sat Oct 24 2020 11:33 AM
Joined: Feb 2012
Posts: 36
S
Wrench Fetcher
Tim Tygarts is no longer in business. Apparently now doing business as "Chevs of the 40's"

Re: Rebuilding a sm420 transmission
Snowman51 #1382047 Sat Oct 24 2020 01:23 PM
Joined: Sep 2006
Posts: 3,773
B
Shop Shark
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.

Re: Rebuilding a sm420 transmission
Snowman51 #1382176 Sun Oct 25 2020 09:03 PM
Joined: Mar 2019
Posts: 1,148
G
Shop Shark
Truth of the matter is sm420s last fot years with "no" care. They last forever with "normal care". nono

Re: Rebuilding a sm420 transmission
Snowman51 #1382214 Mon Oct 26 2020 12:26 AM
Joined: Jan 2008
Posts: 4,698
C
Shop Shark
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?

Re: Rebuilding a sm420 transmission
Snowman51 #1382219 Mon Oct 26 2020 02:11 AM
Joined: Sep 2011
Posts: 806
B
Shop Shark
[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?


BC
1960 Chevy C10 driver 235 T5 3.73 dana 44
1949 GMC 250 project in waiting
1960 C60 pasture art
G.M dealer tech since 1980 & counting
Re: Rebuilding a sm420 transmission
Snowman51 #1382743 Thu Oct 29 2020 10:27 PM
Joined: Mar 2018
Posts: 141
S
Shop Shark
Clutch is engaged. Vibration/noise seems to stop if I put in the clutch.

Re: Rebuilding a sm420 transmission
Snowman51 #1382744 Thu Oct 29 2020 10:28 PM
Joined: Mar 2018
Posts: 141
S
Shop Shark
Foot off accelerator.

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