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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
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| | | | Joined: Oct 2012 Posts: 130 'Bolter | | 'Bolter Joined: Oct 2012 Posts: 130 | What wt oil/fluid and make does everyone use in a SM 420 transmission?
Also are there any additives that could be used to improve shifting?
thanks Doug | | | | | Joined: Feb 2004 Posts: 29,746 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,746 | 90 weight straight mineral oil- - - -NOT any of the synthetics, multi-weight lubes like 85/140 or "mouse milk" additives, or anything fancy. The gears in a 420 are heavy and have a lot of inertia, and the super slippery lubes don't let the synchronizer blocker rings get enough traction on the gears to match up speed during shifting and prevent grinding. The 90 weight mineral oil Tractor Supply sells for Ford N-series tractors is the correct lube for the 420 transmission.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | | | | | | Joined: Apr 2008 Posts: 899 'Bolter | | 'Bolter Joined: Apr 2008 Posts: 899 | NAPA also carries 90w mineral oil, part# 65-201. Runs about $35/gal.
Ed
| | | | | Joined: Apr 2008 Posts: 899 'Bolter | | 'Bolter Joined: Apr 2008 Posts: 899 | Jerry, Do you have a part # for Tractor Supply? I can't seem to find it. Tks Ed
Ed
| | | | | Joined: Feb 2019 Posts: 4,472 AD Addict & Tinkerer | | AD Addict & Tinkerer Joined: Feb 2019 Posts: 4,472 |
Phil Moderator, The Engine Shop, Interiors and Project Journals1952 Chevrolet 3100, Three on the Tree, 4:11 torque tube Updated to: ‘59 235 w/hydraulic lifters, 12v w/alternator, HEI, PCV and Power front Disc BrakesProject JournalsStovebolt Gallery Forum | | | | | Joined: Apr 2008 Posts: 899 'Bolter | | 'Bolter Joined: Apr 2008 Posts: 899 | Thanks, guess NAPA is pretty reasonable after all. Ed
Ed
| | | | | Joined: Feb 2016 Posts: 1,982 'Bolter | | 'Bolter Joined: Feb 2016 Posts: 1,982 | I used Valvoline SM transmission lube, got it Qt. bottles at one of the auto chain stores, works great.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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