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#1456384 Sat Jun 25 2022 06:11 PM
Joined: Dec 2020
Posts: 70
I am cleaning my sm420 trany up and installing new gaskets. I saw online that there is an upgrade to the front shaft by Novak that replaces the front input housing, and they grind a new seal area on your own shaft. I have the ability to do this myself, as, I have a universal tool and cutter grinder, a nice Romi lathe and Bridgeport mill. I can grind the shaft between centers, make a new input housing, and install a new lip seal. Has anyone done this upgrade recently? I would need the seal number, and the shaft dimension, that way I can do it myself. This is a 1948 sm420 that is in really great shape. I am also considering a Gear Vender overdrive unit. The truck is a 47 3600 with the 4:56 ratio, but in very good shape too. I found a 4:11 center carrier, but the guy is a little hard to deal with, so an overdrive will be my next choice for better road drivability. Any info will be appreciated.

lumbersawyer #1456385 Sat Jun 25 2022 06:28 PM
Joined: Feb 2004
Posts: 24,752
Kettle Custodian (pot stirrer)
Instead of fabricating a new bearing retainer, why not simply machine the old one to accept a seal? I'd be tempted to clean up the OD of the input shaft to get it round, and press fit a thin machined ring onto the shaft to act as an inner seal surface, also. Bearing and seal manufacturers such as National, Chicago Rawhide, and SKF publish progressive size charts for seals that list the ID and OD, as well as the lip material and design. Just pick one that meets your needs. Since the SM 420 has survived for decades without a front seal, not all modifications are "improvements"- - - -especially if you're not going to encounter extremes of up and downgrades like the rock crawlers do that might let the lube leak out of the front of the housing.

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lumbersawyer #1456421 Sat Jun 25 2022 10:45 PM
Joined: Dec 2020
Posts: 70
Well, I went out just for fun and machined a left-hand internal sleeve to fit on the input shaft. It fit the first time trying the shaft into it...That has happened more than once when I make threads for special nuts. So, I'm not sure what I'm going to do yet, but it was fun to run my lathe and challenge myself. Jerry, the bearing retainer does not look to be thick enough for a seal counter bore. It is only about 1/8" thick between all the dimensions. With the bearing nut and the oil slingers, not much room left to fit a seal in it.

lumbersawyer #1456424 Sun Jun 26 2022 12:07 AM
Joined: Sep 2010
Posts: 1,408
The newer sm420 have a seal you may be able to find the front retainer for a newer one mine is a 67 or 69 transmission ,also has the reverse switch on the top cover the seal is available.

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lumbersawyer #1456773 Wed Jun 29 2022 01:25 AM
Joined: Jul 2014
Posts: 362
I've run an SM420 for 50 years with no seal. It doesn't leak as far as I can tell, even when parked nose-down on a steep hill. Does yours leak now?

51 3800 PU, 55 235 (w/cam, headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
lumbersawyer #1456782 Wed Jun 29 2022 02:03 AM
Joined: Dec 2020
Posts: 70
I have not run the truck yet. The transmission was covered in greasy dirt so I thought it might need some new gaskets and maybe a upgrade to the front seal.

Last edited by lumbersawyer; Wed Jun 29 2022 02:52 AM.
lumbersawyer #1457486 Mon Jul 04 2022 08:09 PM
Joined: Dec 2020
Posts: 70
I ordered a new bearing retainer front for my SM420 From Novak. It came with a standard input seal in it, but mine being an older trany the seal will be replaced with a 1 1/8" bore instead of 1 1/4" that the newer trany must have. I machined the left-hand threads back to get a very nice area for the new seal. So my new seal is 1.750 OD X 1.125 ID X .250

Last edited by lumbersawyer; Mon Jul 04 2022 08:10 PM.

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