This was one of my primary reasons for tearing the old truck apart. When I purchased the truck, I was told the engine had been rebuilt and it was also rather apparent given the “relative” cleanliness. But it appeared like the SM-420 4-speed transmission had not been touched recently, if ever. It was completely coated in oil/grease/dirt, and I knew it was also leaking because I had to add oil – not to mention the puddles on the floor.
Here’s what it looked like on the garage floor immediately after removal.

In doing my research, I learned that these SM-420 transmissions were used in a range of applications, and my little 1/2-ton truck was one of the least severe applications. The only functionality issue I really had was a tendency to slip out of 3rd gear when I backed off the throttle. This is apparently a common trait for these transmissions, and not necessarily solvable.

The rebuild kit included all new bearings, synchronizers, and gaskets. All the primary gears were in good enough condition for reuse – amazing after 70 years! Here are a few pictures taken during disassembly. Notice the large amount of sludge sitting in the bottom of the case.

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Here are a few pictures I took during the reassembly process.

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And here’s the final product, painted in the OEM color and then reinstalled.

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