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Old August 16th, 2024, 06:32 PM
Prowbar Prowbar is offline
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Join Date: Dec 2020
Location: Friesland, the Netherlands
Truck: 1965 GMC 1500, 478 V6, SM420
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Default Re: SM420 transmission rebuild with upgrades

Quote:
Originally Posted by Jim A View Post
I've had my SM 420 out of my '60 1500 twice for clutch work in 54 years. Seemed even heavier the second time.
It didn't seem to need anything but a little cleanup and fresh oil either time.
It is interesting to see the innards of this super tough old masterpiece.
I hope the ones you've gone over work well for a long time.
Glad to hear they serve you well. Indeed, they are tough and heavy beasts!
The 2 that I've owned needed to be serviced.
The original trans from the truck (this transmission) was extensively abused in its life as a farm truck. The bearings in the trans were completely shot, the magnet was full of metal and the 2nd gear synchronization was almost non existent. The gear lube looked like it had been there for the life of the truck.

The other transmission was fine and shifted great. No issues with the 305 V6. When I installed the 478 V6 the bearings started to make a little noise. Enough to convince me to rebuild the thing.
I've grown to like rebuilding transmissions.

The funny thing about that particular noise is that there are plenty of "quiet" (gear whine not included) SM420's. Both of mine have that annoying grinding noise, and I've serviced basically everything in the driveline to rule anything else out. Think I've found the issue, hopefully mine is nice and quiet from now on.
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James (August 16th, 2024)