I recently bought a SM420 trans out of a 54 1/2 ton. The trans passes the bench test for shifting, the gear oil was black but no water in it. Sounds quiet spinning it at 600 rpm no load in all gears. Before I pop the covers to take a look see, I was planning on dropping the old oil and refilling with ATF and run it with a 1/2 drill motor on its snout to clean the gears and case. Can anyone see why I shouldn't do this? ATF has detergents and I think it will give it a good cleaning this way. If everything looks good, I'll probably run it without the rebuild.I should say after it has cleaned, I will replace with GL1 90 weight
Thanks in advance,
Steve

Last edited by sstock; 01/07/2018 2:41 PM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration