By and large the biggest rotational mass in this transmission is the giant first gear. It travels continually at the speed of the main shaft regardless of the speed of the truck or what gear it is in.
That said, a vibration could be caused by excessive clearance between the first gear and the main shaft. There is a check on this that requires the disassembly of the main shaft. It is measuring the rock of the first gear to be no more than .015.
Good time to inspect the friction ring and neoprene strip inside the diameter of the first gear for wear. Also, I have seen the second gear syncronizer ring retainer broken on a few transmissions that would cause the second gear synchronizer ring to flop around on the inside of the first gear, that could cause a nice noise and vibration too.
Steve
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Last edited by sstock; 02/17/2020 3:38 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