[Zeke3]

It sounds like the torque you are using is too low. If the bolt is a 5/8"-18 and a grade 8 capscrew, the torque should be in the 235 lb-ft range, reference Craftsman generic torque table.

There is a lot of torsional vibration going through that joint to the crank pulleys. Increasing the grip length of the bolt will get more stretch in the bolt and resistance to loosening. You can increase the length by adding a thick wall spacer.

I do not recommend using a lock washer, it is just another spring in the system.