Ed's comment about flywheel balance rings quite true. I recently changed the drive connections for a Sunset P5 boxcab from tubing to solid brass couplings between the gearbox shafts. These couplings are approx. 5/16" in diameter and 3/4" long with two set screws. I mounted them in plane with the set screws lined up for ease of maintenance. The driveline wound up creating vibration because of the tiny amount of missing brass in the screw holes. Once I rotated one of the couplings 180° on the shaft, the vibration disappeared. Who knew?
