I remember when they sent civilian contractors to do an inspection on the entire US Army CH-47C fleet, one of the things they were checking was torque on the spiral bevel gear bolts - my ship failed, quite a few were loose. Had to replace the aft transmission...
The crash referenced by 212man was a different cause though, related to using walnut shell media blasting during the transmission overhaul process - oil passages were being left plugged when things weren't cleaned out properly.
Helicopters are unforgiving of maintenance errors, some more than others.