Check the condition of the gears, ring gears, hubs, couplings and synchronizer blocking rings, internal surfaces and ends of the gear teeth. Check for excessive wear on the locking rings. If the axial clearance between the end face of the blocking ring and the end face of the toothed rim of the gear synchronizer is 0.8 mm or less, replace the rings.
Check the condition of the shafts.
Check the condition of the bearings.