My 371xp ran fine for a long time and started showing a crankcase leak. I replaced the bearings and seals and I still was finding an air leak under the oil pump. This little, expensive o-ring seats under the sleeve that is beneath the clutch. It will prevent air from escaping between the crank and sleeve.
Often overlooked on disassemble and not listed in the Husqvarna part diagram. This O-Ring goes in between the PTO side of the crankshaft and the bushing.