The cap closes the chain lubrication tank of the Good Luck chainsaw and keeps the oil from leaking out. It often has a vent channel so that oil flows freely to the pump.
Replace it if the seal has hardened, the thread is stripped, or oil seeps out along the neck. A clogged vent channel leads to weaker lubricant delivery.
It is best to do the work on a cooled engine with the ignition off, and to tighten the fasteners without overtightening, so as not to strip the thread in the body.
