If you have stripped or ruined the sump plug thread in your gearbox, you have 2 easy alternatives. You can have a helicoil fitted or use this self cutting taper thread plug which must only be tightened enough without use of a washer to hold in the oil safely.
Fitting instructions:
This tapered plug must be fitted by using it as an engineering tap. Half a turn in and half a turn out again so it cuts its own thread. The plug will not go all the way in and should stop short of the hexagonal head. Therefore no washer is required. Never overtighten the bolt as it will crack the alloy casing.