Thanks for the suggestions! Picked up a couple different sized npt plugs from Home Depot to find the right size. For anyone else in the future, that model use 1/2” npt. Plenty on eBay for a few bucks, search for “1/2” pressure release valve”
I was given a 5’ Countyline brush cutter, square back, that has had a rough life. It doesn’t look pretty but works. But due to the previous owner’s abuse of it, there are next to zero identifying labels on it, or the gearbox. We recently lost the vent plug on the gearbox and I have no idea how...