Hi, The A plug has a 3/4 inch hex and is washer mounted. The F plug is for a tapered seat and has a 5/8 inch hex. Putting a tapered plug into a cylinder head that is not designed for it has been done but it's not the best thing to do. There's a lot of stess on the head and the seal is not so good. Try some other plugs and see what fits. Hopefully the seat will match the cylinder. Dan.