Looks nice, makes me a little misty eyed for the days when I was learning how to run lathe myself. I saw people were asking about threading. It looks to me that if you have any problems with the cap working loose that you have enough wall thickness that you could cut a ring into the ID to match the inner o-ring. That way it would "snap" into the groove and keep it from walking. If you do it don't cut very deep, and go easy with a sharp boring bar ground to match the o-ring and it should still hold the seal.
