Drill and tap a upper tel-tale for a Mercury Mark 58, Use a 21/64 drill, and a 1/8"-27NPT tap, buy a 90 degree, brass nipple at auto parts store, or I bought mine at Mettle Air, part # 139-3A, It will give you a second tel-tale at the top of the cylinders, and also works if the original one is plugged and you can not get it open. Rick