[vector1]

lot of people put a hole in the thermostat. usually one, maybe two 180 deg. apart, up and down as installed, 3/32" is what stewart puts in theirs. if your cap is on the pressure side or outlet from the waterpump you have to hold the pressure of the water from the water pump plus the pressure from the heat buildup, i wouldn't be afraid to use a 20-24# cap myself if everything is in good working order.