Anyone know the thread size on a dipstick base for a 351W. The pan has a large cast bulkhead for the dipstick. I want to plug the hole. I can not get a 1/4 NPT plug into it easily, I before I torque it harder and ruin something, I thought I would ask. I plan on just adding 5 quarts every 3,000 any, so no dipstick is OK with me.