I was wondering how many thermactor plugs are required for the 351w heads; some people say 4 per head?
84 heads will need 1 per head, the thermactor passages are drillled the length of the head from front to back. The front of the passage had a threaded bushing to step down the size of the hole to 7/16NC for the accessory bolts to thread into. The rear of the passage was open and covered by a crossover tube secured by a 5/16 bolt above each hole. So unless someone removed the threaded plug from the frontend, you should only need one plug per head. You can use a short bolt to plug the hole, but I don't recall the thread size and pitch for this. It's something like a 3/4" NC bolt though.
