If it is a 14mm thread, which is the newer type smaller thread, you may have yourself a Heisler head there, Take the valve cover off and look at the casting numbers. From the outside the Heisler looks just like an IH head but the giveaway is the smaller spark plugs. Also the one we have has an X cast in between the center 2 cylinders. You are a lucky man if that is what you have. They have bigger intake valves and a smaller combustion chamber to raise compression, comparable to using hi altitude 8000 ft pistons.