Be careful which number you are looking at. My '62 261 has a head that says B51 848 over 850, and the real casting number says 3837850. You want the casting number on the top next to the temperature sending unit that starts with 383. The only differance between the 848 and the 850 heads is the compression and the steam holes, they flow exactly the same and use the same size valves and rocker arms. The combustion chamber and ports are the same shape.

The steam holes are necessary because the 261's have siamesed cylinders and need more water flow to the head to avoid hot spots. It is a good idea to run the bypass thermostat or provide circulation back the the water pump via the heater core when the thermostat is closed to avoid hot spots in the head and block when the water isn't up to temp.

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A lone amateur built the Ark. A large group of professionals built the Titanic.