Modifying the Pierce "PM-I" 5-port cylinder head for the MGB.

The Pierce PM-I cylinder head is a popular, stock replacement type cylinder head for the MGB. Because it is made from aluminum rather than cast iron, it is less prone to cracking and lighter in weight than the factory head it replaces. To achieve better than stock performance however, the PM-I requires just as much effort and attention to detail as it's cast iron sibling would.

Disassembly of the head will reveal the port short turn shaping and seat detail. We do not believe the factory sharp-edged short turn shaping or two angle seat detail is optimal for proper airflow.

Shown above is our modified inlet port short turn and hand blended intake seats (which now accept larger 1.625" inlet valves). The larger valves and modified port increase the airflow potential of the cylinder head considerably.

The PM-I chambers are similar to the later style OEM heads for depth and volume. Owners of pre-18v blocks should be aware that valve relief notches may be needed in the block if larger than stock valve lifts are planned in conjuction with this cylinder head.

Our modifications to the combustion chamber are minimal, the CNC work done by Pierce leaves little room for additional creativity.

Shown above is the PM-I inlet port from the manifold face. We've found these heads to be pretty generous in size right from the factory, necessitating material removal from only some key locations for proper modification (you can't port what isn't there).

Our fully ported version of the above intake port shows mostly cleanup work and minor reshaping. We focus on the divider shape and short turn detail above all. Getting the air around the bend without trauma is a difficult task at best and the majority of our time is spent dealing with this aspect.

Another view of our modified inlet port from the manifold face reveals the modified port divider form. We've re contoured the divider adjacent each port's short turn for better airflow performance.

In contrast to a factory cast iron head, the PM-I exhaust ports are much more even in cross sectional area throughout, allowing for more efficient gas flow once modified.

Once modified, the PM-I exhaust ports are capable of flowing nearly as well as those of a cast iron head.