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 optimum performance however, the PM-I should be modified just as it's factory 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.

Our modifications to the combustion chamber are minimal because of the CNC work done by Pierce during manufacture.

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 only minor amounts of material removal 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 at 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.