The Airsoft GI PE POM Polycarbonate Piston is an extra strong poly piston with 8 metal teeth. The second to last tooth on the piston was not molded because it helps reduce stress on the gears. A strong poly-carb piston, like the Airsoft GI PE POM, is beneficial over an aluminum piston because it is light weight, which transfers less energy to the gearbox shell, thus reducing the probability of cracking it.
The use of aftermarket airsoft parts is not an exact science. Some parts may require fitting even thought they were designed to fit without modification. Professional installation of upgrade parts is highly recommended.