PDI AEP/CMG Piston Head. 

This upgraded piston head is virtually indestructible and works with the original piston head retention nut and screw to replace the stock head in all Tokyo Marui AEP and CMG gearboxes. 
This piston head is ported for superior air pressure equalisation on the reverse stroke and comes with an enlarged o-ring to create an excellent seal. 
 
Loctite 266+ threadlock should be applied to the screw.