This beefy double o-ring piston head is machined from duralumin EN AW 6012 T6.
Nine holes are drilled on the front side to allow the negative pressure to quickly equalize, reducing friction on the rear stroke, and helps expand the o-ring on the forward stroke. 
 
Advantages:
Perfect sealing
Axial ball bearing for easy spring rotation
Sleek surface
Plastic nut that reduces total weight