These Rocket piston heads are made from POM. The O-rings in these are of good quality with very clean seams. A good seam is very important to a consistent seal in with a piston head.
Material: POMThrust bearing: YesBacker washer material: AluminumMass w/bearing: 8.99gramsMass w/o bearing: 5.33gramsThickness: 0.308 in (7.83mm)