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.
Thrust bearing: Yes
Backer washer material: Aluminum
Mass w/bearing: 8.99grams
Mass w/o bearing: 5.33grams
Thickness: 0.308 in (7.83mm)