New polymer model with Double oring This piston head has ports on the front to push out the o-ring as it moves forwards, this ensures that all of the air in the cylinder is being pushed out to propel the BB. This piston head allows to adjust the length of the piston of 1-2-3 or 4 mmand it can also be used without shims. This piston head is the ligthest of our production and improves the performance of your aeg, double o-ring and ball-bearing. Airbrake piston heads are much like standard piston heads but with the addition of a second 'piston' on the front which acts as a suppressor in the gearbox.