Superior Parts SP CN81200 Aftermarket Main Piston Unit Fits Max CN55 (CN55A2-19). This is a generic replacement driver with o-ring for the Max CN55 coil nail guns. This comes with a patent pending locking mechanism which will not allow the driver to unscrew from the piston. Aluminum is crimped into the unilok grooves of the shaft to prevent unscrewing. Guaranteed never to unscrew.