Superior Parts SP EA0138-2 Aftermarket Piston Driver Assembly for FinishPro 41XP Two Groove Piston (identical to OEM) without O-Ring Replaces EA0138-2. This is a generic replacement driver without o-ring for the Senco FinishPro 41XP Finish Nailer. 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.