Superior Parts SP EA0138 Senco Aftermarket UNILOK Piston/Driver Assembly for FinishPro 41XP Compatible with FinishPro 41XP 15 Gauge Angle Finish Nailer Heat treated tool steel 6150 Rockwell hardness 50-52 Aerospace grade number 2024 Aluminum piston Mechanically locked piston driver interface Re-designed for a single groove rubber O-Ring