PRO SOURCE Air Engraving Pen: Steel Housing, 3,800 BPM, 90 psi, 1/4 NPT Inlet, 59" Long Hose, 6.2 bar Air Pressure. Lightweight for easy operation. On/off power control. Piston operation for trouble-free use. Writes on materials up to a 60 Rc hardness. Air inlet 1/4" NPT; hose size: 1/4"; 3,800 BPM; 90 psi.