This is the Piston for the OS Max 108FSR engine. FEATURES: Aluminum construction INCLUDES: One Piston REQUIRES: None, item is a stock part SPECS: Outside Diameter: 29mm (1.14") Inside Diameter: 27mm (1.06") Depth: 28mm (1.10") Piston ring groove: 1.3mm (.05") COMMENTS: If the piston ring is needed, order OSMG7812. Piston Ring Groove features a "locating" pin to lock the Piston Ring in place. When installed, the hole in the side of the piston should be facing away from the exhaust port. NNA 2/07/02