Patent ID: 7322501

Claim:
A fastener driving tool comprising: an outer frame portion; a cylinder disposed in the outer frame portion, wherein the cylinder having a larger inner diameter portion and a small inner diameter portion formed downward from the large diameter portion; a piston which is capable of being reciprocated in an up and down direction in the cylinder between an upper dead center and a lower dead center, and including a groove in a ring-like shape at an outer periphery thereof, the groove having an upper surface, a lower surface and a side surface; an O-ring provided in the groove to be slidable on an inner periphery of the cylinder; a driver blade extended downward from the piston; a bumper brought into contact with the piston when the piston is disposed at the lower dead center; a returning chamber provided at an outer periphery of the cylinder; a first air path for permitting a space downward from the piston in the cylinder to communicate with the returning chamber; a nose having an injecting portion which is extended downward from the outer frame portion for guiding the driver blade; a second air path formed between the driver blade and the injecting portion when the piston is at the upper dead center so that compressed air in the returning chamber is exhausted through the second air path wherein a fastening margin is provided between an outer periphery of the O-ring and the inner periphery of the cylinder even when the piston is disposed at any position from the upper dead center to the lower dead center positions, and a clearance is provided between an inner periphery of the O-ring and an outer periphery of the groove, wherein even when the piston is disposed at any position between the large diameter portion and the small diameter portion, compressed air from the returning chamber is brought into the clearance to apply a force to the O-ring, thereby bringing the O-ring into contact with the inner periphery of the cylinder and the upper surface of the groove to seal the piston to the inner periphery of the cylinder.