Abstract: A down hole hammer comprising a substantially cylindrical casing having a top sub at one end and a drill bit support at the other; a central passageway extending through the top sub, casing and drill bit support; a central tube coaxial with the casing and extending from the bottom of the top sub to the top of the bit support thereby defining an annular space between its outer surface and the internal face of the casing, its inner bore forming part of the central passageway; an annular piston reciprocatively accommodated within the annular space for movement between an impact position at which said piston abuts the drill bit support, and a raised position at which said piston is spaced from the drill bit support; fluid porting being provieded to said annular space to effect the reciprocation of the piston between the raised and impact positions, the exhaust from the annular space during reciprocation of the piston being exhausted into the central passageway; a fluid by-pass extending between the top sub and th