Down-hole hammer

A down hole hammer comprising

• a cylindrical casing having a top sub at one end which is adapted to be mounted to a drill string and a drill bit support at the other end to define a piston chamber,
• said drill bit support being adapted to receive a drill bit, a piston received within the piston chamber for reciprocation between the one end and the other end, said piston being formed with a central portion which is slidingly and sealingly engaged with the internal wall of the casing,
• said central portion providing a first shoulder at the end proximate the top sub,
• said piston having one end portion which is intended to impact on the drill bit on movement of the piston to an impact position and the one end portion being configured to be sealingly engaged with the drill bit support and/or the internal face of the piston chamber when the piston is in the impact position to define a first space between the internal wall of the piston chamber, the one end portion and the drill bit support,
• a portion of the length of the internal wall of the casing adjacent the top sub being provided with two sets of fluid delivery ports at locations spaced axially along the portion of the length of the casing,
• the external radial face of the piston and or the radial face of the piston chamber being formed to enable the piston to cooperate with the internal wall of the casing and with a first set of said fluid ports to admit fluid into the first space when the piston is in the impact position to cause movement of the piston to a raised position at which position the piston is in spaced relation from the drill bit support,
• the external radial face of the piston and or the radial face of the piston chamber being formed to enable the piston to cooperate with the internal wall of the casing and with the second set of said set of ports to admit fluid into a second space between the piston and the top sub when the piston is adjacent the top sub to cause movement of the piston to the impact position, wherein the admission of fluid to the second space is at least reduced as the piston moves to a fully raised position.