Abstract: A down hole percussion drill having an inner tube located within an outer tube guiding a ram for reciprocating movement relative to a drill steel. The drill steel and/or a wear bit secured thereto as well as retainers for them are provided with matching disharmonic polygonal profiles for sliding fit under load, free of edge pressure and with full power transmission by the lateral faces. The outer tube top end and a piston therein may have like polygonal connection. The inner tube is rigidly attached in the center of the outer tube by a unitary supporting head into which a coaxial control cylinder projects that has a valve seat for a mushroom valve with peripheral notches at its outer edge and further has an inner control tube with a slidable spring-loaded control sleeve for governing pneumatic fluid through suitable flow links.