Double head piston downhole drill

Claim 1

A PERCUSSIVE DRILL APPARATUS COMPRISING: (A) A DRILL BODY INCLUDING A PAIR OF AXIALLY ALIGNED CYLINDERS AND AN INTERPOSED SPACER RING BETWEEN THE ADJACENT ENDS OF THE CYLINDERS; (B) A PISTON HAVING A PAIR OF HEADS SLIDING IN THE RESPECTIVE CYLINDERS AND AN INTERMEDIATE SHANK SLIDING IN SAID SPACER RING; (C) A BIT HAVING AN ANVIL SLIDING IN THE OUTER END OF ONE OF SAID CYLINDERS TO CLOSE THE CYLINDER AND MOVABLE BETWEEN A FULLY RETRACTED POSITION AND A FULLY EXTENDED POSITION IN SAID DRILL BODY; (D) A PLUG IN THE OUTER END OF THE OTHER CYLINDER; (E) SAID PLUG, SPACER RING AND BIT ANVIL COOPERATING WITH SAID PISTON HEADS TO FORM FLUID PRESSURE CHAMBERS AT THE OPPOSITE ENDS OF SAID PISTON HEADS ADAPTED TO RECEIVE PRESSURE FLUID FOR RECIPROCATING SAID PISTON AND CAUSING IT TO IMPACT ON SAID BIT ANVIL; (F) VALVE MEANS FOR ALTERNATELY ADMITTING PRESSURE FLUID TO THE CHAMBERS AT THE OPPOSITE ENDS OF EACH PISTON HEAD; (G) SAID PISTON AND BIT CONTAINING CENTRAL PASSAGES SERVING AS CENTRAL EXHAUST PASSAGES FOR SAID FLUID PRESSURE CHAMBERS; (H) SAID DRILL BODY CONTAINING A SIDE PASSAGE LOCATED OUTSIDE OF SAID CYLINDERS AND SERVING AS A SIDE EXHAUST PASSAGE FOR SAID FLUID PRESSURE CHAMBERS; AND (I) MEANS FOR CLOSING SAID SIDE EXHAUST PASSAGE WHEN SAID BIT IS MOVED TO ITS FULLY EXTENDED POSITION SO THAT ALL THE EXHAUST AIR FLOWS THROUGH THE CENTRAL EXHAUST PASSAGES.