An improved drilling rig (10) and conversion apparatus for drilling rigs is disclosed. The rig includes a percussion assembly (32) rotatably mounted to a ram (30) above the ground in which drilling is to be made. The rig (10) includes a conversion mandrel (40) structured for insection in, and retention by, the percussion assembly (32) in place of a normally retained bit element (34). The mandrel (40) has an anvil surface (56) to which percussive strokes of a piston (50) disposed within the percussion assembly (32) for reciprocating movement therein are applied. The mandrel (40) further includes a hammer portion (68) to transfer both the percussive force of the piston (50) and continuous downward pressure applied to the percussion assembly (32) by the ram 30 to a length of casing (20) encircling the drill rod (22) which accomplishes the drilling. The conversion apparatus can, therefore, convert a bottom-hole drilling rig to a top-of-the-hole rig.