have downwardly extending sleeves respectively designated as 72 90 92 94 96 and 98 the sleeves are slidably received in seating rings designated as 73 106 108 110 112 and 114 which are each connected to the casing by means of a plurality of hollow screws 67 one of which is shown communicating drilling fluid through the casing 18 between each piston and flange referring to fig1 d a detailed description of the piston and seating rings will be given only for the first stage piston 121 the arrangement and function of each piston stage is identical it is also to be noted that in the preferred embodiment the casing is described as a unitary member however it may be constructed in segments each segment housing a single piston with a seating ring forming an integral part of the casing segment seating ring 73 is held in position by hollow screws 67 one of which is shown above anvil 26 the seating ring 73 seals the casing 18 by means of a unidirectional lip seal 74 which is fitted in annular groove 75 due to the hydrostatic balance a bidirectional seal might also be used each screw 67 has a passageway or orifice 67a communicating from outside the casing 18 to a chamber annulus 79 formed between the.