the interface of intermediate casing 45 and extension rod 41 intermediate casing 45 preferably includes an inner bore which receives a portion of inner sleeve 49 and prevents inner sleeve 49 from moving relative to intermediate casing 45 closer to crankshaft 25 extension rod 41 also preferably has an inner bore which receives a portion of inner sleeve 49 which prevents inner sleeve 49 from moving relative to extension rod 41 in the preferred embodiment an extension rod annulus 50 is defined between piston 35 inner sleeve 49 the end of pony rod 39 extending away from crankshaft 25 and the interior of extension rod 41 piston 35 connects to the end of extension rod 41 extending away from rod clamp assembly 45 in the preferred embodiment a plurality of passages 51 extend longitudally through inner sleeve 49 between rod annulus 40 and extension rod annulus 50 around the threaded portion of pony rod 39 so that rod annulus 40 and extension annulus 50 are in fluid communication through rod clamp assembly 43 a piston liner 55 adjoins to an interior surface of cylinder 17 in the preferred embodiment piston liner 55 is in fluid communication with an interior portion of cylinder 17 and thereby defining a pumping chamber of reciprocating pump assembly 11 piston 35 slidingly engages piston liner 55 as piston 35 reciprocates longitudinally toward and away from crankshaft 25 reciprocating piston 35 within piston liner 55 causes the volume of the pumping chamber to increase and decrease as piston 35 reciprocates longitudinally toward and away from crankshaft 25 thereby positively displacing the fluid being pumped.