A pump body is pre-compressed by expanding a displacement plug in a cavity to pre-compress a portion of a pump body comprising a piston bore, an inlet bore and an outlet bore spaced from said cavity, and connected in a pump assembly. A fluid pump assembly is made up of a plurality of pump bodies connected side by side between opposing end plates with a plurality of fasteners tightened to compress the pump bodies between the end plates, wherein each pump body comprises a piston bore, an inlet bore, an outlet bore and an expanded displacement plug in a cavity; and wherein the expanded displacement plug applies a pre-compressive force at the cavity on each of the pump bodies.