A plate segment for a reciprocating pump power end frame assembly, the power end frame assembly having a pair of end plate segments and at least one middle plate segment disposed between the end plate segments. The plate segment consists of the middle plate segment or one of the pair of end plate segments and includes a plate having a front wall, a rear wall, a top wall, a bottom wall and a pair of sidewalls and at least one opening forming a bearing support surface, the opening extending through the plate. The plate segment further includes at least one extension extending from at least one of the sidewalls of the plate at a position to align with and contact a corresponding extension on an adjacently positioned plate.