A piston's stroke length may be restricted by passing a rod through a through hole in the piston. The stroke length's boundaries may be defined by the points where an interior of the through hole contacts an exterior of the rod. Adjusting a position or orientation of the rod may alter this stroke length. If the rod comprises a noncylindrical external geometry, a radius thereof may vary along an axial length of the rod or around a circumference thereof. Adjustment of the rod, via axial translation or rotation for example, may change the position of contact between the rod and the through hole. Alternately, the through hole may comprise a unique geometry in which the rod may radially translate to adjust the piston's stroke length.