the primary piston is at bottom dead center and the crown of the piston is at a distance t u2212 2e 0 from the central axis of the crankshaft segment 8 the range of movement of the crown of the piston is thus equal to 2e 0 now suppose the operating shaft 1 is moved in the direction of the arrow and through the action of the ramp slots 42 and lever 34 the primary rod journal is moved radially outward by a distance e and consequently the eccentricity of the crankshaft changes to e 0 e due to the interaction of the gear rack 50 and gear rack 62 through the pinion 56 the spindle post 60 is displaced axially by a distance e and due to the interaction of the multifaced gear rack 68 and the gear rack 82 through the two sector gear 86 the primary connecting rod carriage 78 p is displaced radially inward by a distance e at top dead center the crown of the primary piston is at a distance t from the central axis of the crankshaft segment 8 and the range of movement of the crown of the primary piston is from t to t u2212 2 e 0 e it will be appreciated that action at the four secondary stations is similar to that at the primary station thus the mechanism shown in fig2 allows the stroke of the pistons to be selectively varied by axial movement of the operating shaft 28 without changing the top dead center position of the pistons it will be appreciated that the invention is not restricted to the particular.