Abstract: An inner, generally circular plate secured to the end of an engine cam shaft is connected by means of a linking spider through a toggle linkage to an outer, annular plate driven directly by the engine crankshaft. The drag on the cam shaft during normal operation urges the inner plate toward a normal or "retard" position as no outward bias in the opposite direction is exerted on the spider. However, during selected phases of crankshaft operation, such as during low engine speeds, pressure sufficient to overcome the engine drag is brought to bear against a relatively large surface of the linking spider and biases the spider to an axially extended position. The axial movement of the toggle linkage then causes the inner plate to rotate relative to the outer plate to an "advance" position in which it remains so long as the outward bias remains on the spider. When the outward bias against the spider is eliminated, the engine drag draws the spider back to the normal or "retard" position.