Abstract: Disclosed is a mechanism for use to progressively vary the angular relationship of the cam shaft of an internal combustion engine relative to the crank shaft of this engine. The mechanism comprises a first disk provided with two radially extending cavities aligned with respect to each other, each of the cavities containing a spring-biased, small weight slidably mounted therein with the spring uring said small weight toward the center of the first disk. The first disk is connected to the crank shaft in such as a manner as to be driven by, and rotated at the same speed as this crank shaft. The mechanism also comprises a second disk coaxially mounted with respect to the first disk. The second disk is provided with two arcuated slots of indential curvatures that are symetrical with respect to the center of the second disk and extends from a short distance away from the center of this second disk towards its periphery in opposite directions.