A transmission gear unit (10) for the timing system control in internal combustion engines comprises two gears (11, 12) interconnected with each other by at least one gearing chain (13). A removable insert (14) is interposed between the gears (11, 12), which insert (14) has on one side thereof, one element (19) for fitting between at least two teeth of the first gear (11) and on its opposite side, at least two elements (16, 17) each fitting between two teeth of the second gear (12). In this way the insert (14) keeps the chain (13) stretched over the gears and prevents the gears from mutually rotating. The insert (14) is provided with a grip handle (18) for removing it by a movement normal to the plane of the gears thereby releasing the rotation of said gears after they have been set in place.