Abstract: The rotary drilling jar includes inner and outer mandrels releaseably locked against relative longitudinal movement. The inner mandrel is connected to the drilling string and the outer mandrel is connected to the drill bit or other well tools suspended below the jar in the well bore. A detent mechanism releaseably connects the inner and outer mandrels and includes a stack of Belleville springs that is configured to determined the amount of force required to move the detent mechanism to unlock the mandrels to permit relative longitudinal movement. In one aspect, the jar also includes an upwardly facing shoulder on the inner mandrel and outwardly facing shoulder on the outer mandrel that are in engagement when the jar is being lowered into, suspended in, and pulled from the well bore to prevent the imposition of string weight on the detent mechanism.