a bidirectional hydraulic drilling jar includes an elongated mandrel telescopingly disposed within a body and defining spaced apart fluid chambers separated by a restricted bore portion of the body . the mandrel is provided with spaced apart positive mechanical seal assemblies engageable with the restricted bore portion to form a hydraulic dashpot to retard movement of the mandrel in each direction over a limited stroke length so that the spring tension or weight of the drill stem may become effective to deliver an impact blow when the seal assemblies move out of sealing engagement with the restriction . back - to - back check valves are arranged to bridge the respective seal assemblies to reduce the length of stroke required to cock and trip the jar in each direction . a separate orifice or flow restricting passage is provided across the seal assemblies so that the retarding effect is independent of coacting seal surfaces between the mandrel and the jar body . rotary driving torque and impacting in the upward direction are absorbed by a replaceable mandrel sleeve member . the positive mechanical seals include a spiral cylindrical seal ring which is backed on its inner diameter by a piston ring type seal member so that a radially expandable substantially zero gap seal is provided between the opposed fluid chambers when the seal assembly is effectively engaged with the bore restriction in the body .