A double acting hydraulic drilling mechanism, i.e. drilling jar, is disclosed for use in a tubular drilling string for applying upward or downward jarring forces for dislodging a "fish" from a well. The jar is adapted to be run into a well on a drill string and connected to a fishing tool in the well bore. This jar comprises outer and inner housing members positioned in telescoping relation for longitudinal movement of one relative to the other and having a hammer member on one and an anvil member on the other. The housing members enclose a chamber adapted to contain a fluid for controlling application of jarring forces. A pair of pistons are positioned in spaced relation for movement of each relative to the other in the hydraulic fluid chamber. The pistons are arranged so that movement of one housing relative to the other in one direction will move the first piston toward the second and movement in the opposite direction will move the second piston toward the first. The chamber between the pistons has an outlet controlled by a suitable valve member. Valve actuating means are provided which are operated upon predetermined relative movement of the housing members in either direction to open the valve to permit flow of fluid from the fluid chamber to substantially eliminate the resistance to further movement of the housing members and thus permit movement of a hammer member into engagement with an anvil member with an impact force. An alternate embodiment utilizes one or both pistons in a dash-pot action in lieu of the valve member permitting fluid flow from the fluid chamber.
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