Abstract: An improved bidirectional hydraulic jar. The jar comprises a two-ring valve assembly supported preferably on the inner mandrel for reciprocal movement inside a fluid chamber. The chamber comprises two larger portions joined by a narrowed restrictor portion. The valve obstructs passage of fluid through the restrictor portion except for a bleed passage, which comprises an adjustable metering space between the two valve rings. The jarring force can be adjusted by varying the size of the metering space, which is done simply by axially repositioning one of the valve rings. All impact surfaces are enclosed in the housing to prevent downhole debris from dampening the blows. The shoulder on the exposed end of the mandrel is champfered to prevent build-up of debris and to facilitate removal of the tool from the well. This jar can be re-cocked easily, without firing in the reverse direction, for unidirectional jarring operations.