Abstract: Earth drilling hydraulic shock absorber having an anvil reciprocally mounted in a casing and operatively connected with a plurality of independently operating pistons, the anvil being reciprocally mounted within the casing through a spline connection with unidirectional sealing means for equalizing pressure across the sealing means with the hydrostatic pressure in a borehole for retaining a lubricating fluid around the spline connection and to prevent abrasive matter in drilling fluid from coming in contact with the spline connection. The anvil of the shock absorber may include a reduced effective pressure area so that the anvil engages the first piston at a lower downweight.