A hydraulic jar for delivering impacts downhole. The jarring mechanism includes a cup-type piston that moves through a seal bore. The cup is deformable in response to fluid pressure so that it expands each time it moves through the seal bore to compensate for wear on the lip. Since only minimum interference is required between the piston and seal bore, less material may be used. This reduces the force required to reset the tool, which is particularly advantageous in horizontal wells. The seal bore may be slightly wider at the exit; the cup expands to this exit diameter, ensuring an interference fit as the cup travels back through the smaller diameter entry section. The entry end of the seal bore may include longitudinal grooves. This prevents the piston cup from rupturing by preventing a fluid seal around the lip until the most of cup is inside the seal bore.