Abstract: A mechanical well jar is disclosed having inner and outer tubular members movable longitudinally relative to each other a limited distance for connecting in a pipe string. Annular shoulders on the members engage to limit the relative longitudinal movement of the members. A plurality of laterally spaced, arcuate cam plates are attached to the inner surface of the outer member. Each plate has a laterally extending U-shaped notch opening into the space between the plates and formed with curved outwardly flaring sidewalls. Rollers are mounted on the outer surface of the inner member to extend into the spaces between the cam plates for movement into the U-shaped notches to hold the members from longitudinal movement relative to each other.