Abstract: A bi-directional snap-action register mechanism for an odometer-type mechanical register display. The mechanism includes at least one register display wheel and a bi-directional cam mounted coaxially along a common shaft with the register display wheel. The bi-directional cam engages a helical groove formed along the shaft to allow lateral displacement of the bi-directional cam in a direction parallel to a rotational axis of the shaft. A pin is biased by a spring into contact with the bi-directional cam. Rotation of the shaft causes the bi-directional cam to be moved with respect to the pin. The cam abruptly rotates through a predetermined angular distance when the point of contact of the biased pin with the cam moved from a smooth portion to a step transition portion of the cam. The abrupt rotation of the cam is communicated to the register display wheel to cause a new display position to be moved into view.