The guard pin and fork horns are essential components for providing safe action of the escapement when the watch receives a strong shock. The escapement will function normally without them in a stable environment. In most modern watches the guard pin and fork horns are not adjustable, but their function is essential and can be affected by damage, wear, or the installation of replacement parts and therefore must be checked.
Guard pin clearance must be smaller than the clearance of the fork horns at their extremity (the point where the impulse pin first passes the horns when returning to the dead point); otherwise, the impulse pin may not be able to enter the fork notch during the moment when a shock has forced the guard pin against the roller. Guard pin shake should be equal on both sides of the dead point (i.e., for both the ascending and descending supplementary arcs of the balance wheel) and is measured at multiple locations on the safety roller.
Horn clearance must be equal on both sides of the dead point and is measured at the point where the impulse pin passes the fork horns. Horn clearance must be greater than guard pin clearance, present along the entire length of the horn, and less than total lock. The tooth of the escape wheel must not transition from the locking face to the impulse face when the fork horn is pressed against the impulse pin.