Abstract: Recess and protrusion features on a brake pad backing plate and an air disc brake tappet create a unique interface that keys the backing plate for use with the tappet. If the backing plate and/or tappet move during braking, the force applied by the protrusion of the tappet against the inner wall of the recess, or vice versa, can cause stress or damage to the backing plate, tappet, and/or other parts of the braking system. To accommodate such movement, the inner dimensions of the recess can be made greater than the outer dimensions of the protrusion. In one embodiment, the minimum dimensions of the recess are determined from a stack-up of tolerances of various components in the braking system. In another embodiment, maximum dimensions of the recess are determined to avoid overlapping step-down transitions in the backing plate and tappet during braking.