NOCTI Automotive Practice Test

The number cast on a brake drum shows the discard, or maximum, diameter. It’s the largest diameter the drum is allowed to have and still be considered usable. If the drum wears down to that size (or is machined to it), it must be replaced because there’s no safe material left to maintain proper clearance and strength. This isn’t an outer circumference or bolt pattern spec, and the minimum diameter is a different limit used to judge wear, not the cast number you’d read on the part.