During a brake component check, what crack length in a brake drum or disc is considered a defect serious enough to be flagged under out-of-service criteria?

AAMVA CDL Manual, Section 5, subsections 5.3.1-5.3.2, p. 5-6; FMCSA/CVSA North American Standard Out-of-Service Criteria

A badly cracked brake drum or disc can fail catastrophically under the heat and pressure of hard or repeated braking, especially on a long grade. Knowing the specific defect threshold helps a driver decide during a pre-trip or roadside check whether a crack is cosmetic or a trip-ending problem.