How to Measure Brake Drums
Learn how to measure brake drums accurately, including drum diameter, brake surface, overall height, hub bore, PCD, and bolt hole dimensions.
If you need to measure a brake drum, there are six measurements you should take.
Brake surface diameter
the inner diameter of the drum, measured with a brake drum gauge. This is the most critical number.
Bolt circle diameter
the circle that passes through the center of all bolt holes.
Brake surface flatnes
the inner diameter of the drum, measured with a brake drum gauge. This is the most critical number.
Bolt hole count and size
count them and measure their diameter.
Overall depth
from the top of the drum to the bottom of the center bore.
Pilot diameter
the center hole where the drum mounts to the hub.
The most important number on the entire drum is the one stamped into the casting: “MAX DIA”. That’s the maximum allowable diameter. If your gauge reading exceeds that number, the drum is done. No exceptions. No “just one more trip.” It’s like measuring the bottom of your wok and finding it’s worn through. Time to replace it.
Semi Truck Brake Drums: What Every Owner and Fleet Manager Needs to Know
