Answer:
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Step-by-step explanation:
The base drum diameter is the diameter of the brake drum when it is brand new and has not been worn down by use. When using a brake micrometer to measure a brake drum, the instrument is set to a base drum diameter of 10 inches plus four notches. This means that the micrometer has been calibrated to measure brake drums with a diameter of 10 inches plus an additional four notches. The notches are used to account for wear on the brake drum and allow for accurate measurements even if the drum has been worn down from use.