When checking radial play by moving the S-cam up and down, what is the allowable measurement?

The correct answer reflects the precision required in maintaining the S-cam's radial play within a very tight tolerance for optimal performance. When checking for radial play, the movement should demonstrate that the S-cam is secure and functioning correctly without excessive looseness. Allowing movement of "no more than a few hundredths of an inch" ensures that the components are closely fitted, which is crucial for effective brake operation and to prevent premature wear on brake components, such as the brake shoes and drums. This minimal play indicates that the S-cam is properly aligned and adequately engaging with the brake mechanism, thus maintaining safety and reliability in the braking system of a school bus.

Wider tolerances, such as those mentioned in the other choices, would allow excessive movement, which could lead to ineffective braking, increased wear, or potential brake failure. Keeping the radial play consistently within the specified minimal range is essential for ensuring the integrity of the braking system and the safety of the vehicle.