What is a bilateral tolerance?

• A. A tolerance that allows variation in only one direction from the basic size.
• B. A tolerance that allows equal variation in both directions from the basic size.
• C. A tolerance that fixes the size exactly.
• D. A tolerance that applies to runout.

Answer: (B)

Bilateral tolerance means the permissible size variation from the nominal value can occur in both directions and is centered around the basic size. In practice, this is shown as a plus/minus value, such as a basic size of 10.000 with a tolerance of ±0.05, which allows dimensions from 9.950 to 10.050. This symmetrical allowance ensures parts can vary equally above or below the nominal size, promoting interchangeability when a symmetric fit is desired. It’s different from a unilateral tolerance, which allows variation only in one direction; from a zero tolerance, which fixes the size exactly; and it isn’t specifically about runout, which is a different type of tolerance focused on surface rotation.