FAA Aviation Mechanic General (AMG)Aircraft Drawings, Weight and BalanceMedium
An engineering drawing specifies a hole diameter as 0.250 +0.005/–0.000 inch. What does this tolerance callout mean?
- AThe hole may be machined between 0.250 and 0.255 inch, but not smaller than 0.250
- BThe hole must be exactly 0.250 inch with no variation allowed
- CThe hole may be machined between 0.245 and 0.255 inch
- DThe hole may be machined between 0.250 and 0.245 inch, but not larger than 0.250
Show answer & explanationAnswer & explanation
Correct answer: A. The hole may be machined between 0.250 and 0.255 inch, but not smaller than 0.250
A unilateral tolerance of +0.005/–0.000 means the dimension can only vary in the positive direction, so the acceptable range is from the nominal 0.250 inch up to 0.255 inch, with no undersize allowed. This is common for hole sizes where undersizing could prevent fastener insertion.
Why the other options are wrong
- B. The +0.005 allowance shows some variation is permitted, so it is not an exact dimension.
- C. This describes a bilateral tolerance, but the –0.000 shows no undersize is permitted.
- D. This reverses the tolerance direction; the callout allows oversize, not undersize.
Unilateral Tolerance
A tolerance that allows variation in only one direction from the basic dimension, expressed as plus or minus but not both.
- +0.005/-0.000 allows oversize only
- Basic dimension is the minimum acceptable size in this example
- Used when undersizing would cause a fit or function problem
Memory trick: Unilateral = 'one-way street' for size variation.