FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesHard
A mechanic is installing an AN bolt through a fitting requiring a grip length of 0.75 inch, but the shortest available bolt with that shank diameter has a grip length of 0.625 inch. What is the correct procedure to complete this installation?
- ADrill the hole oversize to compensate for the shorter bolt grip
- BAdd one or more washers under the nut to make up the difference in grip length
- CInstall the bolt without washers and torque to the maximum allowable value
- DSelect a bolt with a longer grip length so threads extend well into the bearing surface
Show answer & explanationAnswer & explanation
Correct answer: B. Add one or more washers under the nut to make up the difference in grip length
When an exact-grip-length bolt is unavailable, standard practice allows the use of one or more washers (typically up to three) under the nut to make up the difference, ensuring the bolt shank—not the threads—bears within the clamped material. Bolt threads should never bear within the joint's grip area.
Why the other options are wrong
- A. Drilling the hole oversize would weaken the joint and doesn't address the grip length mismatch.
- C. Installing without proper grip length compensation would allow bolt threads to bear in the joint, risking shear damage.
- D. A longer-grip bolt would place threads within the bearing surface, which is not acceptable.
Bolt Grip Length Selection
The grip length of a bolt should closely match the thickness of the material being joined so that the smooth shank—not the threads—bears within the joint; washers may be added to compensate for slight mismatches.
- Grip length = unthreaded shank length of bolt
- Should match or slightly exceed material thickness
- Washers can be added under the nut to fill gaps (typically up to 3)
- Threads must never bear within the clamped material
Memory trick: "Shank in the stack, threads stay back."