Fasteners, Threads, Bolts & Screws
Screw threads, aircraft bolts, studs, screws and nuts, grip length, and torque loading.
Fasteners, summary notes
- Thread terms: pitch (distance between adjacent threads), major/minor diameter, crest, root and thread angle. Series include Unified (UNC coarse, UNF fine) and metric (ISO). Fine threads resist vibration loosening better and allow finer adjustment.
- Aircraft bolts follow AN/MS/NAS standards and are identified by head markings; the callout encodes diameter, length and material. The grip length must match the joint thickness so that only a thread or two shows and the plain shank, not the threads, fills the hole.
- Bolts must be loaded correctly: SHEAR loads act on the plain shank, never on the threads; close-tolerance bolts and Hi-Lok/Hi-Shear pins are used in shear joints. A washer goes under the turning element to spread load and set grip.
- Nuts and studs: plain, castellated (locked with a split pin through a drilled bolt), self-locking (fibre insert or all-metal deformed thread), and anchor/gang nuts; studs are threaded both ends and fixed into a casting.
- Torque converts the wrench moment into bolt preload (clamp force). Use a calibrated torque wrench to the specified dry/lubricated value: over-torque stretches or yields the bolt, under-torque lets the joint fret, fatigue and loosen. Adjust for any extension bar.
- ⚠ Exam trap: bolts are designed so shear acts on the plain shank, with the correct grip length leaving only a thread or two in the joint. Fibre-insert self-locking nuts have a reuse limit, replace them once the insert no longer grips.
- Torque ↔ preload
- T ≈ K · F · d (K = nut factor, d = dia., F = preload)
During assembly a technician finds the bolt threads run right down into the shear plane of a fitting. Why is this wrong?
In a shear joint the load must be carried by the smooth, full-diameter shank. If the threads lie in the shear plane the reduced (minor-diameter) threaded section carries the load and the notch-like thread roots concentrate stress, a fatigue and shear failure risk. The correct grip length puts the shank in the joint with only a thread or two protruding.
Torque & bolt preload
Fasteners concept map
Fasteners, Threads, Bolts & Screws
Fasteners quiz
Fasteners, Threads, Bolts & Screws, quiz
1. A force of 50 N is applied at the end of a spanner 0.3 m long. The torque applied is:
15 N·m167 N·m50 N·m2. A torque of 30 N·m is required. Using a 300 mm torque wrench fitted with a 100 mm extension in line with the handle, the wrench should be set to:
22.5 N·m40 N·m30 N·m3. The pitch of a screw thread is the:
Distance between adjacent threadsDiameter of the boltLength of the bolt4. Compared with a coarse thread, a fine thread:
Resists vibration loosening better and adjusts more finelyIs always weakerCannot be used on aircraft5. In a shear joint, the load should be carried by the:
Plain (unthreaded) shank of the boltThreaded portionSplit pin6. Correct bolt grip length is shown when, through the joint:
Only a thread or two protrudes beyond the nutThe whole thread is inside the jointNo threads are visible at all7. Applying torque to a bolt primarily creates:
Preload (clamp force) in the jointShear in the threadsHeat only8. Over-torquing a bolt can:
Stretch or yield it, weakening the jointAlways make it saferOnly affect the washer