Screw threads, aircraft bolts, studs, screws and nuts, grip length, and torque loading.
Notes
Fasteners — summary notes
Free
Main ideas
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.
Key formulas
Torque ↔ preload
T ≈ K · F · d (K = nut factor, d = dia., F = preload)
Solved examples
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.