📖 The lesson
Screw threads
- Major diameter — the largest diameter (crests of an external thread); the nominal size. Minor (root) diameter — the smallest. Pitch diameter — where thread width equals gap width; it controls fit.
- Pitch — distance from one crest to the next. Threads per inch (TPI) = 1 ÷ pitch. Lead — distance a screw advances in one revolution: equal to the pitch on a single-start thread, twice the pitch on a two-start thread.
- Thread angle — the included angle between flanks; crest, root and flank name the parts of the profile. Right-hand threads tighten clockwise; left-hand threads (marked by grooves or notches) are used where rotation could loosen a right-hand thread, and in turnbuckle barrels.
Thread forms
| Form | Angle | Notes |
|---|---|---|
| Unified — UNC (coarse), UNF (fine), UNEF | 60° | Standard for US aircraft hardware; AN bolts use UNF. UNJ has a rounded root for better fatigue life. |
| ISO metric (M8 × 1.25) | 60° | Size × pitch in millimetres; European and newer aircraft. |
| British Standard Whitworth / Fine (BSW, BSF) | 55° | Older British equipment. |
| British Association (BA) | 47.5° | Small screws in instruments and electrical equipment. |
| Acme (and square, buttress) | 29° | Power transmission — jackscrews and actuators. |
| Pipe threads (NPT) | 60°, tapered 1 in 16 | Seal by the taper — fluid fittings. |
- Classes of fit: 1 (loose), 2 (general purpose) and 3 (close) — A for external, B for internal threads. Aircraft bolts are generally class 3A.
- Measuring: a screw-pitch gauge identifies pitch/TPI; a thread micrometer or the three-wire method measures pitch diameter; go/no-go ring and plug gauges check fit.
Aircraft bolts
- Standards: AN (Army-Navy), MS (Military Standard) and NAS (National Aerospace Standard) part numbers, plus manufacturers' own and metric standards on European aircraft. Aircraft bolts are made from heat-treated alloy steel (usually cadmium-plated), corrosion-resistant steel or aluminium alloy, with rolled threads.
- General-purpose bolts: hex-head AN3 to AN20, drilled-head versions for safety wire.
- Close-tolerance bolts (triangle on the head, e.g. AN173–AN186, NAS): ground shanks for tight-fitting holes where joints see reversed loads; tapped in, not hammered.
- Internal wrenching bolts (MS20004-series, NAS): high-strength, with a hexagonal socket; used with special countersunk washers or chamfered holes under the head.
- Clevis bolts (AN21–AN36): round head with slot or recess, short thread — for shear loads only (control linkages). Eyebolts carry external tension loads (cable and turnbuckle attachment).
- Specialised fasteners: Hi-Lok and Hi-Tigue pins with collars, lockbolts, taper-lok bolts.
Grip length, washers and installation
- The grip length (unthreaded part of the shank) should equal the thickness of the material, so threads do not bear in the hole. Small differences are taken up with washers (AN960 plain; AN970 large-area for wood).
- Enough thread must project: with self-locking nuts, at least the full chamfer (about one to three threads) must protrude through the nut.
- Where the manual does not say otherwise, install bolts with the head upward, forward or outboard, so a lost nut does not let the bolt fall out.
- Torque to the value in the manual or standard tables, on clean, dry threads unless lubrication is specified. Tighten the nut, not the bolt head, wherever possible. With self-locking nuts, add the nut's measured friction drag (prevailing) torque to the table value.
- When an extension lengthens a torque wrench in line: wrench setting = required torque × L ÷ (L + E), where L is the wrench length and E the extension length.
Studs and thread inserts
- A stud is threaded at both ends: a coarse thread screwed tightly into a casting (cylinders, crankcases, gearboxes) and a fine thread for the nut. Oversize and stepped studs (identified by marks or colours) repair damaged holes.
- Studs are installed to a specified height with a stud driver, and removed with a stud extractor or two nuts locked together.
- Thread inserts — helical coil (Heli-Coil) wire inserts and solid key-locked inserts — give strong, wear-resistant threads in soft aluminium or magnesium and repair stripped holes.
Screws
- Structural screws (AN509 100° countersunk, AN525 washer head) — made of heat-treated alloy steel with a definite grip length; they may carry structural loads like bolts.
- Machine screws (AN500/501 fillister head, AN515/526 round and truss head, AN507 100° flat head) — general use, not for primary structure.
- Self-tapping screws (AN504, AN530/531) — cut their own thread; only for non-structural attachments (nameplates, light covers), never as a substitute for a standard screw.
- Recess types: slotted, Phillips, Reed & Prince (Frearson) — use the correct driver — Torx, hexagon socket and torque-limiting recesses.
- Dowels and taper pins locate parts accurately; quarter-turn fasteners (Dzus, Camloc, Airloc) secure cowls and panels.
Nuts
- Self-locking nuts may not be used where the bolt or nut is subject to rotation (pulleys, bellcrank pivots that turn), and fibre-insert types are limited to about 120 °C (250 °F). A self-locking nut that can be run down by hand, or that is below its minimum prevailing torque, must be replaced.
- Anchor nuts (nutplates) are riveted to structure so panels can be screwed on from one side; channel nuts carry rows of floating nuts.