Module 6.5 Fasteners: screw threads, bolts, locking devices and rivets

Knowledge levels

Item	A	B1 / B3	B2 / B2L
6.5.1 Screw threads: nomenclature, forms, dimensions, tolerances, measuring	2	2	2
6.5.2 Bolts, studs and screws: types, identification, nuts, machine screws, studs, self-tapping screws, dowels	2	2	2
6.5.3 Locking devices	2	2	2
6.5.4 Aircraft rivets: solid and blind, specification, identification, heat treatment	1	2	1

6.5.1 Screw threads

Term	Meaning
Major diameter	largest diameter of the thread
Minor (root) diameter	smallest diameter
Pitch diameter (effective)	where thread width equals space width
Pitch	distance from a point on one thread to the same point on the next
Lead	distance the screw advances in one turn (= pitch for single-start threads; 2 × pitch for two-start)
Crest / root / flank	top / bottom / side of the thread
Thread angle	angle between flanks (60° for unified and metric, 55° for BSW/BSF)

Thread forms

Thread	Angle	Notes
UNC / UNF (Unified coarse/fine)	60°	standard aircraft threads; UNF most common on structure (e.g. 1/4-28 UNF = 28 threads per inch)
UNJ	60°, larger root radius	higher fatigue strength - high-strength aerospace bolts
ISO metric (M)	60°	M6 × 1.0 = 6 mm diameter, 1 mm pitch
BSW, BSF, BA	55° (BA 47.5°)	older British equipment
Acme, buttress, square	—	power transmission, jacks, screw actuators
Taper pipe (NPT)	60°	pipe fittings

• Fit classes: 1A/1B loose, 2A/2B normal (general aircraft), 3A/3B close tolerance (aircraft bolts and nuts). "A" = external, "B" = internal thread.
• Measuring: thread pitch gauge (screw-pitch gauge), thread micrometer, three-wire method, ring and plug gauges (go/no-go).

Worked example - lead

A two-start thread has a pitch of 1.5 mm. How far does the nut move in 4 turns?

• Lead = 2 × 1.5 = 3 mm; 4 turns × 3 = 12 mm

6.5.2 Bolts, studs and screws

Aircraft bolts

• Specified by standards: AN (Air Force-Navy), MS (Military Standard), NAS (National Aerospace Standard), and metric/European standards (e.g. ASNA/EN).
• AN3-AN20 hex-head bolts: the number after "AN" gives the diameter in sixteenths of an inch; the dash number gives the length in eighths of an inch (last digit = eighths, any digit before it = whole inches: -7 = 7/8 in, -12 = 1 2/8 = 1 1/4 in). Example: AN4-7A = 1/4 in diameter, 7/8 in long, "A" = shank undrilled; no letter = shank drilled for a cotter pin; "H" after the diameter = drilled head.
• Head markings identify material: a cross or asterisk = standard AN alloy steel; a single raised dash = corrosion-resistant steel; two raised dashes = aluminium alloy; a triangle = close-tolerance bolt.
• Grip length should equal the material thickness; threads must not bear on the structure (in shear).
• Special bolts: internal wrenching, clevis bolts (shear only, screwdriver slot, used with control cable fittings), eyebolts (tension), close-tolerance and Hi-Lok/lockbolts (permanent).

Nuts

Nut	Locking	Notes
Plain hex (AN315)	needs separate locking (lock washer, wire)
Castellated (AN310) / castle shear (AN320)	cotter pin through slots and bolt hole	castellated for tension, shear nut for shear loads only
Self-locking, non-metallic insert (nylon, AN365/MS21044)	friction of the insert	temperature limit about 120 °C (250 °F); not to be used on rotating parts or where the bolt can turn
Self-locking, all-metal (MS21042)	distorted thread/slotted section	high temperature (engine areas)
Anchor (plate) nuts	riveted to structure	allow removal of panels from one side
Wing nuts, sheet-spring (speed) nuts, channel nuts		non-structural

A self-locking nut must have at least the specified minimum prevailing torque; replace if it can be run on by hand. At least one and a half to two threads (per manual) must protrude.

Machine screws, studs, self-tapping screws, dowels

• Machine screws: round, pan, flat (countersunk 100°) heads; slotted, Phillips, Reed and Prince, Torx/hexalobular drives.
• Studs: threaded both ends; one end (often coarse thread) screwed into a casting, the other (fine) takes a nut. Removed with a stud extractor or two locked nuts; oversize/step studs and inserts (Helicoil) used for damaged threads.
• Self-tapping screws: cut their own thread - non-structural only.
• Dowels: precise location of two parts (not for clamping).