Fasteners
Core concepts
Know the families: AN, MS and NAS bolts, machine screws, self-locking nuts (nylon insert and all-metal), castle nuts with cotter pins, shear nuts, and washers (AN960 flat, AN970 large area). Grip length must suit the joint; identification is by head marking and specification. Torque produces preload, and preload - not torque - holds the joint. Lubricated threads reach the same preload at a lower torque than dry threads, so always use the torque figure for the stated condition.
Rivets: 1100 (A, plain head), 2017-T3 (D, raised dot), 2024-T4 (DD, raised double dash), 2117-T4 (AD, dimple) and 5056 (B, raised cross). Blind rivets, Hi-Loks and lockbolts are used where access is one-sided.
Traps in questions
Threads must not lie in the bearing (shear) plane of the joint - a maximum of about one thread is tolerated. Do not over-torque to "make it safe". Self-locking nuts have limited re-use. Lockwire must be routed so that it pulls the fastener in the tightening direction, with a neat pigtail. Rivet spacing and edge distance are habitually tested: pitch about 3 to 4 diameters, edge distance at least 2 diameters, and the rivet diameter at least three times the thickness of the thickest sheet.
Rule of thumb
Edge distance 2D, pitch 3-4D, rivet diameter at least 3t, no thread in the shear plane.