Aerospace fasteners are specified, not shopped. The part number carries the whole definition — and using a hardware-store equivalent is both a structural and a certification problem.
Read the part number
Legacy AN (Army-Navy), then MS (Military Standard), now
NAS (National Aerospace Standard) and NASM. The number encodes series, diameter,
grip length, material and finish — e.g. in AN4-13A the 4 is the diameter in
1⁄16 inch (¼"), 13 sets the grip, and A means no drilling. The
callout reference decodes the common families.
Grip length is the thing people get wrong
The grip — the unthreaded shank — must match the stack thickness so that threads never bear in the shear plane. Threads in shear cut allowable strength and start fatigue cracks. Aim for at most one or two washers of adjustment; if you need more, you have the wrong grip length, not a washer problem.
A bolt's grip length is slightly shorter than the stack it clamps. What happens?
A short grip puts the thread runout inside the hole, so the shear plane lands on the thread root rather than on the full shank. You lose the difference between the shank and minor diameters in shear area, you introduce a stress concentration exactly where the load is, and the reduced section fretting-damages the hole. Too long a grip is its own failure: the nut bottoms on the shank and the joint never gets clamped however much torque you apply. Grip is selected in 1/16 inch steps for exactly this reason, and it is the single most common error on an assembly floor.
Try it — grip length against the stack
Holes, edge distance and spacing
Structural joints use close-tolerance reamed holes so the fastener actually bears. The rules of thumb: edge distance ≥ 2D (centre to edge) and pitch ≥ 4D between fasteners. Less than 2D and you invite shear-out — check it with the lug tool, and get the joint allowable from the rivet/fastener tool.
Material, finish and galvanic pairing
Match the fastener to the structure: cadmium-plated steel, A286 or titanium in aluminium, but never bare steel against aluminium in a wet environment (see corrosion & the galvanic series and MIL-STD-889). Torque per the spec, not by feel — and where a joint is preload-critical, control it by preload, not torque (bolt torque tool). Use the specified locking method; a plain nut is never acceptable on primary structure.