How to read the number, the head, and the dimensions of an aircraft bolt — and turn what you measured into a part number and an NSN you can actually buy.
The most common sourcing call we get from hangars and MRO purchasing desks starts the same way: someone is holding a bolt — pulled from an assembly, found loose in a kit, or listed on a fifty-year-old parts catalog page under a number nobody stocks anymore — and needs to buy more of it. Aircraft bolts are unusually kind to this problem, because the AN, MS, and NAS numbering systems encode most of what you need directly in the part number, and the head of the bolt carries markings that confirm what the number claims. This guide covers reading those numbers and markings, the traps that produce wrong-part orders, and the path from a measured bolt to a part number to a National Stock Number. One framing note up front, because it matters: everything here is about identification for procurement. Whether a correctly identified part may be installed on a particular aircraft is determined by that aircraft's maintenance data and the certificated person doing the work — never by a sourcing guide.
The AN airframe bolt series — AN3 through AN20 — is the oldest system still in daily use, and its grammar is compact enough to read at a glance once you know the three pieces.
The number after "AN" is the nominal shank diameter in sixteenths of an inch. AN3 is a 3/16-inch bolt (with a 10-32 thread), AN4 is 1/4-inch, AN5 is 5/16-inch, AN6 is 3/8-inch, and so on up the series toward AN20. This is the single most useful fact in aircraft hardware: hand someone an unmarked-length AN bolt and a caliper, and the base number falls out of the diameter measurement immediately.
The dash number encodes nominal length per the AN standard's table. The working shorthand: the last digit of the dash number is eighths of an inch and any digit before it is whole inches — so AN3-5 is nominally 5/8 inch, and AN4-14 is nominally 1-4/8, or 1½ inches. Two cautions. First, length is conventionally measured from under the head to the end of the shank for hex-head bolts, so measure the same way the standard does. Second, the dimension that matters structurally is usually grip — the unthreaded portion that bears in the hole — and grip for a given dash number comes from the standard's table, not from arithmetic. When length and grip both matter, decode against the sheet.
A trailing A means the shank is undrilled — no cotter-pin hole. Omit the A and you get a drilled shank. A letter H appearing with the base number indicates a drilled head, for safety wire. So AN3-5A decodes completely as: 3/16-inch diameter, nominal 5/8-inch length, undrilled shank. Order AN3-5 when the assembly needs AN3-5A and you will receive a bolt with a hole in it — a mistake that is cheap in dollars and expensive in schedule. For the full grammar across MS, NAS, AN, and mil-spec slash-sheet families, see the companion pillar guide, MS, NAS, AN & AS part numbers explained.
The head of an aircraft bolt carries two kinds of information: who made it, and what class of bolt it is. The manufacturer's identification mark — a trademark, symbol, or letter code — appears on essentially all conforming aircraft bolts, and its mere presence is meaningful: a bare, completely unmarked head on hardware represented as aircraft bolt stock is itself a warning sign worth acting on.
Beyond the maker's mark, the classic marking conventions for the AN series — long documented in standard airframe maintenance references — run as follows. Standard cadmium-plated steel AN bolts typically carry a raised cross or asterisk in addition to the manufacturer's mark. Corrosion-resistant steel is conventionally indicated by a single raised dash. Aluminum-alloy AN bolts carry two raised dashes. NAS close-tolerance bolts are conventionally marked with a raised or recessed triangle. Higher-strength bolt families carry their own marking schemes defined by their individual standards.
Treat these as strong conventions, not a decoder ring. Marking practice varies by standard, revision era, and manufacturer, and specific bolt standards define their own required markings in detail. The disciplined workflow is: use the head marking to form a hypothesis about the bolt's family and material, then confirm the hypothesis against the governing standard sheet or the cross-referenced supply record — not the other way around. Do not invent a full identification from a symbol table, and be suspicious of any source that offers one without citing the standard.
NAS bolts are industry standards published by the Aerospace Industries Association, and as a family they sit at the demanding end of the spectrum: tensile strengths typically from 160 ksi upward, and close-tolerance shanks ground to tighter diameter limits for interference or close-fit holes. Designers call out NAS bolts where the joint's strength or the hole's fit requires them — structural splices, fitted shear joints, high-load attachments. The identification consequence is simple and strict: an AN bolt is not a substitute for a NAS callout, however similar the dimensions look, and the reverse substitution is not automatic either. Strength class and shank tolerance are part of the part's identity.
MS bolt numbers point at Military Standard sheets, each defining one design plus a variant table — the same base-plus-dash grammar as everything else in the system. The MS families you meet most often include general-purpose hex bolts, internal-wrenching high-strength bolts, and twelve-point external-drive bolts for high-torque applications. As with the rest of the MS world, some sheets have migrated to SAE AS or NASM custody over the years, usually keeping their digits, so an "inactive" MS number is frequently alive under a new prefix. The decode discipline is identical: base number identifies the sheet, dash number selects diameter/length/material from the sheet's table, and the dash logic does not transfer between base numbers.
Two bolts of the same dash length can carry different thread lengths across standards and revisions. If threads end up bearing in the hole, the joint is wrong even though the "length" matched. Decode grip from the standard's table when the application is structural.
The trailing-A convention is small enough to miss in a parts catalog line and significant enough to matter. Drilled-shank and drilled-head variants exist for a reason; the safetying method belongs to the design.
AN-for-NAS substitutions, commercial-grade-for-aircraft substitutions, and dimensional lookalikes from the hardware store are all identification failures, not judgment calls. A bolt's family and standard are part of what the part is.
Old drawings call out numbers that have been superseded or transferred — AN designs reissued under MS numbers, MS sheets migrated to NASM or AS custody. The item usually still exists; the string you search for has changed. A shared NSN between old and new numbers is the cleanest arbitration available. Our part number lookup resolves the legacy and current forms to the same stock records.
Corrosion-resistant versus plated alloy steel, and one plating versus another, are different items of supply with different suffixes and different NSNs. Quote the full number, letters included, every time.
Here is the end-to-end workflow when you start with a physical bolt and no paperwork. Measure the shank diameter (that gives the AN base number candidate, or the nominal size for MS/NAS families), the length under-head, and the thread. Read the head: manufacturer's mark plus the material/family symbols above. Form the part number from the governing standard's table — diameter base, length dash, drilled/undrilled letters. Cross-reference to the NSN: when the government stocks a standard bolt, DLA assigns it a 13-digit National Stock Number, and aircraft bolts land mostly in FSC 5306 (bolts), with screw-form fasteners in FSC 5305 (screws) and the mating nuts and washers in FSC 5310. The NSN is the identity that survives supersessions, which is why it is worth carrying through to your purchasing records even when your catalog runs on part numbers.
Our part number lookup and NSN catalog resolve to the same records with published pricing on every page — our stock hardware is commercial-equivalent standard hardware, country of origin Türkiye, with origin stated plainly on each listing. If you have already worked a bolt down to a number, you can get a price on it below without leaving this page. For the wider sourcing picture — owner-produced-parts questions, distributor no-quotes, widening a search through cross-references — see the pillar guide on sourcing aircraft hardware for owners and MRO shops. And to say it once more, plainly: identification and procurement are our lane; installation eligibility on any aircraft is determined by that aircraft's maintenance data and the certificated personnel performing and approving the work.
The base digit gives diameter in sixteenths of an inch — AN3 is a 3/16-inch bolt with a 10-32 thread. The dash number encodes nominal length per the AN table, last digit in eighths and any preceding digit in inches, so AN3-5 is nominally 5/8 inch. The trailing A means an undrilled shank; without it, the shank has a cotter-pin hole. Grip for a given dash number comes from the standard's table.
By long-established convention: standard plated-steel AN bolts typically carry a raised cross or asterisk plus the manufacturer's mark; corrosion-resistant steel a single raised dash; aluminum alloy two raised dashes; NAS close-tolerance bolts a raised or recessed triangle. Conventions vary by standard and era, so confirm against the governing sheet rather than the symbol alone.
NAS bolts generally cover higher-strength (typically 160 ksi and up) and close-tolerance hardware, called out where joint strength or hole fit demands it. An AN bolt is not a substitute for a NAS callout, and the reverse is not automatic either.
Stocked standard bolts carry 13-digit NSNs, mostly in FSC 5306 for bolts and FSC 5305 for screws. Our part number lookup resolves AN, MS, and NAS numbers to their NSN records at published prices.
No. Identification is for procurement. Installation eligibility on any aircraft is determined by that aircraft's maintenance data and the certificated person performing and approving the work.
The full grammar of military standard numbering.
A practical guide for owners and MRO shops.
AN, MS, and NAS bolts with published pricing.
MS and NAS machine screws in stock.
Reading the source codes on drawings and solicitations.
Custom machining to your print, made in the USA.
