How to Read Boeing BAC Part Numbers: A Practical Guide

BACB30LU5K12. BACB28V10P. BACS12GU3-8. If you source hardware for Boeing platforms, callouts like these are the bulk of your day, and they are the single largest standards family in our catalog: more than 76,000 BACB parts alone, plus BACS screws, BACN nuts, BACW washers, and a long tail of other BAC prefixes. A BAC number is not an arbitrary SKU. It is a compressed pointer into a Boeing engineering standard, with a family code, a base number, and a string of dash numbers and letters that pin down size, material, and finish.

This guide walks through the anatomy of BAC part numbers the way a distributor reads them at the quoting desk and on the receiving dock. It also stays honest about the limits. BAC standards are Boeing-proprietary, the dash-code semantics vary family by family, and the governing Boeing standard sheet or engineering drawing, not a blog post, is always the final authority.

Where BAC Numbers Come From

BAC stands for Boeing Aircraft Company, and the BAC series is Boeing's own catalog of internal aerospace standards. Unlike the National Aerospace Standards (NAS) published by the Aerospace Industries Association, or the AN and MS military families, BAC standards are proprietary to Boeing. They define the fasteners, bushings, and hardware detailed on Boeing engineering drawings, and they exist because a manufacturer at Boeing's scale needs its own controlled definitions for parts that a generic industry standard does not cover in enough detail.

That proprietary origin has a practical consequence: the authoritative decode always lives in a Boeing standard sheet or drawing, not in a public specification you can pull off a shelf. When you buy to a BAC number, you are buying to Boeing's definition of that part, which is one reason a BAC callout and a dimensionally similar NAS or AN part are not the same thing. More on that in the pitfalls section below.

The Anatomy of a BAC Part Number

Most BAC callouts have three layers:

1. The prefix and family code, which always begins with BAC and continues with one or more letters (B, S, N, W, R, C, F, P) that identify the hardware category.
2. The base number (30, 28, 12), which identifies the specific Boeing standard within that family: head style, thread form, general configuration, and every fixed characteristic.
3. Dash numbers and letter codes, which select the variable characteristics: diameter, grip or length, material option, and finish.

So in BACB30LU5, the BACB tells you the family is bolts and bushings, the 30 tells you which bolt standard, and the trailing characters set the specifics. As with NAS, the boundary between layers is not fixed across families, and the trailing string is where meaning gets family-specific. There is no single universal parsing rule: you learn the families you buy most, and you look up the rest against the governing standard.

The Family Codes: BACB, BACS, BACN, and the Rest

The letter block immediately after BAC is the fastest way to know what kind of part you are looking at. The categories that come up most often in procurement:

• BACB: bolts and bushings. This is the largest group by far. The BACB30 series covers bolts; the BACB28 series covers bushings. The base number after BACB tells you which.
• BACS: screws.
• BACN: nuts.
• BACW: washers.
• BACR: rivets.
• BACC: clamps, couplings, and connectors.
• BACP: pins.
• BACF: another Boeing hardware category you will encounter in the catalog.

Because BACB covers both bolts and bushings, the family code alone is not enough. You have to read the base number to know whether a BACB part is a fastener or a bushing. You can browse these families directly in our bolts and bushings catalogs, or search the full parts catalog by BAC callout.

Dash Numbers and Letters: Size, Material, and Finish

The characters after the base number carry the specifics: diameter, grip or length, material, and finish. This is where most reading errors happen, because the meaning is family-specific and Boeing packs a lot into a short string. A few patterns are common enough to be useful defaults, as long as you treat them strictly as defaults, not guarantees:

• A dash number often expresses diameter in a nominal size step, and a following number often expresses grip or length, frequently in sixteenths of an inch, but the increment is standard-specific.
• Letters within the string typically select material: alloy steel versus corrosion-resistant steel (CRES) versus titanium or aluminum. Which letter maps to which material is set by the individual standard.
• Additional letters or a trailing code often select finish or a coating option, or add a feature such as a locking element.

The trap is assuming these meanings are universal. They are not. The same letter can designate different materials in different BAC families, and the increment behind a dash number can differ from one standard to the next. Do not treat any precise decode as a universal rule. When in doubt, pull the Boeing standard sheet for that specific base number and confirm.

A Worked Example: A BACB30 Bolt

Take a callout like BACB30LU5K12 and read it layer by layer:

1. BACB: the family is Boeing bolts and bushings.
2. 30: the base number places it in the BACB30 bolt series, so this is a bolt rather than a bushing. The base standard fixes the head style and thread form for the series.
3. LU: letters that, for this standard, select a material and configuration option, for example an alloy-steel or CRES variant. Confirm which against the BACB30 standard sheet rather than assuming.
4. 5: a size code that typically indexes nominal diameter for the series.
5. K12: additional characters selecting a finish or feature and the grip or length. Grip is often expressed in sixteenths of an inch, but confirm the increment for this family before ordering.

The point of the exercise is not the exact decode, which belongs to the Boeing standard, but the discipline: every letter and digit is load-bearing. If a requisition drops or adds one character, it can select a different material, a different length, or a different part entirely. Confirm before ordering.

A Second Example: A BACB28 Bushing

Now take a bushing callout such as BACB28V10P and notice how the same BACB family reads completely differently once the base number changes:

1. BACB: same family code as the bolt above, bolts and bushings.
2. 28: the base number places it in the BACB28 bushing series. Same family letter, entirely different kind of part. This is exactly why the base number matters.
3. V: a letter that, for the BACB28 standard, selects a bushing style or material option. What V means here is defined by this standard and should not be carried over from any other family.
4. 10: a size code that typically indexes a nominal dimension such as bore or outside diameter for the series.
5. P: a trailing letter that often selects a finish or a feature. Confirm its meaning on the standard sheet.

The contrast is the lesson. BACB30 and BACB28 share a family code but describe a bolt and a bushing respectively, and their dash and letter codes do not decode the same way. For a deeper look at the bushing side, see our BACB28 Boeing bushing guide.

Common Pitfalls

• Dropped or transposed characters. BAC strings are long and dense, and a single missing letter can turn one orderable part number into a different, equally orderable, wrong one. Copy callouts character-for-character, and have receiving verify part markings against the purchase order rather than against memory.
• Assuming dash and letter semantics transfer. A letter that sets material in BACB30 may mean something else in BACB28 or BACS. Parse each family, and each base number, on its own terms against the Boeing standard.
• Superseded Boeing standards. BAC standards get revised, canceled, and replaced over time. If a drawing calls an older standard, buy exactly what your engineering authority requires and check the standard's current status rather than silently substituting a successor.
• Lookalike NAS, AN, and MS numbers. A BAC bolt and a dimensionally similar NAS or AN bolt can look interchangeable, but BAC parts are Boeing-proprietary standards with their own material, tolerance, and marking requirements. They are not interchangeable unless the engineering authority for your application says so in writing.
• Reading the family code without the base number. Because BACB covers both bolts and bushings, the family letter alone will not tell you what you are holding. Always read the base number.

Part number literacy is half of buying Boeing hardware well; the other half is making sure the parts that arrive match the paperwork. If NAS callouts also cross your desk, our companion guide on how to read NAS part numbers covers that family, and our overview of AS9120 vs AS9100 explains what a distributor's certification actually covers.