Mil-Spec fastener buyer’s guide.

// IN THIS GUIDE

A practical starting point

Buying a specified fastener starts with a simple question: what does the drawing or purchase order actually require? A familiar shape or a matching thread size can narrow the search, but neither identifies the complete part.

This guide explains how to carry the requirement from the drawing into an RFQ. Use the linked series references for selected part numbers and dimensional examples, and the purchasing FAQs for quick answers.

// THE REQUIREMENT

What does “Mil-Spec fastener” mean?

Mil-Spec is a common purchasing term for hardware made to a military specification or standard. Buyers also use it when discussing related AN and NAS aerospace hardware. The exact document and part designation matter more than that broad label.

A fastener standard may define geometry, material, finish, thread requirements and performance, or refer to other documents that control those details. Your drawing and purchase order can add source restrictions, inspection requirements and delivery documentation.

For that reason, “stainless screw, 4-40 × 1/4” is a useful description, but it is not a complete substitute for a specified part number. Keep the original designation and any drawing notes together when requesting a quote.

// IDENTIFICATION

AN, MS, NAS and NASM: what the prefixes tell you.

A prefix does not establish material, strength, certification or interchangeability by itself. There is no universal dash-number decoder that works across all four groups.

// WORKED EXAMPLES

Read the complete part number, not just its size.

Copy the designation exactly from the requirement. A dropped suffix can change a locking feature, finish or configuration while leaving the nominal thread and length unchanged.

These two selected stocked references share a nominal size, but they are different requirements. The LP designation in this example identifies patch-type locking. That code belongs to this series; it is not a rule to apply to every aerospace screw.

The head style matters too. MS51957-13 is a pan-head screw, while MS51959-13 is an 82-degree flat-head screw. Both selected references list 4-40 UNC-2A threads and a 1/4-inch nominal length. Their head forms and length measurement conventions differ.

// SPECIFICATION DETAILS

Check the details that a short description leaves out.

Before comparing a proposed part with the required item, review the features that affect fit and performance. A useful comparison includes more than diameter and length:

• Thread: nominal diameter, pitch or threads per inch, thread form and class. UNC, UNF and UNJ callouts are not interchangeable descriptions.
• Length convention: a raised-head screw is generally measured beneath the head; a flat-head screw includes the head. Rivet grip, bolt grip and shoulder length are separate measurements defined by the selected family.
• Head and drive: keep the head profile, countersink angle, recess and any drilled features in the requirement.
• Material and condition: CRES means corrosion-resistant steel, not one particular alloy or strength level. Preserve the specified grade and condition.
• Finish and lubrication: passivation, plating and dry-film lubricant describe different treatments. Retain the specified process and any type or class.
• Locking and retention: a patch-locking screw, all-metal locknut and insert locknut use different constructions. Select the configuration your requirement specifies.

Our tables are identification aids. A nominal dimension does not include every tolerance, inspection limit or acceptance requirement in the applicable specification.

// QUALITY AND RECEIVING

Match the documentation to the order.

A distributor’s quality management certification and a part’s delivery documentation answer different questions. Webson’s AS9120B and ISO 9001:2015 registration covers its quality management system for distributing fastening products and installation tooling. It does not replace the certificates or reports your order requires for a particular item.

These requirements can affect both supply options and the quote. Include them at the beginning of the inquiry; a general request for “certs” may leave the required scope unclear. Documentation availability is part- and order-specific.

// CROSS-REFERENCES

Separate a successor document from an approved alternative.

A canceled or superseded document does not by itself tell you whether a part is unavailable, or whether a different part is acceptable for your application. A successor document may retain the original part numbers. Do not automatically replace an MS prefix with NASM on an order.

Our document cross-reference identifies standard relationships. A manufacturer’s commercial-to-MS cross-reference is another type of resource: it proposes a relationship between product designations. Neither replaces the need to review the governing drawing, source restrictions and technical requirements.

If alternatives may be considered, identify that in the RFQ and keep the proposed alternative separate from the exact requirement. The responsible engineering or quality authority determines whether the change is approved for the intended use.

// STOCK AND AVAILABILITY

What does “stocked” mean on this website?

Entries marked Stocked identify selected items Webson stocks. The tables are not a live inventory feed and do not show on-hand quantities or reserved stock. Request a quote for current availability, quantity, pricing and lead time.

A stocked reference also does not establish which certificates or reports will accompany a particular order. Include your documentation requirements with the inquiry so they can be reviewed with the supply options.

// PART IDENTIFICATION

Can a photo identify the exact part?

A clear photo can help communicate head shape, drive style and visible features. Include dimensions and any package markings or part numbers you have.

A photo cannot establish alloy, heat treatment, thread class, strength or certification. If the full designation is missing, send the available information and explain which details are unknown. Those details need to be resolved before a compliant replacement can be selected.

// SOURCING QUESTIONS

Can I request parts that are not listed?

Yes. The website covers selected hardware rather than Webson’s complete sourcing range. Send the full AN, MS, NAS or NASM designation, quantity, required delivery date and documentation requirements for review.

You can include several parts in one inquiry. Use a separate line for each complete designation and quantity, and identify any requirements that apply only to particular lines.

// NEXT STEP

Turn the requirement into a usable RFQ.

Use a separate line for each complete part number. Include the quantity, required delivery date and any recurring demand or scheduled releases. Then add the governing drawing or specification revision, material and finish notes, required records and source restrictions.

If you have an incomplete designation, send what you have and identify the missing information. A photo can help communicate visible features; it cannot establish the material, strength or certification of the part.

Paste your part list into the Webson RFQ form. Email drawings, spreadsheets or supporting specifications to info@websonfasteners.com with your company name and part number so we can match the files to your inquiry. If your documents require a particular transfer method, contact us before sending them.

Webson stocks and sources military and aerospace hardware from Deer Park, New York. Our selected stocked tables help identify parts; contact us for current availability, pricing, lead time and documentation details.

Talk through a requirement: (631) 243-1800.