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What does AN, NAS, MS and others mean?

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AN- ARMY-NAVY specification series started in the early 1940s as a means to standardize military items for World War II.  Mostly canceled in the 1950s, a few have survived to only a few years ago, AN3-AN20 bolt is one of the longest lived specs.

NAS- National Aerospace Standards, started in 1941 is handled by the Aerospace Industries Association a group of aerospace companies

From their brochure: The NAS series is best known for its state-of-the-art, high strength, precision fasteners. In addition to all types of screws, nuts, and rivets, NAS standards define high pressure hose, electrical connectors, splices and terminations, rod end bearings, and many other types of hardware and components.

MS- Military standard started around the 1950s and for the most part replaced the AN hardware series.  However, a few of the AN standards have stayed around.  The MS series was canceled in 1994 by the Secretary of Defense, at the request of contractors in order to save money.  However, many of the commercial companies used MS standard hardware for all there products.  The cancellation caused the aerospace community many problems, and there was a rush to create new standards to replace the MS ones.  Some of the following specs were the result.

NASM- approximately 500 military standards were converted by the NAS group to commercial specifications, but retain the original MS part number.  The spec that defines the part is NASM and then the numerical portion of the MS number.  Example, MS20426 rivet spec went to NASM 20426, but part number stayed MS20426.

AS- Aerospace Standards created by SAE International (originally Society of Automotive Engineers) some MS specifications were replaced by AS standards.  Unfortunately, the part number changed to AS then the number of the MS part.  MS21919 cushioned clamps changed to AS21919.

 

NOTE: Certified General Aviation aircraft many times uses the old part number as that is what the aircraft was certified with, and experimental aircraft typically follows the old numbers because the certified ones do.  Various hardware supply companies will ship the hardware with different numbers from what you order on the website.  

Example: Aircraft Spruce will list AN365 in the catalog, but ship it in bags labeled MS21044.

In aviation, course threads are standard for #2 through #8 screws, fine thread is standard for #10 (3/16”) and larger bolts.