AN, CRES, DH, DS, Grip, "J" Thread, MS, NAS, Nylock, Pitch, "R" Thread, T.P.I., UNC, UNF

Glossary of Terms and Abbreviations

AN
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World War II era hardware standards system. Literally "Air Force/Navy" or "Army/Navy." A very logical numbering system now being bastardized by government changeover to NAS/MS series. We insist on using it because it is convenient and logical and because almost everybody speaks the language. Also, most old manuals and parts lists are in AN speak, not Nush-Mush (NAS/MS).

NAS
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National Aerospace Standards. Korea era standards systems now almost totally duplicated by the MS system, but still in use anyway.

MS
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Military Standard specification. Cold war era standards required as airplanes got more complicated. Unfortunately, this numbering system lacks much of the logic of the AN system it replaces. Dash numbers vary from part number to part number and seems to bear no resemblance to size. Only the Guv'ment.......!

Grip
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The un-threaded length of a bolt or screw. This is the important dimension in shear applications and, in fact, all NAS shear bolt and screws are dash numbered as to grip, not under head length.

Length or UHL
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Always the Under Head Length. Overall length, including the head thickness, is unimportant to us. For countersunk and flathead bolts, screws and rivets, the UHL and overall length are the same dimension.

CRES
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Corrosion REsistant Steel or Stainless Steel to non-military types.

DH
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Drilled head.

DS
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Drilled shank.

NYLOCK
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Contraction for nylon lock nut, although actually a corporate trade name for a nut manufacturer. Like teflon, now universally accepted as a descriptive adjective rather than a proper noun.

T.P.I.
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Threads per running inch. This is not pitch, but its reciprocal (t.p.i. = 1/p), although everybody calls it pitch, including us in this catalog.

PITCH
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Pitch is not threads per inch, but the distance across a full thread. Pitch is the reciprocal of t.p.i. (p = 1/t.p.i)..





UNF
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Unified National Fine thread series or just "Fine Thread (FT)" Used mostly in through fastener applications. All AN bolts and screws. Truncated thread root. MIL-B-7742

UNC
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Unified National Course thread form. Coarse thread screws and bolts are mostly in blind holes into soft castings of iron, aluminum, etc. when fine threads would easily strip. AN 73A series bolts use UNC threads as do MS20074 series. Otherwise the same as UNF.

"R" THREAD
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"R" thread form, as in UNRF. The Unified Standards Board goofed in the '40s when they allowed a truncated thread root for UNF/UNC instead of radiused. This essentially replaced 1 sharp corner with 2 shallower corners, and the only reason they got away with it was because of the low tensile strength, thus higher tolerance notch sensitivity, of the fasteners. When higher strength (thus more brittle) NAS and MS fasteners were introduced this had to be corrected. The "R" thread form was mandated in the late '60s, replacing the truncated root with a radius of .144 x pitch. This still allowed the theoretical thread engagement, and the increased radius alone nearly doubled fatigue strength. "R" threads were used on 160k psi tensile NAS bolts until recently and are slowly being replaced by the "J" thread form. You will also find "R" threads on name brand commercial socket head cap screws (Unbrako, Holo Krome) and the better grade 8 fasteners (watch out for offshore brands though!). You will see the threads specified as UNRF or UNRC. Mil spec number MIL-B-7838.

"J" THREAD
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"J" Thread form. The "R" thread taken farther. The root radius has been increased to .180 x pitch. This reduces thread engagement to 75% and, in theory, can have thread interferences if the nut is on the tight side of its tolerance. In practice this is not an issue. Most nuts tend to run on the loose side and the thread length on nuts are actually longer than idealized design, giving it the margin to cope with the reduced engagement height. "J" thread form is found on 160k psi and up MS bolts and later NAS shear bolts. Specified as UNJF. Mil spec number MIL-S-8879.


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UNF/UNC UNRF UNJF
Mil Spec MIL-B-7742 MIL-B-7838 MIL-S-8879
% thread height engagement 75% to 88% 83% 75%
Root Radius NA (Truncated Root) .114 x pitch .180 x pitch
Fatigue life factor 1X 2X 4X