NAS1351 Decoder: Socket Head Cap Screw Part Numbers

What Does NAS1351C4LL24P Mean?

It is a 1/4-28 socket head cap screw in corrosion-resistant steel, with a strip self-locking element, 1.500 in long, cadmium plated. Type any NAS1351 part number below. The decoder splits it into its parts, checks each one against the NAS1351 tables and says what is wrong if a code does not exist. Spaces, dashes and lower case are fine.

You can also type a screw size, such as 1/4-28 x 1 or #10-32 x 3/4, to build the part number, or part of a number (for example NAS1351C3) to list every preferred length. For washers to go under the head, see the NAS1149 decoder.

NAS1351 DECODER

material · size · type · length · finish NAS1351 Rev 10, Mar 2009

Part number or size

Examples:

Part number	Thread	L in	L mm	Head Ø max	Head ht max	Socket	Min break lb

How a NAS1351 Part Number Is Built

NAS1351 standard C material: CRES 4 size: 1/4-28 LL type: strip self-locking 24 length: 24/16 = 1.500 in P finish: cadmium

Alloy steel has a dash as its material code. With no type code, a dash separates size and length: NAS1351-02-8, NAS1351C3-8. No finish letter is a finish too (see below).

The Sheet's Own Examples

Part number	Meaning given on the sheet
NAS1351-02-8	.0860-64 UNRF-3A, alloy steel, undrilled head, plain, .500 in long, black oxide
NAS1351C04H12	.1120-48 UNRF-3A, corrosion-resistant steel, drilled head, plain, .750 in long, passivated
NAS1351-08LE16P	.1640-36 UNRF-3A, alloy steel, self-locking (optional type element), 1.000 in long, cadmium plate, undrilled head
NAS1351C4LL24P	.2500-28 UNRF-3A, corrosion-resistant steel, self-locking (longitudinal strip), 1.500 in long, cadmium plate, undrilled head
NAS1351N10LN32	.6250-18 UNRF-3A, heat-resistant steel, self-locking (pellet), 2.000 in long, passivated, undrilled head
NAS1351N12LB36S	.7500-16 UNRF-3A, heat-resistant steel, self-locking (patch type), 2.250 in long, silver flash, undrilled head
NAS1351N4LB16B	.2500-28 UNRF-3A, heat-resistant steel, self-locking (patch type), 1.000 in long, black oxide, undrilled head

Material and Finish Codes

Material code	Material	Finish code (after the length)
– (dash)	Alloy steel Alloy steel per FF-S-86	no letter black oxide; P cadmium plate
C	Corrosion-resistant steel Corrosion-resistant steel per FF-S-86	no letter passivate; P cadmium plate; S silver flash
N	Heat-resistant steel (A286) Chemistry of AMS5731 or AMS5737 (UNS S66286), cold worked and age hardened to meet FF-S-86 and NAS1351	no letter passivate; S silver flash; B black oxide; P cadmium plate

Type Codes (after the size)

Type code	Meaning
none	Plain, undrilled head
H	Drilled head (safety wire holes) Drilled hole data per FF-S-86. Not for sizes below 04.
LE	Self-locking, optional type element Optional type locking element per NAS1283
LL	Self-locking, longitudinal strip element Longitudinal strip locking element per NAS1283, Type L
LN	Self-locking, pellet element Pellet locking element per NAS1283, Type N
LB	Self-locking, patch type element Patch type locking element per NAS1283, Type P

Dimensions by Size (Table I), inches

Size code	Thread (UNRF-3A)	Body Ø D	Head Ø A	Head height H	Head side S min	Socket J nom	Key engage. T min	Wall G min	Chamfer K max
00	.0600-80 No. 0-80	.0568–.060	.091–.096	.057–.060	.054	.050	.025	.020	.003
01	.0730-72 No. 1-72	.0695–.073	.112–.118	.070–.073	.066	.062	.031	.025	.003
02	.0860-64 No. 2-64	.0822–.086	.134–.140	.083–.086	.077	.078	.038	.029	.003
03	.0990-56 No. 3-56	.0949–.099	.154–.161	.095–.099	.089	.078	.044	.034	.003
04	.1120-48 No. 4-48	.1075–.112	.176–.183	.108–.112	.101	.094	.051	.038	.005
06	.1380-40 No. 6-40	.1329–.138	.218–.226	.134–.138	.124	.109	.064	.047	.005
08	.1640-36 No. 8-36	.1585–.164	.262–.270	.159–.164	.148	.141	.077	.056	.005
3	.1900-32 No. 10-32	.184–.190	.303–.312	.185–.190	.171	.156	.090	.065	.005
4	.2500-28 1/4-28	.2435–.250	.365–.375	.244–.250	.225	.188	.120	.095	.008
5	.3125-24 5/16-24	.3053–.3125	.457–.469	.306–.312	.281	.250	.151	.119	.008
6	.3750-24 3/8-24	.3678–.375	.550–.562	.368–.375	.337	.312	.182	.143	.008
7	.4375-20 7/16-20	.4294–.4375	.642–.656	.430–.437	.394	.375	.213	.166	.010
8	.5000-20 1/2-20	.4919–.500	.735–.750	.492–.500	.450	.375	.245	.190	.010
10	.6250-18 5/8-18	.6163–.625	.921–.937	.616–.625	.562	.500	.307	.238	.010
12	.7500-16 3/4-16	.7406–.750	1.107–1.125	.740–.750	.675	.625	.370	.285	.010
14	.8750-14 7/8-14	.8647–.875	1.293–1.312	.864–.875	.787	.750	.432	.333	.015
16	1.0000-12 1-12	.9886–1.000	1.479–1.500	.988–1.000	.900	.750	.495	.380	.015

Thread Length, Drilled Heads and Breaking Load (Table II)

Size code	Lt min thread length, in	Drilled holes	Hole location W, in	Hole Ø B, in	Alloy steel, lb	CRES, lb	Heat-resistant, lb
00	.500	n/a	n/a	n/a	324	140	288
01	.625	n/a	n/a	n/a	500	220	445
02	.625	n/a	n/a	n/a	710	310	631
03	.625	n/a	n/a	n/a	940	420	836
04	.750	2	.026–.040	.033–.039	1,190	530	1,058
06	.750	2	.035–.050	.033–.039	1,820	810	1,620
08	.875	6	.040–.060	.044–.050	2,650	1,180	2,360
3	.875	6	.045–.065	.044–.050	3,600	1,600	3,200
4	1.000	6	.065–.085	.044–.050	6,550^h	2,910	5,820
5	1.125	6	.084–.104	.044–.050	10,400^h	4,640	9,280
6	1.250	6	.103–.123	.061–.067	15,800^h	7,020	14,080
7	1.375	6	.121–.141	.061–.067	21,400^h	9,500	19,000
8	1.500	6	.140–.160	.061–.067	28,800^h	12,800	25,600
10	1.750	6	.178–.198	.061–.067	43,500	20,500	41,000
12	2.000	6	.215–.235	.091–.097	63,400	26,100	59,700
14	2.250	6	.253–.273	.091–.097	86,500	35,600	81,400
16	2.500	6	.290–.310	.091–.097	113,000	46,400	106,200

Breaking loads are minimums in pounds. ^h Based on 180 ksi heat treatment. The sheet lines through the older 170 ksi values: 6,200, 9,850, 14,900, 20,200 and 27,000 lb for sizes 4 to 8. Drilled holes: two at 180° for sizes 04 and 06, six at 60° for 08 to 16, none below 04 (note d).

Preferred Lengths (Table III)

Length, in	Length, mm	Length code	Sizes with this length tabulated
.125	3.17	2	00, 01
.1875	4.76	3	00, 01, 02
.250	6.35	4	00, 01, 02, 03, 04, 06, 08
.375	9.52	6	00, 01, 02, 03, 04, 06, 08, 3, 4, 5
.500	12.70	8	02, 03, 04, 06, 08, 3, 4, 5, 6
.625	15.88	10	03, 04, 06, 08, 3, 4, 5, 6
.750	19.05	12	04, 06, 08, 3, 4, 5, 6, 7, 8
.875	22.22	14	06, 08, 3, 4, 5, 6, 7, 8
1.000	25.40	16	06, 08, 3, 4, 5, 6, 7, 8, 10
1.250	31.75	20	08, 3, 4, 5, 6, 7, 8, 10
1.500	38.10	24	08, 3, 4, 5, 6, 7, 8, 10, 12
1.750	44.45	28	3, 4, 5, 6, 7, 8, 10, 12
2.000	50.80	32	3, 4, 5, 6, 7, 8, 10, 12, 14
2.250	57.15	36	4, 5, 6, 7, 8, 10, 12, 14
2.500	63.50	40	5, 6, 7, 8, 10, 12, 14, 16
2.750	69.85	44	6, 7, 8, 10, 12, 14, 16
3.000	76.20	48	6, 7, 8, 10, 12, 14, 16
3.250	82.55	52	12, 14, 16
3.500	88.90	56	12, 14, 16
4.000	101.60	64	14, 16
4.500	114.30	72	14, 16
5.000	127.00	80	16

Other lengths are special order, minimum run: any 1/16 in step up to 3.500 in, then 1/8 in steps up to 6.000 in.

Length Tolerance (all +.000)

Nominal length	Size up to .375 in	Over .375 to .750 in	Over .750 to 1.000 in
Up to 1.000 in	−.030	−.030	−.050
Over 1.000 to 2.500 in	−.040	−.060	−.100
Over 2.500 to 6.000 in	−.060	−.080	−.140

"Size" is the nominal screw diameter: size 6 (3/8 in) is in the first column, size 7 (7/16 in) in the second.

Reference Guide: Reading NAS1351

The Rules of the Code

Five parts, fixed order

Material code: "-" alloy steel, C corrosion-resistant steel, N heat-resistant steel (A286).
First dash number: the thread size (Table I).
Type code: H drilled head; LE, LL, LN or LB self-locking; nothing for a plain, undrilled screw.
Second dash number: the length in sixteenths of an inch. 8 = .500 in, 24 = 1.500 in.
Finish code: P, S or B, or nothing for the default finish.

The type code takes the place of the second dash. So NAS1351-08LE16P has no dash before 16, but NAS1351-02-8 needs one.

The size codes

Numbered sizes use two digits: 00 (No. 0-80), 01, 02, 03, 04 (No. 4-48), 06 (No. 6-40), 08 (No. 8-36).
No. 10-32 is size 3 (about 3/16 in).
Fraction sizes are the diameter in sixteenths: 4 = 1/4-28, 5 = 5/16-24, 6 = 3/8-24, 7 = 7/16-20, 8 = 1/2-20, 10 = 5/8-18, 12 = 3/4-16, 14 = 7/8-14, 16 = 1-12.

The leading zero matters. 04 is a No. 4 screw (.112 in). 4 is a 1/4 in screw. The same goes for 06 and 6, 08 and 8.

No Letter Still Means Something

NAS1351 leaves the letter off for the most common choice. The decoder treats these as complete numbers.

No type code: undrilled head, no locking element.
No finish letter, alloy steel: black oxide (MIL-DTL-13924, Class 1).
No finish letter, CRES or A286: passivated (AMS2700, Method 1).

So NAS1351C3-8 is a passivated CRES screw, and NAS1351-3-8 is black oxide alloy steel. Compare the NAS1149 washer, where passivated CRES needs its own letter (R).

Which finish letters go with which material

Alloy steel (-): P cadmium. Nothing else.
CRES (C): P cadmium, S silver flash.
Heat-resistant (N): S silver flash, B black oxide, P cadmium.

B is valid only on heat-resistant steel. NAS1351-3-8B is not a valid number: black oxide alloy steel has no letter.

Drilled Heads and Self-Locking Elements

Drilled head (H)

Holes through the head for safety wire, per FF-S-86. Sizes 04 and 06 get two holes 180° apart. Sizes 08 to 16 get six holes 60° apart. Sizes 00 to 03 cannot be drilled.

The sheet gives one type code per number. It has no code for a drilled head with a locking element, and every self-locking example has an undrilled head.

Self-locking (LE, LL, LN, LB)

LE: "optional type" element per NAS1283 (the type is not fixed by the number).
LL: longitudinal strip, NAS1283 Type L.
LN: pellet, NAS1283 Type N.
LB: patch, NAS1283 Type P.

The element meets MIL-DTL-18240. It must give the required torque within M, the first 5 thread pitches of full engagement, and it is not effective in the first thread at the tip (N). Sizes 04 and up carry at least six dots on the head. Heat-resistant screws 04 and up also carry the letter N.

Lengths and Thread Length

Table III lists the preferred lengths for each size, from .125 in (No. 0 and No. 1) to 5.000 in (1-12). The decoder accepts other lengths but flags them: the sheet allows them on special order, minimum run, in 1/16 in steps up to 3.500 in and 1/8 in steps from 3.500 to 6.000 in. An odd length code above 56 (for example 57) or a code above 96 is not a NAS1351 length.

Length is measured from under the head and has a plus tolerance of zero. A 1/4-28 x 1 screw (NAS1351-4-16) is 0.970 to 1.000 in long.

How much is threaded

Lt is the minimum full thread length. From size 3 (No. 10) up it is close to 2D + 0.50 in, and exactly that from 1/4 in up (1.000 in for 1/4-28). The numbered sizes are rounded to .500 to .875 in. Screws shorter than Lt are threaded as close to the head as practicable. Longer screws have the body and grip lengths of ASME B18.3. NAS1351 does not print those, so the decoder does not show them.

Sources and Limits

Everything on this page comes from NAS1351 Revision 10 (revision date 10 March 2009, first issued April 1962), Aerospace Industries Association, sheets 1 to 5: Table I (dimensions), Table II (thread length, drilled holes, breaking load), Table III (lengths), the length tolerance, material, finish and code notes, and the examples.

Millimetre values and hex key fractions are converted here; they are not printed on the sheet. Inch values are as printed.

Check the current revision of NAS1351 before ordering.
Plain CRES and A286 numbers follow the alloy-steel form: NAS1351C3-8, NAS1351N3-8.
Procurement specification: FF-S-86. Dimensions not given on the sheet follow FF-S-86 Type VI and ASME B18.3.
Related: NAS1149 washer decoder, AN960 decoder.