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Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and height, pilot holes + envelope models

Name: Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and height, pilot holes + envelope models dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

What are the head size and pilot hole for a 3/8 lag screw? · Standard part · basis: AFT Fasteners' ASME B18.2.1 table with Engineers Edge, Fastenermart and McMaster-Carr as cross-checks; pilot holes from Fastenermart (by wood group) and from a Portland Bolt / Haydon table; five envelope models as files

Also called: lag screw head size, 3/8 lag screw pilot hole, 1/4 lag screw wrench size, lag bolt dimensions chart, 5/16 lag screw head width, 1/2 lag screw pilot hole hardwood, lag screw head height, lag screw threads per inch, 7/16 lag screw head size, lag screw thread length, b18.2.1 lag screw

Exact dimensions. Every file is drawn at true size with no tolerance added. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your process's clearance before making the part. Mating features

length: 22.0 mm
width: 19.0 mm
height: 84.9 mm

Notes

Dimensions are inches, hex lag screws (the AFT table cites ASME B18.2.1-1996), 1/4 to 1/2: threads per inch; shank diameter E max / min; width across flats F (basic, then max / min); width across corners G max / min; head height H (basic, then max / min). AFT's flattened table was read by assigning columns from its layout, so check the standard if a figure matters. The same sizes agree with Fastenermart (F 7/16, 1/2, 9/16, 5/8, 3/4; H 11/64, 7/32, 1/4, 19/64, 11/32) and with McMaster-Carr's head heights (0.172, 0.219, 0.25, 0.344 for 1/4, 5/16, 3/8, 1/2).
One disagreement: Engineers Edge prints a width across flats of 3/8 for the 1/4 lag screw, against 7/16 in AFT and Fastenermart (and 0.425–0.438 in AFT's max / min). Both are shown; the model uses 7/16. Thread length is a rule, not a column: AFT gives a minimum of half the screw length plus 0.50 in, capped at 5.00 in; Engineers Edge gives the same rule capped at 6.00 in. Both caps are as printed and they differ. Screws too short for the rule are threaded close to the head. AFT also prints length tolerance ±0.12 in to 6 in and ±0.25 in over 6 in, and a minimum shoulder length S (0.094, 0.125, 0.125, 0.156, 0.156 in).
Pilot holes are not part of B18.2.1. Fastenermart: shank hole (equal to the nominal size), then threaded-portion pilot for softwood groups III and IV, medium wood group II and hardwood group I; the page calls them pilot, not lead holes, and does not explain a dagger on 7/16. Portland Bolt (credited to OBEC) and Haydon Bolts print one threaded-portion pilot per size, 15/64, 9/32, 5/16 for 3/8, 7/16, 1/2 (no 1/4 or 5/16 row); those equal Fastenermart's medium-wood column. Treat all as a guide, as Haydon does, and follow your project engineer or the screw maker.
Files are Xem-drawn envelopes: a hexagon at the basic width across flats and the basic height (corners are F / cos 30°, which AFT's G max matches, e.g. 0.505 in for 1/4), on a plain cylinder at the nominal diameter; no thread, no chamfer, no washer face, no point. The 3 in length under the head is an example chosen by Xem for all five files, not a published length.
McMaster-Carr sells lag screws with per-part STEP (it files them under hex head wood screws); the page is linked, not copied. Wood screws: wood-screws-flat-head-inch.

Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and… size chart

size (in)	TPI	shank E max / min (in)	across flats F, basic (AFT)	F max / min (AFT, in)	across flats F (Engineers Edge)	across corners G max / min (in)	head height H, basic (AFT)	H max / min (AFT, in)	shank hole (in)	pilot, softwood III & IV (in)	pilot, medium wood II (in)	pilot, hardwood I (in)	pilot (Portland Bolt / Haydon, in)
1/4	10	.260 / .237	7/16	.438 / .425	3/8	.505 / .484	11/64	.188 / .150	1/4	3/32	5/32	3/16	—
5/16	9	.324 / .298	1/2	.500 / .484	1/2	.577 / .552	7/32	.235 / .195	5/16	9/64	3/16	13/64	—
3/8	7	.388 / .360	9/16	.562 / .544	9/16	.650 / .620	1/4	.268 / .226	3/8	11/64	15/64	1/4	15/64
7/16	7	.452 / .421	5/8	.625 / .603	5/8	.722 / .687	19/64	.316 / .272	7/16	13/64	9/32	19/64	9/32
1/2	6	.515 / .482	3/4	.750 / .725	3/4	.866 / .826	11/32	.364 / .302	1/2	15/64	5/16	11/32	5/16

Mating features

Drawn at the exact size. Open press, slide, run and socket features by your process's clearance (tolerances by process); leave designed gaps and clamp fits as drawn.

feature	exact	mates with	fit	files
shank	1/4, 5/16, 3/8, 7/16, 1/2 in	shank hole = nominal size; pilot softwood III & IV 3/32, 9/64, 11/64, 13/64, 15/64; medium wood II 5/32, 3/16, 15/64, 9/32, 5/16; hardwood I 3/16, 13/64, 1/4, 19/64, 11/32 in (Fastenermart)	designed	lag-screw-*
head	hex across flats × height: 7/16 × 11/64 (1/4), 1/2 × 7/32, 9/16 × 1/4, 5/8 × 19/64, 3/4 × 11/32 in	hex pocket or socket space	slide	lag-screw-*

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

AFT Fasteners	Lag screws dimensions and mechanical properties (ASME B18.2.1-1996): E shank, F width across flats, G across corners, H head height, S shoulder length, thread-length rule	web	Publisher's page; dimensions are facts
Engineers Edge	Standard hex lag screws: shank diameter E, head width across flats F, head height H, threads per inch, thread-length rule	web	Publisher's page; dimensions are facts
Fastenermart	Lag screws: pilot hole sizes by wood group (shank hole, softwood groups III & IV, medium group II, hardwood group I) and head dimensions	web	Publisher's page; dimensions are facts
Portland Bolt (table credited to OBEC Consulting Engineers)	Pilot hole sizes for lag screws: unthreaded shank portion and threaded-portion pilot, 3/8 to 1/2 in	web	Distributor table; numbers only
Haydon Bolts	Lag bolt pilot hole diameter: shank (unthreaded) and pilot hole, 3/8 to 1/2 in (same values as Portland Bolt's table)	web	Maker page; numbers only
McMaster-Carr	Stainless steel hex head lag screws (McMaster files these as hex head wood screws) (free STEP/IGES per part, for your own design use; no redistribution)	STEP, IGES, PDF	McMaster-Carr site terms: personal design use, no redistribution verify

Free files

★ Free files 5 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

lag-screw-1-4-hex-3in-envelope	1/4 hex lag screw envelope: hex head 7/16 in across flats (basic) × 11/64 in high (basic) on a plain Ø6.35 mm (1/4 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point	STEP · STL
lag-screw-5-16-hex-3in-envelope	5/16 hex lag screw envelope: hex head 1/2 in across flats (basic) × 7/32 in high (basic) on a plain Ø7.94 mm (5/16 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point	STEP · STL
lag-screw-3-8-hex-3in-envelope	3/8 hex lag screw envelope: hex head 9/16 in across flats (basic) × 1/4 in high (basic) on a plain Ø9.52 mm (3/8 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point	STEP · STL
lag-screw-7-16-hex-3in-envelope	7/16 hex lag screw envelope: hex head 5/8 in across flats (basic) × 19/64 in high (basic) on a plain Ø11.11 mm (7/16 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point	STEP · STL
lag-screw-1-2-hex-3in-envelope	1/2 hex lag screw envelope: hex head 3/4 in across flats (basic) × 11/32 in high (basic) on a plain Ø12.70 mm (1/2 in nominal) shank, example length 3 in (76.2 mm) under the head (Xem's choice); no thread, chamfer, washer face or point	STEP · STL

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Plain text: sheet.txt · JSON: /api/geometry?object=lag-screws-hex-inch

Hex lag screws, 1/4 to 1/2 in (ASME B18.2.1): shank, head width and height, pilot holes + envelope models, isometric render of a generated part — Render of lag-screw-3-8-hex-3in-envelope from the free STEP; not to scale.

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Questions

What size wrench for a 3/8 lag screw?

9/16 in across flats (basic) in AFT's B18.2.1 table, Engineers Edge and Fastenermart; head height 1/4 in.

What pilot hole for a 1/2 lag screw?

Fastenermart: 15/64 softwood (groups III & IV), 5/16 medium wood, 11/32 hardwood; Portland Bolt and Haydon print 5/16 for the threaded portion. The shank hole is 1/2 in.

How much of a lag screw is threaded?

At least half the length plus 0.50 in, capped at 5.00 in (AFT) or 6.00 in (Engineers Edge); short screws are threaded close to the head.