Square lag screws are used in wood construction and are often ordered for aesthetic purposes and provide a rustic look. Hex lag screws are more readily available in the marketplace as a standard stock item.

Manufacturing

Portland Bolt manufactures square lag screws from 1/2″ diameter through 1-1/2″ diameter in most ASTM, AASHTO, and SAE specifications. Special tooling can be created to produce nonstandard head dimensions or bolt heads with your company’s name and/or logo.

Unless otherwise specified, nonstandard lag screws manufactured by Portland Bolt will possess a semi-cone point rather than a gimlet or cone point. A full point is unnecessary since a pilot hole nearly as large as the root diameter of the lag screw will need to be drilled prior to installation. Additionally, a gimlet point is designed for small diameter screws so they can be installed without a pilot hole. Lag screws in diameters listed in the above table do not require a gimlet point since too much torque would be required to install the large screws without drilling a pilot hole first. Without a pilot hole, the heads of the screws will often break before the lag is properly installed.

Standard Thread Length

The minimum thread length for square lag screws shall be equal to half the nominal screw length plus 1/2″ or 6″, whichever is less. One should consider longer than standard thread lengths for lag screws which are extremely long.

Square Lag Dimensions

Click table to show drawing
Bolt Diameter D F H Threads Per Inch Thread Dimension
Body Diameter Width Across Flats Head Height Pitch TF TD TR
Max Min Flat At Root Depth of Thread Root Diameter
1⁄2 0.515 0.482 3⁄4 11⁄32 6 0.167 0.072 0.064 0.371
5⁄8 0.642 0.605 15⁄16 27⁄64 5 0.200 0.086 0.077 0.471
3⁄4 0.768 0.729 11⁄8 1⁄2 41⁄2 0.222 0.096 0.085 0.579
7⁄8 0.895 0.852 15⁄16 37⁄64 4 0.250 0.108 0.096 0.683
1 1.022 0.976 11⁄2 43⁄64 31⁄2 0.286 0.123 0.110 0.780
11⁄8 1.149 1.098 111⁄16 3⁄4 31⁄4 0.308 0.133 0.119 0.887
11⁄4 1.277 1.223 17⁄8 27⁄32 31⁄4 0.308 0.133 0.119 1.012
13⁄8 1.404 1.346 21⁄16 29⁄32 3* 0.333 0.143 0.128 1.119
11⁄2 1.531 1.469 21⁄4 1 3* 0.333 0.143 0.128 1.244
Dimensions per ASME B18.2.1 1996
* ASME only provides dimensional data for lag screws through 1¼” diameter. Lag screws over 1¼” diameter use custom thread pitches.