Lag Bolt sizes vary by strength, material and head shape. Even within these broad categories, however, there are significant differences in finish, materials and head shape that can make a difference to the performance of a lag bolt. The most noticeable differences between lag bolts come in the form of their heads and threads. Hex lag bolts have six-sided heads that can be installed using wrenches or hex drivers and have coarse threads that create a type of friction joint, which helps them to maintain their grip in wood materials. Square lag bolts, on the other hand, have flat heads that can be installed with an impact driver and have fine threads that are less likely to damage the wood materials when screwing in the fastener.
Regardless of the head shape, there is one thing that all lag bolts have in common: they don’t require nuts. While a hex lag bolt can be used as a self-tapping screw, lag screws must be pre-drilled to ensure that their head is securely fastened to the material being secured. A lag screw that is installed correctly can hold up to nine times more weight than a basic nail that is hammered into a stud.
The first step to installing a hex or square lag screw is to clamp together the two materials that are being fastened. A power drill can be used to create a preexisting hole through which the lag screw will be installed. The hole should be sized to match the diameter of the lag bolt, which can be found on a hex or square head lag bolt chart. The lag bolt body, which is the unthreaded portion of the shank between the head and the threaded section, should have a pilot hole that matches the size of the hex or square lag screw (see a table of Species Groups for Sawn Lumber for a list of recommended pilot holes).
A stepped pilot hole is also necessary for the threaded portion of the hex or square lag bolt. This size varies depending on the bolt size and density of the wood and can be determined by looking at a hex or square lag bolt chart. A pilot hole that is too small may cause the screw to break during installation, while a pilot hole that is too large may reduce its withdrawal resistance.
Once the hex or square head lag bolt is properly aligned with the material it is being fastened to, the bolt can be installed by turning it with a wrench or hex driver. For most applications, the bolt should be at least twice as long as the thickness of the attached material. Ideally, the bolt will be a little longer than this in order to ensure that it can penetrate both the material and into the surrounding wood. If you’re unsure what size of hex or square lag bolt is best for your application, contact the team at Old West Iron for help finding the perfect fastener for your next project.