Lag Bolt Pilot Hole Size
As with any type of screw or bolt, it’s important to use the correct sized drill bit for each installation. This allows the screw or bolt to be driven in and secured properly without damaging it.
Lag bolts (also known as lag screws) are designed for a specific purpose, connecting wood together without weakening the structure of the assembly. This is accomplished by using a large diameter shaft and threaded part which engages into the second piece of wood and creates a strong hold.
The proper hole size for a lag bolt depends on the material it will be used to fasten together and the thickness of the materials. In general, a lag bolt should penetrate at least 1.5 times the thickness of the material it will be joining.
To determine the correct hole size for a lag bolt, I will first make a hole with a twist drill bit marked with tape at the proper depth matching the unthreaded shank length of the screw (which does not include the threaded portion). Next, I will mark the location with a pencil or marker where the threads should be cut with a hex wrench.
Lastly, I will make the final hole with a hex drill bit matched to the threaded end of the screw. With all three holes drilled, the hex head of the lag bolt can now be installed and tightened with a socket wrench or ratchet. pilot hole for 5/16 lag screw