Part wood screw & part bolt—lag screws are a beast of their own. Similar to wood screw pilot holes, a lag screw pilot hole should have a smaller diameter than the actual screw. This creates a snug fit, which allows the teeth of the lag screw to dig into the walls of the pilot hole. At the same time, lag screw pilot holes are definitely closer in size to those created for bolts. Thus, you’ll likely need to use either a spade bit or a large twist bit to create them.In terms of the drill bit’s coating or material, it doesn’t really matter. Since the screw holes are so large, the drill bits won’t be prone to breaking. Thus, HSS drill bits and other affordable options are perfectly acceptable for drilling lag screw pilot holes. With that being said, larger bits also require more time to dissipate heat. To speed this up, clear the saw dust out of the drill bit’s flute between each stoke. This will ensure that the heat isn’t trapped against the metal.

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Pilot Hole Size for SoftwoodLag Screw DiameterPilot Hole Size for Hardwood

Lag Screws in Softwoods

When drilling into softwoods, it’s always worth trying a smaller sized bit. The lower density wood fibers allow for greater compression when receiving the screw. This results in an overall tighter fit and more secure fastener. There are a few exceptions to this like aromatic cedar and other hard softwoods. If you aren’t sure how if you should test a smaller drill bit, we have a chart with all the hardness ratings for North American softwoods. Though there’s no need to countersink your lag screw, we recommend smoothing the surface.

Lag Screws in Hardwoods

The most challenging part with hardwoods is driving in the actual lag screw. If you have a pack of nut setter bits, you’re in good shape…at least for the smaller sizes. For the other users out there that will be using a socket on a 1/4″ hex shank, it’s a different story. As the lag screw enters deeper into the pilot hole, the socket will likely ground out before the screw is fully inside. When you encounter this, stuff some small washers into the back of the socket. This will effectively make the socket shallower when it wraps around the lag screw’s head. Do be mindful that it is of course safer and quicker to use nut setter bits.

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