What Drill Bit Size for a 3/8 Lag Bolt? (Size Guide)

In this article, I’ll help you to identify the appropriate drill bit size for your 3/8 lag bolt.

Thread-cutting or thread-forming screws require pilot holes to get started. As a contractor, I needed the proper drill bits to pre-drill holes to install tapping screws or lag bolts because using the right drill bit helps you fit your lag bolt snugly into whichever material you are drilling.

In general, for a 3/8 lag bolt, use a 21/64 inches drill bit to make the pilot hole. Using the drill bit, you should get a 0.3281 inches pilot hole size.

Check the detailed explanation and illustration below.

What Size Drill Bit for a 3/8 Lag Bolt – Getting Started

To install a lag bolt, drill a pilot hole with a drill bit first. For a 3/8 lag bolt, use a 21/64 inch drill bit to make the pilot hole – you should get a 0.3281 inches pilot hole size.

That’s very critical. If you use a smaller or larger drill bit to make the pilot hole, the lag bolt won’t fit snugly into the hole. You will have to re-drill another hole or switch the material.

The type of drill bit also matters depending on the wood you are drilling. For instance, hardwoods like mahogany require well-suited drill bits, while softwoods like cypress can be drilled with a regular drill bit. (1)

However, self-drilling screws do not require a drill bit. They can drill their own pilot holes as they advance into the material. Drill bits are required for other thread-cutting, tapping, thread-forming, or thread-rolling screws.

How to Choose the Appropriate Drill Bit for Pilot Holes?

Lucky for you, I have a simple trick to help you select the correct size drill bit from your drill bit set. You don’t have to understand any particular concept of drill bit or drill bit chat analysis to use this trick.

Follow these easy steps to choose the precise drill bit for making a hole for a 3/8 lag bolt.

Step 1: Get Your Drill Bit Set and The Lag Bolt

drill bit set and lag bolt

Keep the drill bit set and the 3/8 lag bolt close. Go ahead and describe the spot you want to drive the lag bolt through with a pencil, pen, or marker.

Step 2: Align the Largest Drill Bit on Top of The Lag Bolt

Now lift the 3/8 lag bolt close to your eye level and grab the largest drill bit from the drill bit set. (2)

Align the drill bit with the lag bolt by placing it horizontally on the 3/8 lag bolt – the drill bit should lay on top of the 3/8 lag bolt.

Step 3: View the Threads of The Lag Bolt Perpendicularly

Position your head well and look at the threads of the lag bolt.

If the threads are partially or completely eclipsed, proceed to the next, second-largest drill bit. Align it on top of the 3/8 lag bolt and check the behavior of the threads.

Step 4: Repeat Steps One Through Three

Keep aligning the drill bits progressively from the largest to the smallest until you find the perfect match.

What’s the Perfect Match?

If the drill bit doesn’t eclipse the threads of the lag bolt or expose the shaft/frame of the lag bolt — then it is the perfect drill bit size to drill a pilot hole for the lag bolt. In other words, the drill bit should flash with the shaft of your 3/8 lag bolt.

Once you have the correct size drill bit, you can pre-drill a hole for the lag bolt. I reiterate that you should not use a too-small or too-large drill bit to cut a pilot hole for the lag bolt; the bolt will not fit well, and the connection will be loose.

Take a look at some of our related articles below.

(1) softwoods – https://www.sciencedirect.com/topics/
(2) eye -https://www.webmd.com/eye-health/picture-of-the-eyes

Video References

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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.