What size pilot hole do I need for a 3/8 lag bolt?

Asked By: Lys Marckhoff | Last Updated: 8th May, 2020
Category: hobbies and interests woodworking
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Pilot Hole Diameter for Lag Bolts
Bolt Dia Shank (Unthreaded) Pilot Hole Dia
3/8 3/8 15/64
7/16 7/16 9/32
1/2 1/2 5/16
9/16 9/16 23/64

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Hereof, how big should a pilot hole be for a lag bolt?

3/8"

Furthermore, what size should pilot holes be? For most hardwoods, the pilot hole should be at least as large as the screw's minor diameter. If the screw has deep threads, or the wood is very hard, the pilot hole should be another 1/64-in. larger than the minor diameter. For softer woods, the pilot hole can be 1/64-in.

Moreover, how do you drill a pilot hole for a lag bolt?

When they're lined up, clamp them together so they stay in place. Then, using a bit with a slightly smaller diameter than your lag screw, drill a hole all the way through the materials where you want the screw to go. Since lag screws have hex heads, you can't use a regular screwdriver to tighten them.

What is the shear strength of a 3/8 lag bolt?

Values also are based on lag screw bending yield strength values of 45,000 psi for screws of 3/8 inch and larger diameter, 60,000 psi for 5/16 inch diameter and 70,000 psi for 1/4 inch diame- ter.

39 Related Question Answers Found

Should you drill pilot holes for lag bolts?

Pilot or lead holes are typically used to ease the installation of large diameter lag bolts. Full points are not needed for large diameter lag bolts, because pilot holes should be drilled to insure the heads do not break when torque is applied.

What size hole do you drill for a 3/8 lag bolt?

Pilot Hole Diameter for Lag Bolts
Bolt Dia Shank (Unthreaded) Pilot Hole Dia
3/8 3/8 15/64
7/16 7/16 9/32
1/2 1/2 5/16
9/16 9/16 23/64

How big should a pilot hole be for a 5/16 lag bolt?

For a 5/16-in. lag bolt, that works out to be about 7/32 in. A simpler approach that I've always used is just choosing a drill bit that is the same diameter as the solid part of the bolt minus the threads. The threads are what bite into the wood to hold the screw in place.

How do I know what size drill bit to use?

Hold the shaft of the drill bit up in front of the screw. You should only be able to see the threads of the screw (and maybe a bit of the screw). If you can't see the threads the drill bit is too big. If you can see too much of the screw, the bit is too small.

What is the difference between a lag bolt and a lag screw?


They differ from lag screws because they thread their way through the wood as they're screwed in, whereas lag screws require you to drill a hole first. Also, lag screws take advantage of a nut on one side to help hold things together; the tight fit of the wood screw negates the need for a nut.

What is a pilot hole in drilling?

Pilot holes for screws
By drilling a small pilot hole into the material, into which a screw is then driven, less wedging takes place, thereby reducing the likelihood of the material being split. A hole drilled for tapping machine screws or bolt threads in metal or plastic may also be referred to as a pilot hole.

Should I use washers with lag bolts?

No. No nuts are required to install a lag bolt. A bolt is installed into a through pilot hole that is larger in diameter than the threads. A washer is used for both screws and bolts to increase the surface area in contact with the wood.

How strong is a lag screw?

A single 1/4" Grade 5 lag bolt, in a configuration like this, will fail at roughly 13,000 lbs. Even 1/8" lag screws (well down into "numbered" screw sizes) will have a shear strength of over 3,000 pounds.

How much weight can a 5/16 lag screw hold?

If a lag screw with a 5/16-inch shaft is installed correctly, it can withstand pull-out, or withdrawal, of up to 212 pounds in close-grain hem, fir and redwood, and up to 266 pounds in larch Douglas fir, according to information from the American Wood Council.

What is a shank pilot hole?


A pilot hole works in conjunction with a screw-shank clearance hole. A pilot hole is a small-diameter hole bored into the bottom board. Its purpose is to provide a path for the screw to enter the board without splitting it.

What size pilot hole do I need for a number 10 screw?

Wood Screw Pilot Hole Sizes
Screw Size Hard Wood
Tapered Bit Straight Bit
#8 11/64" 1/8"
#9 3/16" 9/64"
#10 13/64" 9/64"

How do I remove a lag screw?

  1. Inspect the lag bolt head to determine the correct tool to use to remove it.
  2. Apply the tool to the bolt.
  3. Turn the bolt counterclockwise slightly to determine the amount of torque you'll need to remove the lag bolt.
  4. Continue to turn the bolt to the left (counterclockwise) until it is out of the material.

How do you Countsink a lag bolt?

If you don't want to drill out the other side, place a piece of tape on the drill bit to indicate a stopping point 1/4 inch before breaking through. Insert the lag bolt into the hole. Use a 3/8-inch socket and ratchet to drive the bolt flush into the countersunk hole. Remove the clamps.

How far should a lag screw go into wood?

Ledger Board Fastener Spacing Schedule
Joist Span Fastener Spacing, On Center
1/2" Lag Bolt SDWS Timber Screws
0'-6' 30" 12"
6'-8' 23" 9"
8'-10' 18" 7"

How long should screws be?


The most important factor in screw selection is length. The general rule of thumb is that the screw should enter at least half the thickness of the bottom material, e.g. 3/4″ into a 2 x 4.

Do deck screws need pilot holes?

As for those holes -- just about any pilot hole is just about as good as any other, so long as the screw head doesn't fall in. And note that your drill-driver is strong enough to drive those screws without any pilot holes -- though your wrists might get a bit tired. New deck, nail or just screw it!

What size hole do you drill for a 1/4 Tapcon?

The Tapcon drill bit that is used depends on the diameter of the Tapcon screw being installed. The 3/16” Tapcon requires the use of 5/32” diameter bit, and the 1/4Tapcon requires a 3/16” diameter bit.