How to Install Threaded Inserts in Wood: Drill Size by Type

A threaded insert needs a pilot hole sized for that exact insert, not just its bolt thread. For common wood inserts, a 1/4-20 thread calls for anything from 11/32 to 15/32 inch depending on the design, so no single drill size exists. Match the hole to your insert, drive it straight, and check the fit with a bolt. Below: maker-chart sizes by type, plus hardwood versus softwood.

How to Install Threaded Inserts in Wood: Step by Step

These six steps work for any wood insert. The drill size and the driving tool change with the insert type, and the sections below cover both.

1. Pick the insert by its internal thread. The inside thread has to match your bolt, so a 1/4-20 bolt needs a 1/4-20 insert. Rockler’s guide points out that you choose an insert by the thread on the inside of the hardware, not the outside.
2. Look up the drill size for that exact insert. Use the size printed on the package or the maker’s chart in the next section. Charts from different makers are not interchangeable.
3. Test the hole in scrap first. Rockler’s pilot hole guide says hole size can vary with the nature and grain of the wood and recommends a trial installation on a scrap piece.
4. Drill the pilot hole straight. A drill press or a guide block keeps the hole square. Drill a little deeper than the insert is long so the bolt tip never bottoms out, and bore a shallow recess first if a flanged insert needs to sit flush.
5. Drive the insert in. Thread it onto your driver (a hex key, or a bolt locked with jam nuts) and turn it in steadily, keeping it square to the surface. Do not force it past the point where it sits flush.
6. Check the fit with the bolt. Thread the bolt in by hand. It should turn freely and then snug up without wobble. E-Z LOK warns that a hole that is too small causes threading difficulties or damage, while one that is too large will not hold the insert.

Threaded Insert Drill Size Chart: What the Makers Specify

Here is the part most guides skip: the drill size follows the insert’s outside design, so two inserts with the same 1/4-20 thread can need holes that differ by 1/8 inch. The tables below are copied from two maker documents, E-Z LOK’s wood drill chart and Rockler’s pilot hole guide. Use the chart for the brand printed on your package.

E-Z LOK Drill Sizes by Internal Thread

Internal thread	Knife-thread brass insert	Hex-drive insert
#8-32	17/64″	15/64″
#10-24	25/64″	9/32″
1/4-20	25/64″	23/64″
5/16-18	33/64″	7/16″
3/8-16	33/64″	15/32″ (flanged only)
M6	25/64″	23/64″
M8	33/64″	7/16″
M10	33/64″	15/32″ (flanged only)

Rockler Pilot Hole Sizes: Softwood vs. Hardwood

Internal thread	Insert	Pilot hole, softwood	Pilot hole, hardwood
8-32	Steel	17/64″	9/32″
1/4-20	Steel	29/64″	15/32″
1/4-20	Brass	3/8″	13/32″
1/4-20	Hex drive	11/32″	13/32″
5/16-18	Steel	29/64″	15/32″
5/16-18	Brass	1/2″	9/16″
5/16-18	Hex drive	7/16″	1/2″
3/8-16	Steel	1/2″	9/16″
3/8-16	Brass	1/2″	9/16″

For the same 1/4-20 thread, these charts run from 11/32 inch (Rockler hex drive, softwood) to 15/32 inch (Rockler steel, hardwood), a spread of 1/8 inch. Source: Rockler pilot hole guide and E-Z LOK wood drill chart.

E-Z LOK also lists finserts (for example 21/64 inch for a two-fin 1/4-20), which are a separate family that this guide does not cover.

Why 5/16 Inch Is Too Small for a 1/4-20 Insert

Many online charts, including an earlier version of this page, list 5/16″ as the hole for a 1/4-20 insert. None of the maker charts above goes that small: the smallest 1/4-20 hole in them is 11/32″. A hole that undersized forces the insert to cut far more wood than it was designed for, and E-Z LOK says an undersized hole leads to threading difficulties or damage to the insert and the material.

Hardwood vs. Softwood Pilot Holes

Rockler’s chart shows that wood species matters less than insert type. Within one insert, the hardwood hole is only 1/64″ to 1/16″ larger than the softwood hole (steel 1/4-20 goes from 29/64″ to 15/32″, hex-drive 1/4-20 from 11/32″ to 13/32″). Switching insert type changes the hole by up to 1/8 inch. Pick the insert first, then adjust for species.

E-Z LOK handles species differently: its chart gives one size per part number, and the company sells different products by wood hardness. Rockler’s catalog labels its E-Z Hex flanged insert as a softwood insert and its E-Z Knife brass insert as a hardwood insert, and Rockler’s guide says tapered or flanged inserts are best in softwood. The guide’s author also says he sometimes drills a slightly smaller hole in softer woods such as basswood, which makes driving harder but holds better. That is one maker-side preference, not a rule, so test it in scrap. Woodcraft states its E-Z Knife 1/4-20 kit is made for hardwood or softwood but not recommended for plywood.

Types of Threaded Inserts for Wood and How They Hold

The insert type decides both the drill size and how the joint behaves when something pulls on it. Rockler rates the common types as relative strengths, not pull-out numbers, and neither document we read gives pull-out figures, so we do not quote any. For a deeper look at choosing between types, see our guide on how to use thread inserts.

Type	How it goes in	Shear	Tension	Compression
Classic T-nut	Hammered in	Strong	Weak	Strong
Screw-on T-nut	Tapped in, then fixed with three screws	Strong	Moderate	Strong
Barrel-shaped insert	Bolt and jam nuts, or threaded rod	Strong	Strong	Strong
Flanged or tapered insert	Hex key (many have a hex socket)	Strong	Strong	Strong

Tension is the one that separates them. If the joint will be pulled apart, such as a table leg that carries weight and gets bumped, a classic T-nut is the weak choice. Rockler adds that a barrel insert threads in flush with the surface, while flanged inserts and T-nuts sit slightly proud unless you drill a recess for the flange first.

4 Ways to Drive a Threaded Insert

• Hex key: Hex-drive flanged inserts have a socket built in, so an Allen wrench drives them. Rockler says these tapered inserts are the ones best used in softwood.
• Bolt and jam nuts: Run a bolt with the same thread as the insert, add a few nuts so only a little thread shows, thread the insert on, and turn it into the hole with a ratchet or wrench.
• Threaded rod in a drill: Chuck a threaded rod with a nut against the chuck, thread the insert on, and drive it. Run the drill slowly and keep it square. This suits jobs with many inserts.
• Drill press turned by hand: Use the same rod setup in a drill press but leave the power off and turn the chuck by hand. Rockler recommends this when inserts will not start square to the surface.

E-Z LOK also sells a dedicated mandrel tool for its 1/4-20 knife-thread inserts (part DT-4) with a hex shaft that fits power drills and drill presses, which is worth having if you are installing many of one size.

What about the screwdriver slot on top of many inserts? Rockler says driving an insert with a screwdriver is asking for trouble and that the slot is better suited to taking an insert out. Some retailer listings do say a screwdriver works. Our advice: use the slot to back out a crooked insert, and drive with a proper tool.

Common Mistakes When Installing Threaded Inserts in Wood

• Drilling to the bolt size or a generic chart instead of the number for your insert, the same error as the 5/16 inch myth above.
• Mixing charts: Rockler and E-Z LOK list different holes for the same 1/4-20 thread because their inserts are built differently.
• Skipping the scrap test. Grain and density shift the right hole size, which is why Rockler recommends a trial piece.
• Forgetting the recess. A flanged insert and a T-nut sit proud of the surface unless you bore a recess for the flange.
• Choosing the insert by outside size. Select by internal thread so the bolt, thumb screw or knob you already own will fit.
• Drilling the hole crooked. A drill press or guide block avoids an insert that leans and cross-threads the bolt.

For more hole-drilling basics, browse our drilling guides.

When a Threaded Insert Is the Wrong Fastener

Inserts earn their place where a joint must come apart repeatedly. If the part is permanent, a plain wood screw is simpler, and how far a screw should go into wood covers depth. MDF behaves differently from solid wood, so check the MDF pilot-hole sizes before drilling a panel. For heavy post connections, see how to connect 4×4 wood, and for helical or key-locking inserts in metal and plastic, read how to install thread inserts.

Threaded Insert Questions Answered

What drill size do I need for a 1/4-20 threaded insert in wood?

It depends on the insert type. E-Z LOK lists 25/64 inch for its knife-thread brass insert and 23/64 inch for its hex-drive insert. Rockler lists 29/64 to 15/32 inch for steel, 3/8 to 13/32 inch for brass, and 11/32 to 13/32 inch for hex drive, softwood to hardwood. Always use the size on your package.

What drill size do I need for an M8 wood insert?

E-Z LOK lists 33/64 inch for its M8-1.25 knife-thread insert and 7/16 inch for its M8-1.25 hex-drive insert. Rockler’s guide only covers inch sizes, so for any other metric brand use the number printed on that package.

Do hardwood and softwood need different hole sizes for threaded inserts?

Slightly. In Rockler’s chart the hardwood hole is 1/64 to 1/16 inch larger than the softwood hole for the same insert. The insert type matters more, since it can change the hole by up to 1/8 inch for the same thread. Test in scrap first.

Can you install a threaded insert with a screwdriver?

You can try, but Rockler warns that driving an insert with a screwdriver is asking for trouble and says the slot is better for removing an insert. A hex key, or a bolt locked with jam nuts, drives it straighter.

Can you install a threaded insert without drilling a hole?

No. Every insert in the E-Z LOK and Rockler charts lists a drill size, so a pilot hole is part of the installation. Test the hole in scrap, then drive the insert flush.

The short version: find your insert type, use that maker’s drill size, test in scrap, and drive it in square. That one habit prevents most failed inserts.