Use a No. 29 Hole—Unless Your 8-32 Tap Forms Rather Than Cuts

8-32 tap-drill size

Ray Kowalski August 28, 2026 · 12 min read

Quick answer: use a No. 29 drill for a standard 8-32 cutting tap

8-32 tap-drill size

Drill: No. 29

Diameter: 0.1360 inch

Metric equivalent: approximately 3.45 mm

Application: conventional 8-32 UNC cutting tap

These dimensions are listed together by 6G Tools.

For an ordinary 8-32 UNC internal thread made with a conventional cutting tap, drill the hole with a No. 29 bit. Independent tap-drill charts also identify No. 29 at 0.1360 inch for an 8-32 coarse thread, including the chart from American Fastener Technologies.

This is a general-purpose chart selection, not a mandatory hole size for every application. Published tables commonly base the recommendation on approximately 75% thread depth—more precisely, a percentage of full thread height rather than the axial length over which two threaded parts engage. Bolt Depot’s US tap-and-drill table lists No. 29 for 8-32 and states that its drill sizes are for 75% thread depth.

Two distinctions matter before drilling:

A forming or roll tap is not a conventional cutting tap. It displaces material instead of cutting it and generally requires a larger, manufacturer-specified starting hole.
A clearance hole is not a tap-drill hole. If the screw only needs to pass through the component, the hole must be larger than the screw and selected according to the required fit.

Therefore, No. 29 is the direct answer when the hole will subsequently be threaded with a conventional 8-32 cutting tap and no drawing, tooling specification, or other controlled requirement calls for a different diameter.

What 8-32, No. 29, and 0.1360 inch mean

The three labels describe different parts of the specification:

8-32 identifies the intended thread.
No. 29 identifies a size in the numbered-drill system.
0.1360 inch identifies the nominal diameter of that drill.

In 8-32, the “8” means a No. 8 machine-screw thread, while “32” means 32 threads per inch. The size is commonly classified as 8-32 UNC, a coarse machine-screw thread.

The listed major diameter of an 8-32 thread is 0.1640 inch, or about 4.17 mm, and its pitch is approximately 0.794 mm. The same dimensional table lists a 0.1360-inch tap-drill diameter for the thread (MISUMI).

Pitch and drill diameter are not interchangeable:

Pitch is the distance from one thread crest to the next.
Major diameter is the nominal outside diameter associated with the screw thread.
Tap-drill diameter is the diameter of the hole made before tapping.

The tap-drill hole is smaller than the thread’s major diameter by design. Material must remain in the hole wall for the cutting tap to remove and shape into the internal thread. If the starting hole were drilled to a clearance diameter instead, too little material would remain to create the intended thread form.

“No. 29” belongs to the numbered-drill system. It does not mean 29/64 inch, 0.029 inch, or 29 mm. Numbered drills use assigned designations whose diameters must be checked in a drill index or size chart.

For this operation, the following descriptions refer to the same preferred nominal drill size:

No. 29 drill
0.1360-inch drill
Approximately 3.45 mm drill

The metric figure is a rounded conversion. When a drawing specifies an inch diameter or a controlled tolerance, use that requirement rather than assuming that every nearby metric bit is interchangeable.

The complete thread designation matters as well. “No. 8” alone does not identify the pitch. A different number of threads per inch can produce a different tap-drill recommendation, so confirm the complete designation before selecting the drill.

Why the standard chart answer is based on about 75% thread engagement

Tap-drill charts must balance two dimensions: the hole has to be small enough to leave material for the internal thread, but it must also match the intended thread geometry and process requirement.

For a cutting tap, the tool removes material from the drilled hole’s wall. General-purpose charts commonly select a drill intended to leave approximately 75% of full thread height. Lincoln Machine’s chart, for example, recommends No. 29 at 0.1360 inch for 8-32 NC and expressly describes its recommendations as being for 75% depth of thread (Lincoln Machine).

This percentage should not be confused with the axial engagement length between a screw and a tapped component. A hole can have a particular percentage of thread height while still having a short or long threaded length. Those are separate design considerations.

The dimensional tradeoff is straightforward:

A larger starting hole leaves less material for the tap, so the resulting percentage of thread height decreases.
A smaller starting hole leaves more material for the tap, so the resulting percentage of thread height increases.

That relationship explains why a nearby larger drill may be considered as a dimensional substitute but should not be described as an exact equivalent. It also explains why choosing a smaller drill is not automatically an improvement. A smaller hole changes the amount of material the cutting tap must remove and may fall outside the permitted hole specification.

The 75% figure is a conventional chart basis, not a guarantee of strength, tap life, tapping torque, fit, or manufacturing success. The supplied evidence does not establish those outcomes, and it does not support assigning an exact resulting thread percentage to every nearby drill size.

A general chart also cannot override controlled requirements. The appropriate starting-hole diameter may instead be governed by:

a drawing or process sheet;
a permitted finished-hole range;
a tooling specification;
a required thread class or fit;
the tap manufacturer’s technical data;
inspection requirements; or
an applicable engineering or industry standard.

For ordinary work with a conventional cutting tap, the No. 29 chart selection is the practical default. For tightly controlled work, acceptance should be based on the specified hole and thread requirements—not solely on the size marked on the drill.

If No. 29 is unavailable: compare 3.5 mm and 9/64 inch

A 3.5 mm or 9/64-inch bit is sometimes discussed when a No. 29 bit is unavailable. Neither is an exact match. Both are larger than the preferred drill, so both leave less material for the cutting tap.

The nominal comparison is as follows; the substitute dimensions are also listed in this 8-32 drill-size guide:

Drill choice	Diameter, inches	Diameter, metric	Difference from No. 29	Status
No. 29	0.1360 in	≈3.45 mm	—	Preferred chart size
3.5 mm	≈0.1378 in	3.5 mm	≈0.0018 in larger	Close, larger substitute
9/64 in	0.1406 in	≈3.57 mm	≈0.0046 in larger	Larger substitute

The differences follow from the nominal diameters:

A 3.5 mm drill converts to approximately 0.1378 inch. Compared with 0.1360 inch, it is approximately 0.0018 inch larger.
A 9/64-inch drill equals 0.140625 inch, commonly rounded to 0.1406 inch. Compared with 0.1360 inch, it is approximately 0.0046 inch larger.

Of the two alternatives, 3.5 mm is dimensionally closer to No. 29. Its nominal difference is less than half the difference between 9/64 inch and No. 29. That makes it the closer substitute by diameter, but not another name for the same drill.

Because both substitutes make a larger hole, each produces less thread height than the preferred No. 29 drill. The evidence supports the direction of that change but does not establish an exact thread percentage for either alternative.

How to decide whether a substitute is acceptable

Do not assume that a nearby drill is acceptable merely because a tap can be run into the resulting hole. Instead, determine whether the larger hole diameter is permitted for the actual part.

Use the specified drill or verify the allowed diameter when:

the drawing calls out a starting-hole dimension;
the hole has a defined tolerance;
the thread form or fit will be inspected;
the process documentation identifies a particular drill;
the tap manufacturer specifies a hole range; or
the part is governed by engineering or regulatory requirements.

If none of those controls applies, a larger substitute is still a deliberate departure from the standard chart selection. Record the decision accurately: the drill is larger, and the resulting thread height will be lower. Do not label 3.5 mm or 9/64 inch as an exact No. 29 equivalent.

For repeated 8-32 work, obtaining a No. 29 drill removes the substitution question and follows the general-purpose chart recommendation directly.

Cutting taps and forming taps do not use the same default hole

The No. 29 recommendation applies to a conventional cutting tap. Its cutting edges remove material from the wall of the drilled hole to create the internal thread.

A forming tap, also called a roll tap or thread-forming tap, works differently. It displaces the workpiece material into the thread form rather than cutting material away. Because the process requires a different amount and distribution of material, the starting hole is generally larger than the cutting-tap hole.

For orientation, one third-party guide gives an approximate range of 0.1405 to 0.1440 inch for an 8-32 forming tap, while warning readers to use the specific tap manufacturer’s recommendation (StyleCNC). That range should not be treated as a universal specification.

The manufacturer’s data must control because the appropriate hole can vary with the particular tap geometry and application. A general cutting-tap chart does not contain enough information to select the starting hole for every forming tap.

Before drilling, inspect the tool, packaging, catalog entry, or technical sheet for wording such as:

cutting tap
forming tap
form tap
roll tap
thread-forming tap

Do not identify the tap type solely from the 8-32 marking. That marking identifies the thread the tool is intended to create; it does not necessarily state whether the tool cuts or forms the material.

The decision is therefore simple:

If the tool is a conventional cutting tap and no other specification applies, use the standard No. 29 recommendation.
If it is a forming or roll tap, stop and obtain the starting-hole recommendation for that specific tap.

Material and machining conditions are reasons to consult the manufacturer’s documentation, but they do not establish one universal adjustment. Avoid unsupported rules such as always increasing or decreasing the hole in a particular material.

Tap-drill hole versus 8-32 clearance hole

A tap-drill hole and a clearance hole have different functions.

A tap-drill hole is intentionally smaller than the screw’s major diameter. After drilling, a tap creates internal threads in the wall of the hole.

A clearance hole is large enough for the screw to pass through without threading into that component. It must therefore be larger than the screw’s nominal major diameter, with additional clearance selected according to the intended fit.

Consider a typical two-component assembly:

The upper component has a clearance hole through which the screw passes.
The lower component has an undersized tap-drill hole.
The lower hole is tapped to create the internal thread.
Tightening the screw draws the upper component against the threaded lower component.

The same screw passes through both pieces, but the holes serve opposite purposes. The upper hole avoids thread engagement; the lower hole is specifically prepared to create it.

There is no single universal clearance-hole diameter for every 8-32 assembly. One published chart lists No. 18 at 0.1695 inch for a close fit and No. 16 at 0.1770 inch for a free fit. Other charts use different categories and values. For example, the ETSU-hosted tap and clearance chart lists No. 19 for its close-fit category and No. 15 for its non-structural category.

Those differences do not alter the tap-drill recommendation. They show that the proper clearance hole depends on the fit system being used.

Select a clearance-hole diameter from:

the part drawing;
the assembly specification;
the applicable standard; or
a chart whose close, normal, free, structural, or non-structural category matches the design intent.

The key error to avoid is drilling the component that must be tapped to a clearance-hole diameter. Doing so removes material needed to form the intended internal thread. If one component needs clearance and another needs tapping, identify the operations separately before drilling.

A decision checklist before drilling the hole

Use this sequence to confirm that the standard chart answer applies.

Confirm the complete thread designation. Verify that the required thread is 8-32 rather than simply “No. 8.” The pitch is part of the specification.
Determine the hole’s function. If the component must contain internal threads, prepare a tap-drill hole. If the screw only passes through it, choose a clearance hole.
Identify the tap type. Check whether the tool is a cutting tap or a forming or roll tap. Do not infer this from the thread designation alone.
Review controlling documentation. Check the drawing, process sheet, permitted hole diameter, thread requirement, and tap manufacturer’s data before relying on a general chart.
Choose the standard drill when appropriate. For an ordinary 8-32 UNC internal thread made with a conventional cutting tap, and with no overriding specification, select the chart-backed No. 29 drill.
Treat alternatives as substitutions. If No. 29 is unavailable, recognize that 3.5 mm and 9/64 inch are larger. Verify that the resulting diameter is permitted rather than treating either size as identical.
Use manufacturer data for a forming tap. Do not automatically apply the cutting-tap recommendation. Find the hole-size table or technical data for the specific forming tool.
Choose clearance holes by fit. When the screw only needs to pass through the part, use the drawing or the appropriate clearance-fit chart—not a tap-drill chart.
Drill before tapping. Once the correct starting diameter has been established, drill the hole before running the tap.

For critical, structural, safety-related, regulated, or tightly toleranced work, verify the selection against the controlled drawing, applicable standard, engineering requirements, and tool manufacturer’s specification. The ETSU-hosted reference likewise advises users to consult applicable industry or regulatory standards rather than treating a general chart as final authority.

Frequently asked questions

What diameter is a No. 29 drill bit?

A No. 29 drill has a nominal diameter of 0.1360 inch, approximately 3.45 mm. “No. 29” is a numbered-drill designation, not a fractional-inch or metric dimension (American Fastener Technologies).

Can I use a 3.5 mm drill instead of a No. 29 for an 8-32 tap?

It is a close but larger dimensional substitute, not an exact equivalent. Because it produces a larger starting hole, it leaves less material for the tap and lowers the resulting thread height.

Use the preferred drill when the hole diameter is controlled. Otherwise, verify that the larger resulting hole is permitted for the part rather than assuming interchangeability.

Is 9/64 inch the same as a No. 29 drill?

No. A 9/64-inch drill is larger than a No. 29 drill and is farther from the preferred diameter than a 3.5 mm bit. It may be discussed as a substitute, but it is not the same size and will leave less material for the cutting tap.

What drill size should I use for an 8-32 forming or roll tap?

Use the starting-hole size specified by the forming-tap manufacturer. Do not default to the cutting-tap recommendation.

Forming taps displace material instead of removing it, so they generally need a larger hole. The correct value depends on the particular tool and application rather than on the 8-32 designation alone.

Is the 8-32 tap-drill size the same as an 8-32 clearance-hole size?

No. A tap-drill hole is intentionally undersized so that internal threads can be created. A clearance hole is larger so the screw can pass through without threading into that component.

Clearance-hole values vary by fit category. Choose the specified close, normal, free, structural, or non-structural fit rather than using the tap-drill selection for both operations.

Bottom line

For an ordinary 8-32 UNC internal thread made with a conventional cutting tap, drill 0.1360 inch with a No. 29 bit, approximately 3.45 mm (Lincoln Machine).

Treat 3.5 mm and 9/64 inch as larger, lower-thread-height substitutes rather than exact equivalents. Before drilling, confirm that the operation requires a tapped hole rather than a clearance hole and that the tool cuts rather than forms the thread. Where a drawing, tolerance, manufacturer specification, engineering requirement, or applicable standard exists, that controlled requirement overrides the general chart answer.