Choose the Right Drill Bit for a 10-32 Hole

Use a No. 21 drill bit, measuring 0.1590 inch (4.04 mm), before tapping a 10-32 hole with a conventional cutting tap. Milwaukee pairs its 10-32 NF plug tap with a No. 21 drill, while Fastenal specifies the same drill for a Greenfield 10-32 UNF spiral-point tap (Milwaukee Tool; Fastenal). A forming tap or a hole intended to let the screw pass through requires a different diameter.

Choose what the 10-32 hole must do; the selector shows the corresponding drill size.

The applicable sizes are:

Hole Purpose	Drill Size	Diameter
Conventional cutting tap	No. 21	0.1590 in / 4.04 mm
Fractional fallback	5/32 in	0.15625 in / 3.969 mm
Close-clearance hole	No. 9	0.1960 in / 4.978 mm
Free-clearance hole	No. 7	0.2010 in / 5.105 mm

These dimensions are not interchangeable recommendations. The first two are starting holes that will be threaded. The two clearance sizes are finished holes through which a No. 10 screw passes without engaging the material. A forming tap falls into a separate category because it displaces material rather than cutting it.

No. 21 Matches a Conventional 10-32 Cutting Tap

The designation 10-32 identifies a No. 10 screw thread with 32 threads per inch. The 32 is the pitch count, not a drill diameter. The cited Greenfield tap listing identifies the thread as UNF, gives a tapping diameter of 0.1902 inch and specifies a No. 21 tap drill.

A common shop estimate for the starting hole of a Unified cutting thread is nominal major diameter minus thread pitch. For a No. 10 thread with a nominal diameter of 0.190 inch, the calculation is 0.190 − (1 ÷ 32) = 0.15875 inch.

A No. 21 drill measures 0.1590 inch, only slightly above that calculated result. This explains why the numbered drill is paired with a conventional 10-32 cutting tap. A published tap-and-clearance chart also lists No. 21 for a 10-32 hole in its 75% thread column (LittleMachineShop).

The formula is a useful cross-check, not a reason to override the tap maker. Required starting diameter can depend on the tap geometry, work material and required thread engagement. When the manufacturer supplies a drill size or allowable range for the exact tap, use that specification.

A No. 21 bit can be labeled by number rather than decimal diameter. Confirm the marking instead of assuming that a nearby-looking fractional bit is equivalent. Numbered drills provide intermediate diameters that common fractional sets do not include, which is why tap charts frequently call for them.

A 5/32-Inch Bit Is an Undersize Fallback

A 5/32-inch bit is the nearest common fractional option to No. 21, but it is not the same size. Its diameter is 0.15625 inch, compared with 0.1590 inch for No. 21. The fractional bit is therefore 0.00275 inch smaller.

That difference changes the tapping operation. A smaller pilot leaves more material for the cutting tap to remove, so the tap must do more work. The resulting hole should not be described as a No. 21 equivalent merely because both diameters round to similar values in a basic fraction-to-decimal comparison.

For a noncritical job in readily machinable material, 5/32 inch may serve as an expedient fallback when a No. 21 bit is unavailable. The specified No. 21 remains the better choice. This is especially relevant when tap breakage, repeatability or a controlled finished thread matters.

Do not substitute 11/64 inch on the theory that moving to the next familiar fraction is harmless. An 11/64-inch bit measures 0.171875 inch, substantially larger than the 0.1590-inch No. 21. That larger starting hole leaves less material for the tap and produces a shallower cut thread than the usual No. 21 hole.

The two fractional choices therefore create opposite problems. A 5/32-inch hole is undersize and raises tapping effort. An 11/64-inch hole is oversize for the usual cutting-tap recommendation and reduces thread depth. Neither is an equal replacement for No. 21.

Forming Taps Require Their Own Larger Starting Hole

The No. 21 recommendation applies to a cutting tap. A cutting tap creates the thread by removing material and producing chips. A forming tap, also called a roll tap, creates the thread by displacing material rather than cutting it. That difference requires a larger and more tightly controlled starting hole.

For example, one OSG 10-32 UNF forming tap specifies a tap-drill range of 0.174063 to 0.17725 inch (OSG). Even the low end of that specified range is larger than a No. 21 drill at 0.1590 inch.

That OSG range applies to the cited tap model; it should not be treated as a universal range for every 10-32 forming tap. Use the data supplied for the exact forming tap and work material. If the tool or package identifies the tap as roll, form or forming, do not use the cutting-tap recommendation without checking its specifications.

The distinction also explains why selecting a drill solely from the thread designation can fail. Two tools can both produce 10-32 threads while requiring different starting holes because one cuts material and the other forms it. Identify the tap type before selecting the drill.

Clearance Holes Are Larger Than Tap Holes

A clearance hole is appropriate when the 10-32 screw should pass through one part and engage threads in another part, a nut or an insert. The clearance hole itself is not tapped.

For a No. 10 screw, the cited clearance recommendations are No. 9 at 0.1960 inch for a close fit and No. 7 at 0.2010 inch for a free fit (University of Virginia clearance chart). In metric equivalents, those diameters are 4.978 mm and 5.105 mm, respectively.

The close-clearance No. 9 hole provides less extra room around the screw. The free-clearance No. 7 hole provides more room for passage and alignment. Both are larger than the nominal No. 10 screw diameter and much larger than the No. 21 hole intended to be tapped.

Clearance sizing follows the screw diameter, not its thread pitch. A No. 10 screw remains the same screw-number diameter whether it has 24 or 32 threads per inch. Consequently, the cited No. 9 and No. 7 clearance recommendations do not change merely because the screw is 10-24 rather than 10-32.

Tap-drill sizing does change with pitch because the pilot must leave material that the tap can turn into the intended thread. This is why a clearance chart cannot double as a tap-drill chart, even when both charts list No. 10 fasteners.

10-24 and 10-32 Tap Holes Are Not Interchangeable

Check the tap marking before drilling. A 10-32 tap has 32 threads per inch and is the fine-thread option identified as UNF in the cited Greenfield listing. A 10-24 tap has 24 threads per inch and uses a different pilot size.

The shared “10” indicates that both are No. 10 threads; it does not make their pitches or tap-drill requirements identical. A screw can start poorly or damage a thread if its pitch does not match, even though its basic diameter appears correct. For the coarse-thread version, use the 10-24 tap-drill guide.

This check matters before any hole is made because the drill diameter establishes how much material remains for the tap. Discovering a pitch mismatch after drilling may leave a hole unsuitable for the intended thread.

Drill the Pilot Before Starting the Tap

Secure the work so it cannot move while the drill enters. Mark the intended center, center-punch it and align the No. 21 bit square to the surface. Misalignment at the drilling stage makes it harder to start the tap straight because the tap follows the pilot hole.

Drill the hole to the required depth. For a blind hole, the drilled depth must extend beyond the length of full thread needed. The bottom includes space taken by the drill point, and the leading chamfered portion of a tap does not immediately produce full-depth threads. Account for both rather than treating total drilled depth as usable full-thread depth.

Clear drilling chips as needed and avoid forcing a small bit that is no longer cutting cleanly. After drilling, deburr the entrance without excessively enlarging it. Removing the raised edge helps the tap enter at the intended location while preserving the specified pilot diameter below the surface.

Before tapping, verify these three items together: the tap is marked 10-32, it is a cutting tap rather than a forming tap, and the drilled pilot is No. 21. That check separates three common sources of error—wrong pitch, wrong tap type and wrong hole purpose.

Start the 10-32 Tap Straight

Apply a cutting fluid suitable for the workpiece and place the tap square to the hole. Initial alignment is critical because the first threads guide the rest of the cut. If the tap begins at an angle, continuing to turn it does not correct the underlying alignment.

Use a tap wrench unless the tool manufacturer permits powered tapping. Milwaukee states that its cited 10-32 hand plug tap is not recommended for power-tool use and advises using cutting fluid. That instruction applies to the cited hand-tap product; it does not establish the operating method for every tap made for machine use.

Manage chips according to the tap style rather than applying one motion to every tool. The cited Milwaukee product is a straight-flute plug tap, while the cited Greenfield product is a spiral-point tap. Their shared No. 21 drill recommendation does not mean every aspect of their use is identical.

Do not force a tap that suddenly requires markedly more effort. Stop and determine whether the pilot is undersize, chips are obstructing the cut, the tap is misaligned or the tool is unsuitable for the material. In particular, confirm that a 5/32-inch fallback was not mistaken for the specified No. 21 diameter.

After tapping, remove debris and check the finished thread with the intended 10-32 screw. For controlled work, use the specified thread gage rather than relying only on whether one screw can be driven into the hole. The intended screw is a practical fit check, while a specified gage addresses work governed by a defined acceptance requirement.

Match the Drill to the Hole’s Actual Job

Use No. 21 at 0.1590 inch when the hole will receive a conventional 10-32 cutting tap. Use 5/32 inch only as an acknowledged undersize fallback, not as a dimensional equivalent. Obtain the exact manufacturer’s range for a forming tap. If the screw only needs to pass through, use the appropriate No. 10 clearance size—No. 9 for the cited close fit or No. 7 for the cited free fit.