Use a No. 25 Bit Before Tapping a Standard #10-24 Hole

Drill the hole with a No. 25 bit — 0.1495 inch, about 3.80 mm — before cutting a standard #10-24 UNC thread with a conventional cutting tap. That is the size listed on the Lincoln Machine tap-drill chart, and it is based on approximately 75% thread engagement.

A 5/32-inch bit is a larger fractional fallback, not an equal substitute. A 3/16-inch or No. 9 hole is a clearance hole: the screw passes through it without threading.

Pick what the hole must do, your tap type and the bits you own; the drill size updates beside the controls.

No. 25 Is the Chart Drill, Based on About 75% Thread

Multiple tap-drill references specify a No. 25 drill for a standard #10-24 thread. Its nominal diameter is 0.1495 inch, or about 3.80 mm.

The 75% figure is a chart target, not a guarantee. It describes the nominal thread form the chart aims for. The finished result varies with the actual hole diameter, the tap geometry, the workpiece and the process, so no drilled and tapped hole should be assumed to measure exactly 75%.

These dimensions come from published reference charts. Drill Review did not independently measure a No. 25 bit or a finished #10-24 tapped hole for this guide.

5/32 Inch Is 0.00675 Inch Larger, and Clearance Holes Are Larger Still

The dimensions below are listed in or derived from the Mutual Screw & Supply tap and clearance chart.

Role	Drill	Diameter	Use
Tap drill	No. 25	0.1495 in	Before cutting #10-24 threads
Fractional fallback	5/32 in	0.15625 in	When No. 25 is unavailable
Clearance	3/16 in or No. 9	0.1875 in or 0.1960 in	No. 10 screw passes through

Subtracting 0.1495 from 0.15625 shows that a 5/32-inch bit is nominally 0.00675 inch larger than No. 25. That gap is small enough for some charts to name 5/32 inch as the closest fractional option.

It still changes the thread. A larger pilot hole leaves less material for the tap to cut, which lowers both thread engagement and tapping load. Whether that trade is acceptable depends on what the joint has to do. A 5/32-inch bit should not be relabeled as a No. 25.

The two clearance sizes also differ from each other: 3/16 inch is 0.1875 inch and No. 9 is 0.1960 inch. The chart lists both as clearance references for a No. 10 fastener. Neither is a pilot drill for tapping #10-24.

The choice comes down to the finished hole:

• If the hole itself receives the screw threads, drill No. 25 and tap it.
• If the screw passes through the part and engages a nut, insert or threaded part behind it, use the clearance-hole specification.
• If only fractional bits are on hand, 5/32 inch is the larger fallback, and the reduced engagement has to suit the job.

The 24 Decides the Drill: #10-32 Takes a No. 21

The 24 in #10-24 means 24 threads per inch. The No. 10 identifies the nominal screw size; the second number is the pitch.

Both numbers are needed for the lookup. #10-24 and #10-32 share the No. 10 size but take different tap drills: the Lincoln Machine chart lists No. 25 at 0.1495 inch for #10-24 and No. 21 at 0.1590 inch for #10-32. Reading only “#10” off the tap or the fastener is not enough. Confirm the threads-per-inch figure first.

The lookup changes again one screw size down; those figures are in Drill Bit Size for 8 32 Tap: #29, 0.1360 Inch.

Guhring’s 60% to 70% Guidance Explains Deliberately Larger Holes

Thread-engagement percentage describes how much of the theoretical thread form remains after the pilot hole is drilled and the internal thread is cut.

• A smaller pilot hole gives more engagement but makes the tap remove more material.
• A larger pilot hole gives less engagement and reduces the cutting load.

Many conventional charts use approximately 75% as the reference point. Guhring takes a process-oriented view: its tap-drill guidance suggests 60% to 70% thread for most applications, on the grounds that lower engagement can reduce tapping load and improve tap life.

That range is one manufacturer’s guidance, not a universal requirement, and it is not a separate drill-size specification for #10-24. It explains why a machinist may choose a different hole size in a controlled process. It does not make every larger drill an acceptable replacement for No. 25.

Forming, Oversized and Toleranced Work Needs the Tap Maker’s Hole Size

The No. 25 answer covers a conventional cutting tap. Verify a different pilot-hole specification when working with:

• A thread-forming or roll tap
• An oversized tap
• Specialized combination tooling
• A #10-32 tap
• A controlled thread class or tightly toleranced drawing
• A process for which the tap manufacturer supplies a different hole range

A forming tap is not covered by a general cutting-tap chart. Its predrilled-hole requirement should come from the forming-tap manufacturer.

Oversized taps need separate treatment too. A Practical Machinist discussion about a special #10-24 tap oversized on pitch diameter considered No. 24, No. 23 and 5/32-inch holes for that particular application. Those alternatives addressed special tooling and operating conditions. They are not defaults for an ordinary #10-24 tap.

General charts also do not settle adjustments for workpiece material, thread class, hole depth, tap geometry or tolerance-controlled production. For specialized or critical work, check the predrilled-hole range against the tap manufacturer’s instructions and the applicable drawing or engineering requirements.