Choose the Right Drill Bit for 1/4-20 Threads

Use a No. 7 drill bit for a conventional 1/4-20 UNC cutting tap. A No. 7 bit measures 0.2010 inch, or 5.105 mm. Use a larger hole for a forming tap or a bolt-clearance hole; those operations do not use the standard No. 7 tap-drill size.

Choose what the finished hole must do; the matching drill size appears beside it.

The Correct Size Depends on the Finished Hole

The basic chart answer for tapping 1/4-20 threads is No. 7, as listed by American Fastener Technologies. The alternatives below apply to different tooling or a different purpose.

Intended hole	Drill size	Decimal diameter
Cut 1/4-20 threads	No. 7	0.2010 in
Closest metric substitute	5.1 mm	0.2008 in
Common fractional substitute	13/64 in	0.2031 in
Close bolt clearance	Letter F	0.2570 in
Free bolt clearance	Letter H or 17/64 in	About 0.266 in

A hole that will receive internal threads must retain enough material for the tap to cut. A clearance hole has the opposite purpose: it must be larger than the fastener so the bolt can pass through without engaging the part. A forming tap also requires a different starting diameter because it displaces material rather than cutting it away.

Before selecting a bit, identify all three parts of the job: the 1/4-20 thread designation, whether the tap is cutting or forming, and whether the finished hole is actually supposed to be threaded or unthreaded.

No. 7 Is the Standard Cutting-Tap Drill

The “1/4” in 1/4-20 identifies a nominal 0.250-inch major thread diameter. The “20” means the thread has 20 threads per inch. OSG identifies this combination as 1/4-20 UNC in its tap specifications.

A 1/4-inch drill is not the correct starting size for a conventional 1/4-20 cutting tap. Drilling the full nominal major diameter would remove the material from which the internal thread must be cut. The tap drill is intentionally smaller, which is why the standard recommendation is the 0.2010-inch No. 7 bit.

“No. 7” is a numbered drill designation, not a measurement in millimeters and not a reference to a 7/64-inch bit. Read the size stamped on the bit or its index position rather than selecting it by appearance. The decimal diameter is the useful cross-check: the required No. 7 bit should be marked or listed as 0.2010 inch.

After drilling, use a conventional 1/4-20 cutting tap. Keep the drilled hole and tap aligned because the recommended diameter does not compensate for a tap entering at an angle. A light entrance chamfer can help the tap start at the mouth of the hole without changing the required diameter through the threaded section.

The No. 7 recommendation answers a general shop-size question, but a controlled drawing or tap specification can override a general chart. If the drawing gives a hole diameter, tolerance, thread class or inspection requirement, follow those requirements rather than substituting the nearest bit in the box.

Metric and Fractional Substitutes Are Close, Not Identical

A 5.1 mm drill measures 0.2008 inch, making it dimensionally very close to the 0.2010-inch No. 7 drill. It is the closest listed metric substitute in this comparison. The difference is small, but the bits are not literally the same diameter.

If only common fractional bits are available, 13/64 inch is the closest practical fractional choice listed here at 0.2031 inch. It is 0.0021 inch larger than No. 7, so it removes slightly more material and leaves slightly less for the tap to cut into.

That distinction matters most when the hole is governed by a drawing, specified thread class, inspection limit or tap maker’s instructions. Under those conditions, use the specified No. 7 bit rather than treating 13/64 inch as an exact replacement. For work without such a requirement, confirm the substitute against the instructions supplied for the actual tap.

The direction of the difference is also worth checking. The 5.1 mm substitute is slightly smaller than No. 7, while 13/64 inch is larger. They should not be treated as interchangeable merely because both appear near No. 7 on a conversion chart.

Do not round the recommendation to 3/16 inch or 1/4 inch. Neither is the specified fractional substitute in the source data. The useful fractional alternative identified here is 13/64 inch, and even that remains a substitute rather than the direct chart answer.

Tap-Drill Recommendations Can Reflect Thread Percentage

A tap-drill chart does not always list only one possible starting diameter. Recommendations may vary with the intended percentage of thread and the tapping process.

Guhring’s drill table associates No. 7, No. 6 and No. 5 drills with 1/4-20. It also lists 5.18, 5.20 and 5.22 mm drills and states that its recommendations fall within a 65%–75% thread range.

Those additional sizes do not make the basic No. 7 answer incorrect. They show why a manufacturer’s process-specific data can differ from a single-value reference chart. The correct hierarchy is straightforward: use the hole specified on the drawing or by the tap manufacturer when one is provided; otherwise, No. 7 is the conventional chart answer for a 1/4-20 cutting tap.

This is also why a substitute should be evaluated by its decimal diameter rather than by the apparent closeness of its name. Number, letter, fractional and metric drill systems use different labels. The diameter is what determines how much material remains for the thread.

Forming Taps Require a Larger Starting Hole

Do not use the No. 7 cutting-tap recommendation automatically with a roll tap or thread-forming tap. A forming tap displaces material to create the thread instead of cutting material into chips. Its starting-hole requirement is therefore larger and more tightly controlled.

One OSG 1/4-20 forming tap specifies a starting-hole range of 0.2245–0.2296 inch in its product data. That entire range is larger than the 0.2010-inch No. 7 cutting-tap drill.

The cited range applies to that OSG tool, not to every forming tap sold in the 1/4-20 size. Use the dimensions supplied for the particular forming tap, material and process. If the tool is labeled as forming, roll or chipless, do not select No. 7 solely because a general chart lists it beside 1/4-20.

This distinction should be resolved before drilling. Once a No. 7 hole has been made, it is already smaller than the cited forming-tap range. The tool type is therefore not a detail to determine after the hole is drilled.

Bolt-Clearance Holes Must Be Larger Than 1/4 Inch

If the 1/4-20 bolt should pass through the part and engage a nut or a separate threaded component, the drilled part needs a clearance hole rather than a tap hole. Use Letter F at 0.2570 inch for a close fit or Letter H at 0.2660 inch for a freer fit.

A 17/64-inch drill measures 0.2656 inch and is the listed fractional alternative near the Letter H free-clearance size. It is appropriate when freer assembly is wanted and a letter drill is unavailable.

The University of Florida’s tap and clearance drill chart lists Letter F for a close 1/4-inch clearance hole and Letter H for a free fit. These dimensions are for allowing the fastener through the hole, not for producing internal threads.

A nominal 1/4-inch drill is not dependable clearance for a nominal 1/4-inch fastener. It provides no intentional allowance for drill runout, coatings or part misalignment. Letter F provides a close clearance while remaining larger than the nominal fastener diameter. Letter H or 17/64 inch provides more assembly allowance.

The choice between close and free clearance depends on the role of the hole. Use the close size when positional allowance should remain limited. Use the free size when easier bolt insertion and assembly allowance matter more. Neither clearance size should be followed by a 1/4-20 tap because both holes are already larger than the thread’s nominal major diameter.

Keep 1/4-20 Separate From Similar Thread Names

A 1/4-28 UNF thread is not the same as 1/4-20 UNC. Both have the same nominal 1/4-inch major diameter, but the thread counts differ. The basic cutting-tap drill for 1/4-28 UNF is No. 3 at 0.2130 inch, not No. 7, according to the American Fastener chart.

Check the complete designation before drilling. A fastener or tap marked 1/4-28 requires the 1/4-28 recommendation; the presence of “1/4” alone is not enough to select a tap drill.

A 1/4-inch pipe thread is another different system. A 1/4-18 NPT tap cuts a tapered pipe thread rather than a 1/4-20 UNC machine-screw thread. One OSG 1/4-18 NPT tap specifies a 0.4375-inch, or 7/16-inch, starting hole in its product data.

That much larger NPT hole is not an alternative for 1/4-20. Confirm that the tap is marked 1/4-20 UNC before using the No. 7 recommendation.

Drill and Tap the Hole Without Losing Usable Depth

For a conventional 1/4-20 cutting tap, begin with a sharp No. 7 bit and drill straight. The tap will follow the drilled hole, so an angled hole can leave the resulting thread misaligned even when the diameter is correct.

Add a light chamfer at the entrance after drilling. The chamfer should help the tap enter and align without removing the full thread section farther into the hole. Then apply a lubricant suitable for the workpiece and tap before cutting the threads.

Keep the tap aligned with the drilled axis as it starts. The first engagement establishes the path followed by the rest of the tap. For a through-hole, provide an unobstructed exit for the drill and account for the tap moving through the far side.

A blind hole needs additional depth beyond the required full-thread depth. The drilled depth must accommodate the drill point, chips and the tap’s incomplete lead threads. A blind hole drilled only to the nominal required thread depth will not provide that extra space, so the usable full-thread section will be shorter than the drilled depth.

Do not confuse drilled depth with finished full-thread depth. The bottom of a twist-drilled blind hole is not flat, and the leading portion of a conventional tap does not produce complete threads immediately. Plan for both conditions before setting a depth stop.

After tapping, verify that the intended 1/4-20 fastener starts without being forced. If the job is controlled by inspection requirements, use the specified inspection method rather than judging the result only by whether one bolt enters.

For the ordinary cutting-tap job, the selection remains No. 7 at 0.2010 inch. Move away from that size only when the actual requirement is a documented substitute, a forming-tap hole, a clearance hole or a different thread designation.