Choose the Correct Drill for a 5/16-18 Tap

Use a Letter F drill bit for a conventional 5/16-18 UNC cutting tap. Letter F measures 0.2570 inch, or 6.5278 mm, normally rounded to 6.53 mm. It is the standard chart choice for about 75% theoretical thread. A forming or roll tap needs a larger hole and should not be drilled with the Letter F size by default.

Choose your tap type; the selector shows the matching drill size or forming-tap range.

Letter F Is the Standard Cutting-Tap Size

Gühring’s tap-drill table lists Letter F for a 5/16-18 cut tap in its 75% theoretical-thread column. Its decimal drill chart confirms that a Letter F drill has a diameter of 0.2570 inch (Gühring tap-drill chart).

That chart designation matters because “5/16-18” identifies the finished internal thread, not the diameter of the hole drilled before tapping. The starting hole must be smaller than the nominal 5/16-inch thread diameter. The cutting tap then removes material to create the internal thread form.

The 75% figure is a theoretical chart target. It does not guarantee that every completed hole will have exactly that percentage of thread. A drilled hole may run oversize, which can lower the actual engagement. The condition of the drill, the setup and the resulting hole therefore still matter even when the nominal drill size is correct.

For ordinary work with a 5/16-18 cutting tap, the practical specification is straightforward:

• Tap designation: 5/16-18 UNC
• Tap type: cutting tap
• Recommended drill: Letter F
• Drill diameter: 0.2570 inch
• Metric conversion: 6.5278 mm, commonly rounded to 6.53 mm
• Chart target: about 75% theoretical thread

Verify the tap itself before drilling. A 5/16-18 tap and a 5/16-24 tap share the same nominal diameter but do not have the same thread pitch. The Letter F answer here is specifically for the 18-threads-per-inch designation in the draft chart.

A 6.5 mm Drill Is the Closest Common Metric Substitute

When a Letter F drill is unavailable, a 6.5 mm bit is the closest common metric substitute among the sizes discussed here. Its converted diameter is 0.2559 inch, making it 0.0011 inch smaller than Letter F.

Because the 6.5 mm hole is slightly smaller, the cutting tap must remove slightly more material. That can require a little more tapping torque than the nominal Letter F setup. The difference is small, but it points in the tighter direction rather than providing an exact decimal equivalent.

A 6.5 mm drill is therefore a practical substitute, not a relabeled Letter F drill. If the drawing, process sheet or tap manufacturer specifies Letter F, use Letter F when it is available. If the job permits a close metric replacement, 6.5 mm is the nearest option identified in the supplied figures.

Do not confuse the rounded metric description of Letter F with a substitute size. Letter F converts to 6.5278 mm and is normally stated as 6.53 mm. A physical 6.5 mm bit remains slightly smaller than that converted diameter.

A 17/64-Inch Drill Produces Less Thread Engagement

A 17/64-inch drill measures 0.2656 inch, so it makes a larger starting hole than Letter F. Gühring’s chart places that size at about 65% theoretical thread for a 5/16-18 cutting tap.

The larger hole leaves less material for the tap to cut, which can reduce tapping torque. It may be useful where the thread design permits lower engagement, but it is not the direct fractional equivalent of a Letter F drill. Selecting it changes the intended thread percentage rather than merely replacing one drill-number system with another.

This distinction is especially relevant when a shop has fractional drills but no letter drills. Reaching for the nearest-looking fractional size can quietly change the thread target. With 17/64 inch, that change is toward a larger hole and approximately 65% theoretical thread rather than the Letter F chart target of about 75%.

Use 17/64 inch only when that lower engagement is acceptable for the actual part. The source provides the theoretical percentage, but the draft does not provide a universal minimum engagement for every material, load or application.

A 1/4-Inch Drill Is Too Small for a Casual Substitution

Do not automatically substitute a 1/4-inch drill. Its 0.2500-inch diameter is smaller than Letter F and works out to roughly 87% theoretical thread under Gühring’s cut-tap formula.

The smaller hole requires the tap to remove more material. That increases the thread percentage, but a higher percentage should not be treated as automatically better. Gühring says a 100% thread specification is only about 5% stronger than a 75% specification while requiring three times the tapping torque. The company generally suggests 60% to 70% thread to improve tap life (Gühring tap-drill guidance).

Those figures explain why 1/4 inch is not a harmless stand-in for Letter F. It shifts the setup from the chart’s approximately 75% choice toward roughly 87%, increasing the cutting demand without a proportional strength gain. The exact outcome still depends on the resulting hole, but the nominal size points toward a tighter thread.

The four relevant choices compare as follows:

Drill	Diameter	Practical Consequence
Letter F	0.2570 in	Standard 75% chart choice
6.5 mm	0.2559 in	Closest common metric size; slightly tighter
17/64 in	0.2656 in	About 65% theoretical thread; lower tapping torque
1/4 in	0.2500 in	Roughly 87% theoretical thread; avoid as a casual substitute

The table does not make the alternatives interchangeable. Letter F is the direct chart answer. The other rows show what changes when availability or an intentional thread-percentage decision leads to a different drill.

Forming Taps Require a Larger Starting Hole

The Letter F recommendation applies to a tap that cuts material and produces chips. A thread-forming or roll tap works differently: it displaces material to form the thread. It consequently requires a larger starting hole.

For 5/16-18 form taps, Gühring’s chart lists three drill sizes tied to different target theoretical-thread percentages:

Form-Tap Drill	Target Thread
7.2 mm	75%
7.3 mm	65%
7.4 mm	55%

These are chart choices, not one universal forming-tap answer. Use the drill size specified for the exact forming tap and workpiece material rather than selecting 7.2, 7.3 or 7.4 mm without checking the tap manufacturer’s instructions.

The distinction between cutting and forming taps should be settled before choosing a drill. A Letter F hole intended for a cutting tap cannot be assumed suitable for a forming tap merely because both taps are marked 5/16-18. The thread designation is the same, but the method used to create the internal thread is different.

If the tap style is unknown, identify it before drilling. The draft does not provide a single safe drill size that covers both cutting and forming taps.

Tap-Drill Holes Are Not Clearance Holes

A tap-drill hole is intentionally smaller than a 5/16-inch fastener. Its purpose is to leave material that the tap can cut or form into an internal thread. Letter F is therefore not the correct choice when the bolt is supposed to pass freely through the part without engaging threads.

That second job requires a clearance hole rather than a tap-drill hole. The supplied material does not give clearance-hole dimensions, so no clearance size should be inferred from the Letter F recommendation.

This difference also prevents a common measuring error. The 5/16 portion of the tap designation describes the nominal thread, while 18 identifies the coarse pitch used here. It does not instruct the operator to drill a 5/16-inch hole before tapping. A hole that large would not be the Letter F tap-drill setup covered by the chart.

Prepare Enough Depth for the Tap and Chips

Secure the workpiece and drill the hole straight. A crooked starting hole makes it harder to keep the tap aligned with the drilled axis. Lightly chamfer the entrance so the tap can start cleanly rather than meeting a sharp edge at the top of the hole.

For a blind hole, drilling only to the desired finished thread depth is not sufficient. The hole also needs room for the tap’s lead and for chips produced by a cutting tap. The required extra depth depends on the tap and the job; the supplied sources do not give one universal allowance.

Use a lubricant suitable for the workpiece and keep the tap aligned as it starts. The correct Letter F drill does not compensate for a tap entering at an angle. Initial alignment determines whether the tap follows the drilled hole instead of cutting off-axis.

Chip handling also distinguishes a cutting tap from a forming tap. The cutting tap removes material, while the forming tap displaces it. That difference is why blind-hole chip space appears in the cutting-tap setup and why the two tap styles use different starting-hole sizes.

Before the tap enters the part, check three specific details: the marking must read 5/16-18 rather than 5/16-24, the tool must be identified as cutting or forming, and the selected drill must match that tap style. Those checks resolve the sizing decision before an incorrect hole becomes permanent.

Related Coarse-Thread Tap Sizes Follow the Same Distinction

For another coarse-thread example, compare the No. 7 drill used for a 1/4-20 cutting tap. The same cutting-versus-forming distinction also applies to the 5/16-inch drill used before a 3/8-16 cutting tap.

Those examples do not change the 5/16-18 answer: use Letter F at 0.2570 inch for the conventional cutting tap, use 6.5 mm only as the close metric substitute described above, and follow the exact tap specification when the tool forms rather than cuts the thread.