Ultimate Internal Thread Tool — Tap Drill Chart with Pitch and Minor Diameter Limits for UNC, UNF, UNEF, UN, UNS, UNJ, Metric M and MJ, Pipe and Whitworth Threads

Each row is one internal thread in one tolerance class, cut with one kind of tap. The list is sorted by the recommended drill, smallest first, so the chart reads like the drill chart on a machine shop wall. A thick line marks each change of drill.

• Tap drill — the recommended standard drill: number (#80 to #1), letter (A to Z), fractional, or metric.
• Drill Ø — the drill's decimal size in inches and millimetres.
• Minor Ø min / max — the internal thread's minor diameter limits for the class. Any drilled hole that ends up between these limits makes an acceptable thread crest.
• % thread — how much of the full basic thread depth the drill leaves (card 05).
• Std — the standard the limits come from.

UNC Unified inch   UNJF UNJ   M metric   FORM roll-form tap

A purple ≠UN mark beside a UNJ drill means the UNJ thread needs a different drill from the UN thread of the same size and class. The usual UN drill would leave the minor diameter below the UNJ minimum.

An internal thread has three diameters. The major diameter D is at the roots of the nut threads, the largest diameter. The minor diameter D₁ is at the crests, the smallest: the bore of the nut. The pitch diameter D₂ sits between them, where the groove and the ridge of the thread are equally wide.

The tap drill makes the minor diameter. That is why the chart's drill always lands between Minor Ø min and max. The tap makes the pitch diameter. Its own size, wear and runout decide it, and a thread plug gage checks it: GO at D₂ min and NOT GO at D₂ max. A good drill cannot rescue an oversize tap, and a perfect tap cannot rescue an undersize hole.

Press the ⊞ box at the end of any row. The window shows that thread's profile drawing, every limit of size for its class, the basic geometry, the tap drills, plug-gage sizes and the same size in the other classes and forms. From there you can print a one-thread report.

For Unified threads you can type a known length of engagement into that window. B1.1 bases the pitch diameter tolerance on LE = D for the coarse, fine, 4-, 6- and 8-UN series and on 9 pitches for the others. A long engagement allows a slightly larger tolerance.

Threads such as 1/4-40, 10-48 or 3/8-40 are not in the standard series of ASME B1.1 Table 1. They are UNS (Unified special) threads, and they are common on instruments, optics and aerospace hardware. ASME B1.1 Appendix D lists 114 of the most used ones in Table D-1. All of them are in the UNS specials family (on by default), with UNJS versions under UNJ.

For any other size, type it into Special thread: 1/4-40, #10-48, 0.312-56, 1-1/4-12 UNJ, M7x0.35 or MJ10x1. The row is computed from the same formulas and pinned at the top of the list.

Metric special threads use the B1.13M (ISO 965-1) tolerance tables by pitch and diameter range. Where a pitch is not tabulated for that range, the para 6.5/6.6 formulas are used and the row is marked.

UNJ is the Unified thread with a controlled radius at the root of the external thread (0.15011P to 0.18042P). The bigger root radius cuts the stress concentration and raises fatigue life, which is why aerospace bolts use it. To make room for that fuller root, the internal thread's minor diameter is raised so the thread is only 0.5625H deep instead of 0.625H.

The UNJ minimum is \(0.10825P\) larger. For 1/4-20 that is 0.0054″, enough to move the tap drill from #7 (0.2010″) to #5 (0.2055″). The #7 drill is below the UNJ minimum minor diameter of 0.2013″ and is not allowed.

The UNJ minor diameter tolerance is also tighter. It is capped at \(0.25981P\), compared with \(0.394P\) for UN:

SAE AS8879B covers UNJC, UNJF, UNJEF and the 8-, 12- and 16-UNJ series in Class 3A/3B. ASME B1.1-2024 now covers UNJ in Classes 2A/2B and 3A/3B for every Table 1 size. Its Class 3B limits are the same as AS8879's; AS8879 rounds the last digit of the minimum minor diameter up rather than to nearest.

Class 2B is the general-purpose internal thread. Class 3B is the close-tolerance class used where tight fit, strength or safety matter, and for all UNJ aerospace work. The minimum minor diameter is the same for both classes. Only the maximum, and so the width of the drill window, changes.

The pitch diameter tolerance is \(T_{D2} = 1.30\,T_{d2}(2A)\) for 2B and \(0.975\,T_{d2}(2A)\) for 3B, with \(T_{d2}(2A) = 0.0015\sqrt[3]{D} + 0.0015\sqrt{LE} + 0.015\sqrt[3]{P^2}\). \(LE = D\) for the coarse, fine, 4-, 6- and 8-UN series and \(9P\) for the others. Rounding follows para 8.3.2. Every minor diameter on this page was checked against all 712 Class 2B and 3B rows of Table 2B.

These tolerances suit engagements up to 1.5D. B1.1 para 5.6.1 allows a smaller tolerance for engagements under 0.67D and a larger one for long engagements.

A metric tolerance class such as 6H names the pitch diameter grade, the position letter (H = no allowance) and the minor diameter grade. When the two grades match, the class is written once. 4H5H is grade 4 on pitch diameter and grade 5 on minor diameter, the close-fit class. 6H is the general-purpose class and the metric counterpart of 2B.

The standard rounds these to the R40 series and then smooths the tables by hand (para 6.2.2), so a few tabulated values sit one R40 step from the formula. This chart uses the tabulated values of B1.13M Table 10 (minor diameter) and Table 13 (pitch diameter). It falls back to the formula only for the smallest pitches the tables leave blank, and marks those rows.

MJ (ASME B1.21M, ISO 5855) is the metric counterpart of UNJ. It has the same raised internal minor diameter and the same reason for a larger tap drill. Sizes 1.6 to 5 mm are class 4H6H and 6 mm and up are 4H5H (para 6.8.2). The chart lists the B1.21M Table 3 standard aerospace series.

Metric sizes are the whole ISO 261 diameter/pitch table, 1 to 300 mm. Pitches ISO 261 says to avoid, and the two special-purpose sizes (M14×1.25 spark plugs, M35×1.5 bearing lock nuts), are flagged.

Percent of thread is the classic shop measure of how deep the tapped thread is. 100 % is the full basic thread depth of the external thread, \(0.649519P\) per side for 60° threads:

For 55° Whitworth threads the denominator is \(1.280654P\). The familiar 1/4-20 with a #7 drill comes out at 75 %.

Above about 75 % thread, stripping strength rises very little while tapping torque and the risk of tap breakage rise sharply. That is why tap charts aim near 75 %, and why the class tolerances above leave room for it.

None of these thread standards names a tap drill. The chart takes the minor diameter band of the class, lists every standard drill whose nominal size falls inside it, and recommends the one nearest the chosen aim point. If no standard drill fits the band, which happens on some tight Class 3B sizes, the nearest drill is shown with a warning. Ream or bore to size, or use a special drill.

A forming tap does not cut. It pushes the metal of the hole wall up into the thread form, so the hole must start larger than for a cutting tap. The metal that flows up forms the minor diameter, and the formed crests are cold-worked and usually stronger than cut ones. Forming suits ductile materials: aluminium, copper alloys, low-carbon and many stainless steels. It is not for cast iron, most cast aluminium or very hard materials.

\(P\) is in the same units as \(D\): inches (1/TPI) or millimetres. 60 to 65 % is the usual target. The chart's band runs from the 75 % hole to the 55 % hole.

Too small a hole overfills the crest and can break the tap. Too large a hole leaves shallow, split crests (a "U" at the top of each thread). Forming taps also need more torque and good lubrication, and through holes or extra hole depth help.

Each pipe and Whitworth family has its own filter at the left. All are off by default.

NPT and NPTF (Dryseal) are tapered 1:16 on diameter. There are two rows per size: the drill for tapping straight from the drilled hole, and the drill to use with a taper pipe reamer. Both are the drills NBS Handbook H28 suggests (Tables 7.2 and 7.3). A tapered thread has no single minor diameter, so the diameter columns show the basic sizes at the face of the fitting (E1, the gauge plane) and at the hand-tight depth L1 (E0). They are not tolerance limits.

NPS / NPSF are the straight pipe threads (NPSC, NPSM, NPSL, and the Dryseal NPSF and NPSI). The chart gives their H28 drill only.

BSW, BSF and BSPP (G) use the 55° Whitworth form. Their limits are the American Truncated Whitworth tables of H28 (1944), which mate fully with BS 84 threads. Medium is class 2 and Close is class 3; the minor diameter band is the same for both. The basic minor diameter is \(D - 1.280654P\), so the percent of thread uses \(1.280654P\) as full depth.

The chart chooses from four drill series. You can turn any of them off at the left, for example to see only the drills in your own index.

• Number (wire gauge) — #80 (0.0135″) to #1 (0.2280″).
• Letter — A (0.2340″) to Z (0.4130″).
• Fractional — 1/64″ steps to 1-3/4″, 1/32″ steps to 2-1/4″, 1/16″ steps to 3-1/2″.
• Metric — 0.05 mm steps to 3 mm, 0.1 mm to 14 mm, 0.25 mm to 20 mm and 0.5 mm to 50 mm.

Number, letter and fractional drill sizes are those of ASME B94.11M. Where a thread needs a hole larger than the biggest drill in the enabled series, the row reads BORE with the target hole size. Large threads are normally bored, or thread-milled from a smaller hole.