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Thread-Forming Tap Drill Chart

Starting hole sizes for thread-forming (roll) taps in common inch and metric threads, calculated for about 65% thread, with the formula behind them.

Ref CT-0053ASME B94.9

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A thread-forming tap has no cutting edges. It displaces metal to form the thread, and the displaced metal flows up toward the major diameter. Because material moves up instead of being removed, the starting hole must be larger than a cut-tap drill. Forming taps work in ductile materials such as aluminum, low-carbon steel, copper, brass and many stainless grades. They make no chips and give a strong, burnished thread.

How to use it

The common starting formula for about 65% thread is:

Inch: hole diameter = basic major diameter − (0.0068 × 65 ÷ TPI)
Metric: hole diameter = nominal diameter − (0.0068 × 65 × pitch in mm)

That works out to roughly major diameter minus about 0.44 × pitch. Pick the nearest available drill, then check the first parts with a thread gauge.

Chart

Thread	Calculated hole (65%)	Nearest common drill
#4-40	.1009 in	#38 (.1015)
#6-32	.1242 in	1/8 (.1250)
#8-32	.1502 in	#25 (.1495)
#10-24	.1716 in	11/64 (.1719)
#10-32	.1762 in	#16 (.1770)
1/4-20	.2279 in	#1 (.2280)
1/4-28	.2342 in	15/64 (.2344)
5/16-18	.2879 in	7.3 mm (.2874)
5/16-24	.2941 in	M (.2950)
3/8-16	.3474 in	S (.3480)
3/8-24	.3566 in	T (.3580)
1/2-13	.4660 in	15/32 (.4688)
M3 × 0.5	2.78 mm	2.8 mm
M4 × 0.7	3.69 mm	3.7 mm
M5 × 0.8	4.65 mm	4.65 mm
M6 × 1.0	5.56 mm	5.55 mm
M8 × 1.25	7.45 mm	7.45 mm
M10 × 1.5	9.34 mm	9.35 mm
M12 × 1.75	11.23 mm	11.25 mm

Key points

Hole size is more critical with forming taps than with cut taps. A hole that is too small sharply raises torque and breaks taps.
A larger hole lowers thread percentage and torque. Many tap makers publish a hole range; stay inside it.
Forming taps need good lubrication. Use the forming oil or fluid the tap maker recommends.
The formed thread has a small split or "cleft" at the crest of the minor diameter. This is normal.
Not suitable for brittle materials such as gray cast iron or some cast aluminum alloys and plastics.

Common mistakes

Using a cut-tap drill chart with a forming tap.
Drilling an undersized hole because the "nearest" drill was rounded down.
Using forming taps in material that is too hard or brittle to flow.
Checking only the major diameter; forming tap threads must be checked with go/no-go gauges.

Governing standards: ASME B94.9 (taps, ground and cut threads). ASME B1.1 and ASME B1.13M for thread limits. Tap manufacturer's hole-size data governs for each tap.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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