#6-40 UNF thread pitch and dimensions

The #6-40 UNF thread has 40 threads per inch, a pitch of 0.0250 in (0.64 mm), on a 0.138 in major diameter (ASME B1.1). Its basic pitch diameter is 0.1218 in and its basic minor diameter is 0.1109 in.

Threads per inch (UNF)

40 TPI

pitch 0.0250 in · 0.64 mm

Major diameter (D)

0.138 in (3.51 mm)

Pitch (1 / TPI)

0.0250 in (0.64 mm)

Basic pitch diameter (E)

0.1218 in (3.09 mm)

Basic minor diameter (internal)

0.1109 in (2.82 mm)

Tensile stress area (As)

0.0101 in² (6.5 mm²)

Tap drill (75% chart)

#33 (0.1130 in)

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#6-40 UNF basic dimensions (ASME B1.1)
Dimension	Formula	inch	mm
Major diameter D	nominal	0.1380	3.51
Pitch P	1 / TPI	0.0250	0.64
Pitch diameter E	D − 0.649519 / n	0.1218	3.09
Minor diameter (internal)	D − 1.082532 / n	0.1109	2.82
Thread height (basic)	0.541266 / n	0.0135	0.34
Tensile stress area As	0.7854 (D − 0.9743 / n)²	0.0101 in²	6.5 mm²

#6 threads: UNC vs UNF
Thread	TPI	Pitch Ø E	Stress area As	Tap drill
#6-32 UNC	32	0.1177 in	0.0091 in²	#36
#6-40 UNF (this page)	40	0.1218 in	0.0101 in²	#33

Neighbouring UNF threads
Thread	TPI	Pitch (mm)	Minor Ø	Stress area As
#4-48 UNF	48	0.53	0.0894 in	0.0066 in²
#6-40 UNF (this page)	40	0.64	0.1109 in	0.0101 in²
#8-36 UNF	36	0.71	0.1339 in	0.0147 in²
#10-32 UNF	32	0.79	0.1562 in	0.0200 in²
1/4-28 UNF	28	0.91	0.2113 in	0.0364 in²

Practical notes

Unified threads are named diameter–TPI–series: #6-40 UNF is a #6 nominal diameter (number 6 screw, 0.138 in) with 40 threads per inch in the fine series.

These are basic (zero-tolerance) dimensions. Most screws are made to class 2A and nuts to 2B (ASME B1.1), so measured sizes sit slightly below the basic values on the screw.

40 TPI is a pitch of 0.64 mm, which does not match a metric ISO pitch exactly; metric and Unified fasteners of similar size are not interchangeable.

Tensile stress area As is the area used for bolt strength (proof load = As × proof stress for the SAE J429 or ASTM grade). For #6-40 UNF it is 0.0101 in².

FAQ

How many threads per inch is #6-40 UNF?

40 TPI, a pitch of 0.0250 in (0.64 mm), in the Unified fine series (ASME B1.1).

What is the minor diameter of #6-40 UNF?

The basic minor diameter of the internal thread is D − 1.082532/n = 0.1109 in (2.82 mm).

What is the tensile stress area of #6-40 UNF?

As = 0.7854 × (D − 0.9743/n)² = 0.0101 in² (6.5 mm²).

What tap drill does #6-40 UNF need?

The standard chart drill is #33, 0.1130 in (2.87 mm), about 77% thread.

What is the difference between #6-40 UNF and #6-32 UNC?

Same #6 diameter, different pitch: 40 TPI versus 32 TPI. They are not interchangeable.

Sources: ASME B1.1 — Unified inch screw threads (UN and UNR thread form): basic dimensions, tensile stress area · ASME B18.2.1 — square, hex, heavy hex and askew head bolts and hex, heavy hex, hex flange screws (width across flats)

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