Metric Taps & Pitch Series

What it is

A metric tap cuts an ISO metric thread. The size is written as M plus the nominal major diameter in millimeters. The pitch is the distance from one thread to the next, in millimeters. When no pitch is given, the coarse pitch is meant. Fine pitch threads always show the pitch, for example M12 x 1.25.

How to read it

A tap marked M8 x 1.25 6H cuts an 8 mm thread with 1.25 mm pitch to fit a 6H internal thread class. A starting tap drill for a cutting tap is the major diameter minus the pitch. For M8 x 1.25 that gives 8 - 1.25 = 6.75 mm, often rounded to 6.8 mm. See the tap drill chart for exact drills.

Common coarse and fine pitches

DIN tap styles

• DIN 371: smaller sizes with a reinforced shank about as large as or larger than the thread. Stiff and strong, but the shank cannot pass through the tapped hole.
• DIN 376: coarse thread taps with a reduced shank smaller than the thread, so the tap can go deeper than its thread length.
• DIN 374: the reduced-shank style for fine pitch threads.
• ISO 529: short machine and hand taps, close to the inch-style overall lengths.

Key points

• Metric pitch is a distance, not threads per inch. Do not convert pitch to TPI to pick a tap.
• Square drive sizes and shank diameters differ between DIN and ANSI taps. Check holders before switching.
• 6H is the common general-purpose internal thread class. Tap limits such as D3 or D4 are separate. See the tap limits sheet.

Common mistakes

• Assuming every M10 tap is 1.5 pitch. M10 x 1.25 and M10 x 1.0 are common on automotive and hydraulic parts.
• Mixing up 1.0 mm metric pitch with similar-looking inch threads.
• Using a DIN 371 tap where the tapped depth is deeper than the thread length.

Governing standards: ASME B1.13M and ISO 261/262 (metric threads); ISO 529 (short taps); DIN 371, DIN 374 and DIN 376 (tap styles). See the thread pitch reference chart.

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.