M8 coarse pitch is 1.25 mm. Basic pitch diameter 7.188 mm, external minor diameter 6.466 mm, and a 6.75 mm tap drill. Fine pitches of 0.75 mm and 1 mm are also tabulated.
M8 is an ISO metric thread on the 60° basic profile, so every dimension below follows from two numbers: the 8 mm nominal diameter and the 1.25 mm coarse pitch. Drawings that call up M8 without a pitch mean the coarse series unless the note says otherwise.
Basic profile dimensions to ISO 68-1, with the tap drill for standard thread engagement.
Pitch diameter and minor diameters are derived from the ISO 68-1 basic profile (d2 = d − 0.649519P, d3 = d − 1.226869P, D1 = d − 1.082532P). The tap drill is the arithmetic d − P for 75-80% thread engagement; pick the nearest drill in the preferred series, which is why published charts quote 6.75 mm for some sizes and the adjacent tenth for others.
Fine series for the same nominal diameter. A fine pitch has a larger minor diameter than coarse at the same nominal size, so a gauge for one will not check the other.
| Designation | Pitchmm | Pitch Ømm | Minor Ø ext.mm | Minor Ø int.mm | Tap drillmm |
|---|---|---|---|---|---|
| M8x0.75 | 0.75 | 7.513 | 7.08 | 7.188 | 7.25 |
| M8x1 | 1 | 7.35 | 6.773 | 6.917 | 7 |
A standard M8 hexagon head or nut takes a 13 mm across-flats spanner under ISO 272.
An M8 internal thread is checked with a thread plug gauge: a matched GO / NO-GO pair on one handle. The GO end checks the minimum material limit and must enter the full thread by hand; the NO-GO end checks the maximum material limit and must not enter. Because the two ends are cut and lapped as a pair, the setting between them is preserved when the gauge is reconditioned — which is what keeps a working gauge useful after years of use.
External M8 threads are checked with a thread ring gauge or a snap gauge, and setting plugs are used to keep those rings in calibration. The pitch diameter above (7.188 mm) is the dimension being controlled; the major and minor diameters only matter as clearance.
Full coarse and fine series on the metric thread pitch chart, adopted in India as IS 919 / IS 4218.