Metric Thread Pitch Table

Because the ISO 262 selection is built on two preferred-number series, R10 and R20, and not every millimeter earns a standard thread. The R10 series (1, 1.2, 1.6, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20...) is the primary ladder - the sizes with full tooling support and hardware-store presence. The R20 additions (1.4, 1.8, 7, 14, 18, 22 and friends) fill gaps for special applications, and the in-between integers that appear in neither series - M9, M11, M15 - have no ISO 262 standard thread at all. That is why an M9 bolt is either a special order or an old DIN oddity, and why car makers occasionally exploit the confusion: a few Japanese standards historically specified non-ISO pitches for anti-tamper reasons. Practically, the chart covers the entire R10 ladder through M20 plus the two R20 sizes you actually meet (M14 on flanges and brake lines, M18 occasionally) - which is effectively the whole world of general metric fastening. The system note worth keeping: within the standard, pitch scales with diameter on the same preferred-number logic, which is why the coarse column climbs 0.25, 0.5, 1, 1.5, 2, 2.5 in tidy steps as the diameter climbs.

Four situations, each a genuine engineering reason. Thin walls and short engagement: a fine pitch packs more threads into the same nut thickness, so a shallow aluminum boss gets a usable thread count where coarse would strip - the classic case for M10x1.25 instead of M10x1.5 in castings. Vibration: fine threads have a shallower helix angle and smaller pitch, so transverse shaking - the thing that actually unscrews bolts - walks them loose more slowly; that is why spark plugs, brake adjusters and instrument hardware run fine. Finer adjustment: any use where the thread is a positioning device (lathe cross slides, tensioners, optics) benefits from the smaller movement per turn. And strength in small diameters: below about M6 the coarse thread eats a larger share of the bolt cross-section, so fine pitches preserve more shank. Against all that, coarse wins on speed of assembly, tolerance of dirt and damage, and clamp-thread reliability in soft materials - which is why coarse is the default everywhere and fine is the deliberate exception. The one mistake to avoid: assuming fine and coarse are interchangeable per diameter - M10x1.25 hardware threads exactly zero millimeters into a coarse M10 nut, and forcing it damages both.

The rule: drill diameter equals nominal diameter minus pitch. M6x1: 6 minus 1 = 5 mm - and 5 mm is exactly the drill everyone uses, because metric drills conveniently exist in half-millimeter steps at the small sizes. M10x1.5: 8.5 mm, exact again. The bends appear where the arithmetic lands between drills: M8x1.25 computes to 6.75, but the drill that exists and the chart everyone quotes is 6.8 mm - rounded up - while M12x1.75 computes to 10.25 and the standard answer is 10.2 mm, rounded down. The published charts were settled by which drills actually exist in common series and by what taps tolerate, not by a rounding rule - so treat the formula as the compass and the actual drill inventory as the map. Directionally, the safe error is a slightly larger hole (a marginally shallower thread) rather than a smaller one (a broken tap); when the formula lands midway, hardware-store practice rounds toward the drill you can buy tomorrow. And the same rule runs the whole table: fine pitches subtract less, so fine-series tap drills sit larger than their coarse siblings - M12x1.25 wants about 10.8 mm where coarse M12 wants 10.2, one of the reasons mixing series ruins work so quickly.

Every field carries a decision. M8: nominal diameter, 8 millimeters across the crests. x1.25: the pitch in millimeters between crests - omit it and the world assumes the coarse default (1.25 happens to be M8 coarse anyway; on M10 the same written form would claim the fine 1.25). LH: left hand - threads tighten counterclockwise, used where rotation would loosen a right-hand thread (left pedals, saw arbors, gas bottles in some markets). Longer callouts add class (6H for nuts, 6g for bolts - the manufacturing tolerance band), coating (the cheap zinc that changes friction), and length for studs. The nut side mirrors the bolt: a nut specified as M8 threads only with an M8x1.25 bolt - nut tables and bolt tables share the same pitch column, which is why one chart serves both. Two cross-system warnings round out the reading: American UNC/UNF threads and metric threads share no pitches (a 1/4-20 is 20 threads per inch, about a 1.27 millimeter pitch - close to M12x1.25 but not equal, and the near-miss will strip after a few turns), and whitworth-legacy threads hide in old British and Commonwealth hardware with a 55-degree profile that no metric gauge reads honestly. When a bolt resists but seems to fit, stop and measure the pitch before the threads tell you the hard way.