Metric Screw Threads

§1The world default

The ISO metric thread is the fastening standard of nearly the whole planet — the system the previous section’s bolts, screws and nuts silently assumed, and the one every other page in this section is ultimately compared to.

Its geometry contains nothing new: the profile is the systems page’s 60° vee, the working dimensions are the calculating page’s ladder of H-fractions, and the strength arithmetic is the metric fasteners page’s A_s figures — this is the thread all of those were already describing. What the ISO system adds is administration, and administration is precisely what makes a thread a standard: a designation that fully specifies the thread in a handful of characters (§2), a fixed table pairing every diameter with its coarse and fine pitches (§3), a tolerance language that puts the whole fits-and-limits machinery of the dimensioning pages onto the helix in two characters (§4), and rules for the awkward cases — coatings that fatten the thread (§5), and threads that wind faster or backwards (§6). Where the Unified page told the story of how the inch world converged, this page describes the system that converged everything else; the engineering content is the same triangle, and the value is in how completely two lines of text — M10 × 1.5 – 6g — determine a physical object anywhere on earth.

§2Reading the designation

A metric designation states the profile, the size, the pitch and the fit, in that order — and its most consequential rule is what happens when the pitch is left out.

The hero decodes the tokens. M declares the ISO metric 60° profile. 10 is the nominal major diameter in millimetres — the same physical meaning as the Unified system’s leading fraction, without gauge numbers or reciprocals. 1.5 is the pitch, stated directly in millimetres of advance per thread rather than counted per inch — the deep cultural difference between the systems, and the reason a metric pitch gauge reads in millimetres while an inch one reads in TPI. And 6g is the external tolerance class of §4; an internal thread would carry a capital class, 6H. The consequential rule: a designation with no pitch means the coarse pitch — plain M10 is M10×1.5 by definition, and writing the pitch explicitly is what flags a fine thread (M10×1.25). It is an elegant default and a standing trap in one: a drawing that says M10 and a store bin that holds M10×1.25 will produce parts that start, bind and strip — so the workshop habit is to confirm pitch with a gauge whenever a fine series could plausibly be present, exactly the two-second check the pipe-threads page will make a doctrine of.

§3The pitch series

Each diameter is paired with one coarse pitch and, for most sizes, one or two standardised fine pitches — a fixed menu, not a free choice.

§46g and 6H

The two-character tolerance class is the fits system of the dimensioning pages wrapped around a helix: the number is how wide the tolerance is, the letter is where it sits.

In 6g, the 6 is the tolerance grade — the width of the band, larger numbers looser, exactly like the IT grades of the hole-and-shaft system — and the g is the fundamental deviation, the guaranteed offset from basic size, with lower-case letters for external threads and capitals for internal. The default commercial fit is 6g on the bolt, 6H in the nut: H puts the nut’s tolerance band exactly at basic size with zero allowance, while g steps the bolt a small guaranteed clearance below basic — so a maximum-metal bolt still enters a minimum-metal nut freely, and there is room for a thin protective coating. That small g-allowance is the metric cousin of the Unified page’s 2A allowance, arrived at independently for the same reasons. Around the default sit the letters engineers actually meet: h, zero allowance, for snug precision fits (6h with 6H is the metric analogue of 3A/3B); and e, a much larger allowance, whose entire purpose is §5’s problem — leaving room for heavy coatings. As with the Unified classes, mixing is legitimate: the class describes one side of the fit, and the joint takes whichever pairing the duty needs.

§5The 4× plating rule

Coat a 60° thread and its pitch diameter grows by four times the coating thickness — a pure consequence of the flank geometry, and the reason galvanised fasteners are a tolerance problem before they are anything else.

§6Multi-start and left-hand

The designation extends to the exceptions — threads that advance faster than their pitch, and threads that wind the other way — with the lead stated explicitly so nobody has to infer it.

A multi-start metric thread is written with both numbers: M10 × Ph3 P1.5 declares a lead (Ph) of 3 mm carried on a pitch (P) of 1.5 mm — two starts, stated rather than implied, because the difference is mechanically drastic. The systems page computed it: the single-start M10×1.5 climbs at 3.03°, while the two-start version’s lead angle doubles to 6.04° — twice the advance per turn, half the mechanical advantage, and a substantial bite out of the self-locking margin, all from a thread whose profile and pitch gauge reading are identical to the single-start’s. That identical appearance is why the designation insists on spelling the lead out, and why an unknown multi-start thread is identified by marking one crest and counting the ridge starts at the end face, never by the pitch gauge alone. Left-hand threads simply append LH — M10×1.5-LH — with the same duties as everywhere in the section: turnbuckle ends, spindles whose rotation would unscrew a right-hand thread, and the occasional deliberate mismatch that stops the wrong part being fitted. Both notations exist for one reason: the default (single-start, right-hand) is so overwhelming that any departure from it must be written where it cannot be missed.

§7Quick reference

The working core of the page on one card rack.