Metric Threads Explained: How to Read a Thread Specification, Pitch, Hand and Tolerance Fits

A bolt has two identities. The numbers stamped on its head state its property class — how strong it is. The thread specification on the drawing states its geometry — what it screws into, how it fits, and whether it will assemble at all. Buyers who are fluent in the first and hazy on the second still end up with bolts that do not fit the tapped hole, nuts that seize halfway down, or a "same size" replacement that is not the same thread.

This guide reads a metric thread specification the way you would read it on a drawing: the designation first, then pitch, hand, and the tolerance fit that decides how the finished thread actually assembles.

Reading the designation: M10 × 1.5

The standard metric thread callout has three pieces of information packed into a short string:

• M — the thread is an ISO metric thread, with the standard 60° V-profile.
• 10 — the nominal major diameter in millimetres: the diameter across the crests of an external thread (a bolt), 10 mm here.
• 1.5 — the pitch in millimetres: the axial distance from one thread crest to the next.

So M10 × 1.5 is a 10 mm metric thread advancing 1.5 mm per turn. Two reading rules save most ordering mistakes:

If the pitch is missing, it means coarse. M10 with nothing after it is M10 × 1.5 by convention — the standard coarse pitch for that diameter. The pitch is only written out when it is not the coarse one.

Fine threads must always state their pitch. M10 × 1.25 and M10 × 1.0 are both real, common threads, and both are different parts from M10 × 1.5 and from each other. A fine-thread order that omits the pitch is an incomplete specification.

For reference, the standard coarse pitches across our cold-headed range (all standard ISO values):

One measurement habit worth having: pitch is the metric counterpart of the imperial "threads per inch", but they are read in opposite directions — TPI counts threads, pitch measures the gap between them. Mixing the two systems is how a 1.5 mm pitch becomes a "16 TPI" order that fits nothing. If you are checking an existing bolt, measure the major diameter with calipers and the pitch with a thread pitch gauge (or measure the length spanned by a run of threads and divide by the number of pitches in that span); diameter alone does not identify a thread.

Coarse vs fine: the same diameter, two different threads

Neither is "better". Coarse is the default because most joints want fast, robust assembly; fine is specified when the designer needs the extra stress area, the anti-loosening behaviour, or a shallow thread in a thin wall. What matters for ordering is that the choice is written down: the diameter is only half the specification.

Thread size is geometry; it is not strength. The same M10 × 1.5 exists in classes 4.8 through 12.9, with very different working loads — our strength concepts guide covers what separates them, and the property class guide covers choosing between them.

Hand: the direction nobody writes down

Threads are right-hand by default: the bolt advances away from you when turned clockwise. A right-hand thread carries no marking — the absence of a note is the specification. A left-hand thread must be marked LH (for example, M10 × 1.5-LH), and it will only mate with a left-hand internal thread.

Left-hand threads appear where a right-hand thread would unscrew itself in service — rotating equipment, certain tensioning and adjustment assemblies. The ordering mistake is symmetrical and expensive: if your drawing needs LH and does not say so, you will receive right-hand parts; if a replacement bolt "almost fits but will not start", check the hand before blaming the diameter.

Tolerance and fit: 6H / 6g, and what the letters decide

A nominal M10 thread is never exactly 10.000 mm. The tolerance designation states how far the finished thread may sit from nominal, and it is written as a number plus a letter, for example M10 × 1.5-6g for an external thread or M10 × 1.5-6H for an internal one:

• The number is the tolerance grade — how wide the allowed band is. Smaller number, tighter band.
• The letter is the fundamental deviation — where that band sits relative to nominal. Capital letters (G, H) are used for internal threads, lower-case (e, f, g, h) for external threads.

The combination you will see most often is 6H / 6g, the standard medium fit for general engineering: the internal thread (6H) sits at nominal and above, the external thread (6g) sits slightly below it, leaving a small designed clearance. That clearance is functional, not sloppy — it guarantees the parts assemble by hand across production variation, and it is also the space that surface finishes occupy: an electroplated or coated bolt is thicker than the bare thread that was measured, which is one reason the finish belongs in the same conversation as the thread fit (we work in four finishes — black oxide, Dacromet, colour zinc and zinc plating — and the fit question comes up differently for each).

Two boundary notes. First, thread tolerance is its own standard system; it is not the same thing as the dimensional tolerances on the rest of the drawing — when a drawing we manufacture to carries a ±0.05 mm requirement, that governs the part's linear dimensions, while the thread is governed by its tolerance class. Specifying one does not define the other. Second, omitting the tolerance class from a custom order does not make it "any fit will do" — it makes the fit the manufacturer's guess, and guesses are what incoming inspection disputes are made of.

Rolled vs cut threads: how the thread was made matters

The designation tells you the thread's geometry. It does not tell you how the thread was formed, and the two standard routes behave differently:

Rolling is a cold-forming process, which is why it pairs naturally with a cold heading line: the blank is headed cold, and the thread is rolled into it without cutting — the same material-conservation logic our cold heading guide explains for the bolt head applies to its thread. In production, rolling is the standard partner to a cold heading line (our cold-headed range is M5–M24), while machined parts (our range: M8–M64) have their threads formed as part of the machining work — which is also where specials, unusual pitches and left-hand threads are easiest to accommodate. Whichever route forms it, the finished thread still has to be verified: dimensional and contour inspection are part of our in-house checks, with reports available to accompany an order.

Getting the thread right also decides whether the torque you apply later means anything: thread friction is one of the largest variables in tightening, as our torque and preload guide works through with numbers.

Common mistakes

• Ordering by diameter alone. "M10" without pitch and tolerance is half a specification; fine threads and fits are decided by the parts you did not write down.
• Assuming a missing pitch note means coarse — on a fine-thread part. The convention holds for new drawings, not for identifying an unknown sample. Measure the pitch on the sample.
• Forgetting the hand. An unmarked drawing produces right-hand threads. If the application needs LH, those letters are the entire difference between a usable part and scrap.
• Treating the tolerance class as decoration. Fit decides assembly, galling and how much room a finish has. 6H/6g is a sensible default, not an automatic one.
• Confusing thread tolerance with drawing tolerances. ±0.05 mm on a shank or head dimension says nothing about the thread; the thread needs its own class on the drawing.

FAQ

Does M10 always mean M10 × 1.5?
On a drawing that follows the standard convention, yes — an unmarked pitch means the coarse pitch for that diameter, which is 1.5 mm for M10. On a physical sample from an unknown source, do not assume: fine-thread M10 exists, and the two are not interchangeable. Measure the pitch.

Why does a fine thread resist loosening better?
Because its helix angle is shallower: each turn advances the bolt less, so the thread behaves more like a wedge that stays put and less like a ramp. The trade-offs are slower assembly and shallower, more damage-sensitive teeth.

What fit should I put on a custom bolt drawing?
For general engineering joints, 6H (internal) / 6g (external) is the standard medium fit and the right default for most orders. Move away from it deliberately — tighter for precision location, looser where thick coatings or high-temperature assembly demand it — and write whichever you choose on the drawing.

Can you make left-hand and fine threads?
Yes — pitch, hand and tolerance class are part of the drawing we manufacture to, in property classes 4.8–12.9. State them on the drawing via the custom fasteners page and they are produced and inspected as specified.

HyHaven manufactures custom fasteners to your drawing or sample, in property classes 4.8–12.9, from 2,000 pieces. Request a quote.