ISO metric threads: pitch, designations, tolerances and callouts

The ISO metric thread is the default fastener thread across UK industry. It appears on drawings, in purchase orders and on inspection sheets every day, yet the shorthand used to describe it is often only half understood.

This guide explains the form itself, the difference between coarse and fine pitch, how designations are built, what tolerance classes mean and how to read a typical callout.

What is an ISO metric thread?

An ISO metric thread is a 60 degree V-form thread defined by its nominal diameter in millimetres and its pitch, which is the distance between adjacent thread crests, also in millimetres. It is standardised internationally by ISO, and British Standards adopt those documents in the UK.

The profile is the same for external threads (bolts, studs, screws) and internal threads (nuts, tapped holes). What differs is the tolerance applied to each, which is what allows them to mate.

The designation letter is M. An M10 thread has a nominal major diameter of 10 mm. Do not confuse it with unified threads (UNC and UNF) or BSP pipe threads, which use different forms and cannot be substituted.

Coarse and fine pitch

Each nominal diameter has one standard coarse pitch. If a drawing says M10 with no pitch, it means the coarse pitch for that size. Coarse is the general-purpose choice and is the easiest to source in fasteners and taps.

Fine pitches are available for many diameters. They are shown explicitly, for example M10 x 1.25 or M10 x 1.

Coarse threads are quicker to assemble, more tolerant of damage and dirt, and better in softer materials such as aluminium and cast iron.
Fine threads have a larger stress area for the same diameter, resist loosening under vibration better and allow finer adjustment.
Fine threads are more easily cross-threaded and damaged, and parts are often harder to source.

Check the pitch against a current thread table rather than memory. Pitch values for each size are set out in the relevant ISO standard.

How designations are built

A full designation is made of several parts, always in the same order:

The letter M and the nominal diameter, for example M12.
The pitch, preceded by a multiplication sign, for example x 1.5. Omitted for coarse.
The tolerance class, for example 6g or 6H.
Optional extras, such as thread length, left-hand (LH) or multi-start information.

Right-hand threads are assumed unless LH is stated. Single-start threads are also assumed, so a multi-start thread must be called out, usually by giving the lead and the pitch.

Understanding tolerance classes

The tolerance class is a number and a letter, such as 6g or 6H. The number is the tolerance grade, which sets how large the permitted variation is. Lower numbers mean tighter tolerances.

The letter gives the tolerance position, which is the allowance from the basic size. A lower case letter denotes an external thread and an upper case letter denotes an internal thread.

For general engineering, 6g for bolts and 6H for nuts and tapped holes is the common medium fit. A 6g external thread carries a small allowance, which helps assembly and leaves room for plating or coating. A 6H internal thread has no allowance.

Tighter classes such as 4h or 5H suit precision work, while looser ones such as 8g suit hot-dip galvanised or dirty environments. Specify them only where the application needs it, as tighter classes raise inspection and tooling cost.

How to read a thread callout

Take the callout M12 x 1.5 - 6g. This is an external thread, 12 mm nominal diameter, 1.5 mm pitch, medium tolerance class 6g. Because 1.5 mm is a fine pitch for M12, it is shown explicitly.

Now take M8 - 6H. This is an internal thread, 8 mm nominal diameter, coarse pitch implied, tolerance class 6H.

Mating parts can be shown together as M10 - 6H/6g, with the internal class first, then the external class. Tapped holes on drawings often add depth, such as M8 x 15 deep, and may separately show a drill depth. Check whether the thread depth is full-form or includes the run-out.

Standards also set out tolerance classes by length of engagement (short, normal and long). Where none is stated, normal is assumed.

Practical points for the shop floor

Quote the pitch whenever it is not coarse, on both drawings and orders.
Allow for coating thickness on external threads. Plating can consume the allowance and make a 6g thread tight.
Use go and no-go gauges, or thread plug and ring gauges, matched to the stated class.
Keep taps and dies for different pitches clearly separated, since M10 coarse and fine look nearly identical.
Confirm the standard and issue referenced on the drawing with your supplier.

Where a customer or auditor requires traceable measurement, gauges should be calibrated by a laboratory accredited by UKAS or an equivalent body.

What does M in a thread designation mean?

M denotes an ISO metric thread. The number after it is the nominal major diameter in millimetres.

What is the difference between coarse and fine pitch?

Pitch is the distance between thread crests. Each diameter has one standard coarse pitch, and fine pitches are optional alternatives with more threads per length, shown explicitly in the callout.

What does 6g or 6H mean on a thread?

The number is the tolerance grade and the letter is the tolerance position. Lower case g is for external threads and upper case H is for internal threads. 6g/6H is the common general-purpose fit.

Can I use a metric bolt in an imperial tapped hole?

No. The forms and pitches differ, so the thread may start to engage but will not seat properly and is likely to be damaged. Match the thread system on both parts.