A tapped hole marked M10x1.5-6H looks simple, but every part of that callout decides which tap you buy, which gauge you check with and whether the customer's bolt will go in. Many thread rejections start with a quote that read the callout loosely. This guide walks through each part of an ISO metric thread designation and the questions worth asking before you quote.

The parts of a metric thread callout

A typical designation has up to five pieces of information. As RoyMech's drawing reference lays it out, the callout gives:

  • Thread symbol - "M" for ISO metric.
  • Nominal size - the major diameter in millimetres, for example 10.
  • Pitch - written after an "x", for example x 1.5, the distance from one thread crest to the next.
  • Tolerance class - for example 6H for an internal thread or 6g for an external thread.
  • Thread length - for example "x 30 LNG", if the length is not dimensioned separately.

So M10x1.5-6H reads as: ISO metric, 10 mm nominal major diameter, 1.5 mm pitch, internal thread with tolerance class 6H.

When the pitch is missing

If a callout says only M8-6H, the pitch is not written. RoyMech notes that if the thread is the standard coarse series the pitch "need not be shown", but that it is better always shown. On the shop floor, this is a common source of confusion when a drawing mixes coarse and fine threads, or when the part was redrawn from an older drawing.

A practical rule: if the pitch is missing, treat it as coarse, but write that assumption on your quote. If the mating part is a fine-pitch bolt, you want the customer to correct the drawing before you cut metal, not after a lot fails assembly.

What 6H and 6g actually mean

The tolerance class is a number followed by a letter. As Engineer Fix explains:

  • The number is the tolerance grade. Metric thread grades typically run from 4 to 8, and grade 6 is the standard, general-purpose grade.
  • The letter is the tolerance position, also called the fundamental deviation. Capital letters are used for internal threads (tapped holes, nuts) and lower-case letters for external threads (bolts, studs).
  • Lower-case h means zero fundamental deviation on an external thread. The more common g gives a small allowance, so there is a little clearance even when both parts are at the limit.

RoyMech groups the common combinations into three classes of fit: fine (5H/4h), medium (6H/6g) and coarse (7H/8g). For most commercial work, 6H/6g is what you will see.

Two details that matter when you plan inspection:

  • On an internal thread, a single class such as 6H applies to both the pitch diameter and the minor diameter. A callout like 6H7H means 6H on the pitch diameter and 7H on the minor diameter.
  • On an external thread, a single class such as 6g applies to the pitch diameter and the major diameter.
A tighter or unusual thread class is not just a gauge change. It can mean a different tap, a different process and a different price.

Left-hand threads and other suffixes

Threads are right-handed unless the callout adds LH. RoyMech gives the example M8-6H-LH for a left-hand M8 coarse internal thread. LH taps and gauges are usually not in stock in a job shop, so an LH callout should show up in your quote as a lead-time and tooling item, not be discovered at the machine.

Also check for notes such as thread depth, full-thread length versus drill depth, countersink at the thread start, and any plating note. Plating adds thickness on an external thread, so ask whether the class applies before or after plating.

Which standard sits behind the callout

The tolerance system for ISO general-purpose metric threads is defined in ISO 965. The ISO page for ISO 965-1:2013 states that it specifies the tolerance system for metric threads conforming to ISO 261, and shows that edition as withdrawn, with ISO 965-1:2026 listed as the new version. If a customer drawing or PO references a specific edition, keep the matching gauge certificates ready for audits.

A pre-quote checklist for thread callouts

Before you send a price, run each threaded feature through these questions:

  • Is the pitch shown? If not, have you stated your coarse-pitch assumption?
  • Is the tolerance class shown? If not, is 6H/6g acceptable to the customer in writing?
  • Is it RH or LH?
  • Are thread depth and drill depth both clear for blind holes?
  • Does the class apply before or after plating?
  • Which gauges will the customer use to accept the part, and do you have them?

For example, on a flange drawing with eight tapped holes noted only as "M8 tapped through", these questions turn one vague note into three clear lines on your clarification list.

How SpecReview helps

Upload the drawing PDF or STEP model to SpecReview and it lists spec gaps and unclear notes to clarify before quoting, such as a thread with no pitch or tolerance class. Its layout inspection plan puts each threaded feature on the inspection list, with every item tagged as coming from the spec, the geometry or an assumption.