Guide
What does a metric thread class like 6H or 6g mean, and when is 4H or 5H worth it?
A metric thread class is the grade-and-position symbol after an ISO metric thread size, such as the 6H in M8x1.25-6H or the 6g in M8x1.25-6g, that sets how large the pitch and crest diameters of that thread may be. The digit is the tolerance grade (a smaller digit means a narrower band) and the letter is the tolerance position (capital for internal threads, lowercase for external ones), both defined in ISO 965-1.
By Saola Manufacturing engineering team · Published · Updated
Most engineers already know that 6H/6g is the default metric pair. This guide is about the decision that comes after: what the 4H, 5H and 7H grades actually buy in microns, why a 5H on one drawing can be the ordinary medium-quality choice while on another it is a genuine tightening, what the e, f and g positions are for, and why a narrow internal class is the first thing to fail once a part is plated. For how to write the callout itself, element by element, see the thread callout guide; for the inch counterpart, see class 2B vs 3B. The values below describe the standards, not the capability of any shop.
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How is a metric thread class built from a grade and a position?
The grade sets the width of the tolerance band and the position sets where that band sits relative to the basic profile. Change the grade and the thread gets tighter or looser; change the position and the whole band moves, which is how ISO metric threads create clearance for assembly or coating.
Per ISO 965-1, internal threads use grades 4 to 8 on the pitch diameter, and external threads use grades 3 to 9. Grade 6 is the reference: the other grades are fixed multiples of it, so grade 4 is roughly 0.63 times the grade 6 band, grade 5 roughly 0.8 times, grade 7 roughly 1.25 times and grade 8 roughly 1.6 times, per ISO 965-1. Because the multiples are fixed, the step from 6H to 5H removes about a fifth of the pitch-diameter tolerance, and the step to 4H removes a little over a third, on every size.
The position is a letter that stands for a fundamental deviation. For internal threads, H puts the lower limit exactly on the basic pitch diameter and G lifts it slightly above basic. For external threads, h puts the upper limit on basic, and g, f and e pull it progressively further below basic. Those gaps are the metric version of the inch allowance: they guarantee clearance at maximum material, and they leave room for a coating. Unlike the inch classes, where the allowance is tied to the class number, ISO lets you choose the grade and the position independently.
ISO 965-1 then groups grade-and-position combinations into three tolerance qualities: fine, medium and coarse. Medium is the general-purpose quality, fine is for threads that need a closer fit, and coarse is for threads made under difficult conditions, such as long blind holes or hot-rolled bar. The quality, not the digit alone, is what a designer is really choosing.
How much tighter are 4H and 5H than 6H, in actual numbers?
On an M8x1.25 tapped hole, per ISO 965-1, the pitch-diameter band is 160 µm for 6H, 125 µm for 5H and 100 µm for 4H, and all three start at the same basic minimum. A tighter H grade only removes the loose end of the range; it never makes the smallest permitted hole any smaller.
The table puts one common size side by side across internal and external classes. Read the internal rows first: every H row begins at the basic pitch diameter, so a 4H hole at its best fits exactly like a 6H hole at its best. What the tighter grade buys is a smaller worst case on the loose side, which means less radial play and slightly more flank contact with a given screw. The 6G row shows the other lever: the band is the same width as 6H, but it starts above basic.
The external rows show the positions at work. The 6h row has no clearance at all, 6g sits a small step below basic, and 6f and 6e sit further down. For this pitch, a 6g bolt in a 6H hole always has at least the 6g deviation of clearance at maximum material, per ISO 965-1, which is why the pair assembles freely even when both parts are at their tight limits.
| Class | Fundamental deviation (µm) | Pitch-diameter tolerance (µm) | Pitch-diameter limits (mm) |
|---|---|---|---|
| 4H internal (fine, short engagement only) | 0 | 100 | 7.188 to 7.288 |
| 5H internal (fine) | 0 | 125 | 7.188 to 7.313 |
| 6H internal (medium) | 0 | 160 | 7.188 to 7.348 |
| 7H internal (coarse) | 0 | 200 | 7.188 to 7.388 |
| 6G internal (medium) | +28 | 160 | 7.216 to 7.376 |
| 4h external (fine) | 0 | 75 | 7.113 to 7.188 |
| 6h external (medium) | 0 | 118 | 7.070 to 7.188 |
| 6g external (medium) | −28 | 118 | 7.042 to 7.160 |
| 6f external (medium) | −42 | 118 | 7.028 to 7.146 |
| 6e external (medium) | −63 | 118 | 7.007 to 7.125 |
Why does the right grade depend on the length of engagement?
ISO 965-1 recommends a different grade for short, normal and long engagement within the same tolerance quality, so 5H can be the ordinary medium-quality choice on a short thread and 7H the medium choice on a long one. Before treating 5H on a drawing as a tightening, check how long the thread is.
The reasoning is about errors that are not pitch diameter. Lead and flank-angle errors make a thread behave as if its pitch diameter were larger (on an external thread) or smaller (on an internal thread), and those errors accumulate along the engaged length. A long engagement eats more of the band through lead error, so the standard gives it a wider band; a short engagement has little lead error to absorb, so the same quality of fit is achieved with a narrower band.
For an M8x1.25 thread, per ISO 965-1 the short group (S) covers engagements up to 4 mm, normal (N) runs above 4 mm up to 12 mm, and long (L) is anything beyond 12 mm. In the medium quality the standard recommends 5H for S, 6H for N and 7H for L on internal threads, and 5g6g, 6g and 7g6g on external threads. In the fine quality the steps are 4H, 5H and 6H inside and 3h4h, 4h and 5h4h outside. The two-symbol classes such as 5g6g give the pitch diameter one grade and the crest diameter another, with the pitch-diameter grade written first.
Two practical consequences follow. A thin flange or shallow boss tapped M8 falls in the short group, so 5H there is not a premium callout. And a deep tapped hole in a casting that engages well past the normal group can legitimately be called 7H, which is easier to tap, without weakening the joint in any way the designer would notice. ISO 965-1 adds the group letter only when the engagement is short or long, as in M8x1.25-5H-S or M8x1.25-7H-L; a class with no letter is read as normal engagement.
When does a metric thread class tighter than 6H/6g earn its cost?
Call 4H or 5H at normal engagement only where the thread fit itself does a job: locating, limiting radial play, carrying cyclic load through the flanks, or protecting a thin wall. A cap screw that clamps a cover gains nothing from it, because preload, not pitch-diameter fit, carries that joint.
The cost of a fine-quality internal class shows up mostly in tooling, gauging and inspection:
- Taps. Ground taps for ISO metric threads are made to tolerance classes defined in ISO 2857, and tap makers’ charts typically pair the tightest tap class with 4H and 5H holes, the middle class with 6H, and the loosest with 6G, 7H and 8H. A tighter tap class leaves less room for tap wear and for oversize cutting in soft or gummy material, so taps are typically changed sooner, and some holes move to thread milling, where size is adjusted by a cutter offset. The thread milling vs tapping guide compares the two.
- Gauges. Thread plug gauges are made to the limits of a specific class per ISO 1502. A 6H NOT GO plug does not reject an oversize 5H hole, so every non-default class on the drawing needs its own GO and NOT GO set, with its own calibration interval. The go/no-go thread gauge guide covers how those members are applied.
- Rejects and sorting. With a narrower band, a lot that would have gauged clean at 6H can split into good and borderline parts, and the borderline ones need measured pitch diameter rather than an attribute check to settle.
On the external side, 4h is the fine-quality class, but most purchased screws are 6g, and you rarely control them. If the aim is less play in a tapped hole, tightening the hole to 5H while keeping catalog 6g screws captures most of the benefit at a fraction of the effort; each side of an ISO metric joint is chosen independently. If the aim is a precise turned stud into a hole you do not own, 4h removes the clearance entirely, so the fit then depends on the other party’s hole, and 6g remains the safer call.
Why do tight metric classes fail after plating, and which position fixes it?
H and h positions have no clearance to spend, so any coating on the flanks moves the pitch diameter straight out of tolerance, and a narrow 4H or 5H band runs out first. The fix is to choose a position that carries clearance, 6G inside or 6g, 6f or 6e outside, or to state that the class applies before coating and mask the thread.
A coating of thickness t on a 60° flank changes the pitch diameter by about four times t, which is the relationship ISO 4042 uses to limit coating thickness on electroplated fasteners; the mechanics are worked through in the threads too tight after plating guide. The consequence for class choice is direct. The clearance available for coating is the fundamental deviation divided by four: on an M8x1.25 bolt that is about 7 µm for 6g, about 10 µm for 6f and about 16 µm for 6e, per ISO 965-1 deviations in the table above. Per ISO 965-1 and that quarter rule, a zinc layer of 8 µm on the flanks fits within 6f or 6e but overruns 6g at the tight end, and it has nowhere to go at all on 6h or 4h.
Internal threads are the harder case. A 6H or 5H hole starts at basic, so even a thin layer of plating inside it makes a hole cut at the bottom of the band reject on the GO plug. Plating inside a tapped hole is also uneven, typically heaviest at the mouth and thinner deeper down, so pre-tapping oversize only partly helps. That is why most drawings either mask internal threads, specify 6G where the coating must stay, or put the coating on the external part only. A 4H hole that must also be plated combines the narrowest band with the least predictable build-up, so it deserves a question to the designer before any parts are cut.
Whatever the position, write which state the class applies to. “M8x1.25-6g AFTER PLATING” makes the finisher responsible for landing inside 6g, and “M8x1.25-6H BEFORE PLATING, MASK THREAD” keeps the coating out. A class with no state written is read differently by the machinist, the plater and the receiving inspector. On surface finishing, the finishing page asks buyers to mark threads that must stay bare so they are masked the first time, and the masking guide covers how to draw that note.
What should the drawing, PO and RFQ say about a metric thread class?
Write the full designation with the class on every thread, add the engagement group when the class departs from the normal-group default, state the coating state, and list every non-6H/6g class in the RFQ so the gauges are priced on purpose.
Callout examples that leave nothing to interpret:
M6x1-6H— general tapped hole, medium quality, normal engagement. Coarse pitch may be omitted per ISO 965-1, but writing it costs nothing.M8x1.25-5H-S— short engagement in a thin section; medium quality, not a tightening.M8x1.25-5H— normal engagement, deliberately tightened to limit play; add a short note saying why, so nobody relaxes it later.M10x1.5-6g AFTER ZINC PLATE— external thread that keeps its coating; the finisher must land inside 6g after plating.M8x1.25-6H BEFORE COATING. MASK THREAD.— internal thread kept bare.
Common mistakes are just as specific. A metric class copied from an inch print by “converting” 3B is at best an approximation: ASME B1.13M describes a 4H5H/4h6h fit as only approximately equivalent to 3A/3B, because the two systems use different formulas. Case errors, pair notation and the other designation rules are covered in the thread callout guide. And a fine-quality class applied by a drawing template to every hole on a part multiplies gauge sets for no functional gain; review the template rather than the part.
For the RFQ, list each thread size with its class and coating state, flag any class other than 6H or 6g, and say how the thread will be accepted, for example GO/NOT GO plug gauges per ISO 1502. The machining RFQ guide covers the rest of the package, and the cost of tight tolerances guide explains why each tightened callout should carry a reason.
How does Saola handle metric thread classes on a quote?
On turned parts, thread fit is checked with go/no-go gauges before a lot ships, and a custom thread form means a custom gauge set. The first piece off new tooling is measured against every toleranced dimension on the drawing.
Threads are cut during CNC turning on turned shafts, inserts and fittings, and tapped holes are made during CNC milling on castings and blanks. The turning page explains that a custom thread form means a custom gauge set, and that the cost shows up in the unit price rather than as a separate line, so list every metric class, especially any other than 6H or 6g, on the RFQ. The quality page lists first-article inspection on every new part, with first-article reports and dimensional inspection records available on request. Zinc plating and anodizing are done at partner plants we manage, and the quote names where each step is done, so a thread that must be masked, or must land inside its class after plating, can be planned in one routing; say which on the drawing. ISO 9001 certification is in progress.
When the drawing is ready, send it for a quote with each metric class and coating state marked.
Frequently asked
Is 6H/6g a clearance fit?
Yes. The 6H hole starts at basic and the 6g screw stops below basic by the g deviation, so the pair always has some clearance at maximum material. A 6H/6h pair has zero minimum clearance, so parts at their tight limits make a line-to-line fit with nothing left for coating or dirt.
What is the metric equivalent of class 3B?
There is no exact equivalent. ASME B1.13M describes a 4H5H/4h6h fit as approximately equivalent to a 3A/3B fit, and 6H/6g as the metric class for applications where 2A/2B has been used, but the limits come from different formulas, so choose the metric class from ISO 965-1 directly.
Does a tighter metric thread class make the joint stronger?
Not meaningfully for a preloaded bolted joint, where clamp load does the work. A tighter class reduces play and keeps more flank in contact, which matters for locating threads and some cyclic-load cases, but thread strength is governed mainly by engagement length and material.
Why do fasteners for plating use 6e or 6f?
Those positions sit further below basic than 6g, so they leave room for a thicker coating while the plated thread still assembles into a 6H hole. ISO 4042 sets the coating-thickness rules for electroplated fasteners with ISO metric threads.
Can a supplier relax 5H to 6H?
Only with the designer’s written agreement. A supplier can ask whether the tighter grade is needed, especially if the engagement is short and 5H may simply be the medium-quality choice, but the class on the drawing is the acceptance criterion until the drawing is revised.
Sources
- ISO 965-1:2026, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data, ISO, 2026 (replaces ISO 965-1:2013)
- ISO 1502:1996, ISO general-purpose metric screw threads — Gauges and gauging, ISO, 1996
- ISO 2857:1973, Ground thread taps for ISO metric threads of tolerances 4H to 8H and 4G to 6G coarse and fine pitches — Manufacturing tolerances on the threaded portion, ISO, 1973
- ISO 4042:2022, Fasteners — Electroplated coating systems, ISO, 2022 (Amendment 1, 2026)
- ASME B1.13M-2005 (S2025), Metric Screw Threads: M Profile, ASME, 2005, stabilized 2025