What is the difference between 2A and 2B threads?

A is an external thread — a bolt or stud — and B is internal, a nut or tapped hole. Both are the standard class. A 2A bolt is designed to assemble with a 2B nut, and that pairing covers the great majority of commercial fasteners.

What does 6g mean on a metric thread?

It is the standard metric external thread class. The g indicates a fundamental deviation below nominal, which provides an allowance so that a plated thread still assembles. It pairs with a 6H internal thread, which is the metric default for nuts and tapped holes.

Should I specify class 3 threads?

Only where the thread is a locating feature or a controlling drawing requires it. Class 3 has no allowance, so both parts must be made near the middle of their tolerance zones — more expensive for both, and no functional benefit for a joint that only has to hold two parts together.

Why are metric classes shown as 6g rather than 2A?

Because the two systems name their classes differently. Unified uses numbers 1 to 3 with letters A and B; metric uses numbers 4 to 8 with letters for the fundamental deviation and case for internal or external. They express the same idea — a class number for tightness and a letter for which part — in different notation.

What happens if I plate a 6h bolt?

It may not fit its nut. A 6h external thread has no allowance, so the coating thickness adds directly to the thread size. Where a fastener will be plated after threading, 6g — or a purpose-made allowance — is the correct class.

Thread Tolerance Classes

Thirteen Unified and metric thread classes, what each one means, and which combinations are used in practice.

Data verified 2026-09-29 · based on ASME B1.1-2019

Find a class

Thread Tolerance Classes (ASME B1.1 and ISO 965)

Class^[1]	Applies to^[1]	Fit^[1]	Character^[1]	Where it is used^[1]
1A #	Unified, external	Loose	No allowance control; largest clearance	For quick assembly where threads may be dirty or damaged — ordnance and some agricultural equipment.
2A #	Unified, external	Standard	The default external class	The ordinary commercial bolt. Assembled with a 2B nut, it gives a free-running fit with enough clearance for plating.
3A #	Unified, external	Tight	No allowance; close fit	Advanced, aerospace and instrumentation fasteners. Requires both parts near the middle of their tolerance zones to assemble.
1B #	Unified, internal	Loose	No allowance control; largest clearance	The matching nut or tapped hole for a 1A fastener.
2B #	Unified, internal	Standard	The default internal class	The ordinary commercial nut and the default tapped hole. Pairs with 2A.
3B #	Unified, internal	Tight	No allowance; close fit	Used with 3A fasteners where a close, accurate fit is required.
4g #	Metric, external	Very tight	Large fundamental deviation, small tolerance	Fine-pitch precision work. Rarely used outside instrument and aerospace applications.
6g #	Metric, external	Standard	The default metric external class	The ordinary metric bolt. The g indicates an allowance below nominal so plated threads still assemble.
6h #	Metric, external	Standard, no allowance	Default where a coating is not applied	Common on unplated fasteners and on threads that have been rolled after coating.
8g #	Metric, external	Loose	Large tolerance	Hot-dip galvanised and other heavily coated fasteners, where the coating occupies much of the clearance.
4H #	Metric, internal	Very tight	Small tolerance, no allowance	Precision internal threads for instrument and aerospace work.
6H #	Metric, internal	Standard	The default metric internal class	The ordinary metric nut and tapped hole. Pairs with 6g or 6h.
7H #	Metric, internal	Loose	Larger tolerance	Where tapping is difficult or the material is gummy, and for threads that will be plated after tapping.

The letter tells you which part, the number tells you how tight, and case matters. In the Unified system, A is an external thread (a bolt) and B is internal (a nut); classes 1, 2 and 3 run from loose to tight. In the metric system, lower case is external and upper case is internal — so 6g is a bolt and 6H is a nut, and confusing the two produces a drawing that specifies a nut on a bolt.

Class 2 is the default; class 3 exists for a reason. Class 3 threads have no allowance — both parts are made to nominal, so they assemble only if each is near the middle of its tolerance zone. That makes both parts more expensive to produce and does not make them inherently better. Class 2 is right for almost everything.

The metric allowance letter works differently from the Unified system. The g in 6g places the external thread's tolerance zone below nominal, leaving room for a coating — which is why 6g is chosen for plated fasteners and 6h for unplated ones. A 6h bolt plated after threading can end up too large for its nut.

Choosing a Class

The question is not which class is best but which one the application needs, and the answer is almost always the standard class.

Use class 2 (or 6g/6H) unless you have a specific reason not to. It gives a free-running assembly with enough clearance to tolerate plating, minor damage and ordinary manufacturing variation. If a joint needs to be tighter than that, the right answer is usually a different locking method rather than a tighter thread class.

Class 3 (or 4g/4H) is for applications where the thread itself is a locating feature — instrument threads, precision adjusters, aerospace fasteners where the class is specified by the controlling drawing. It requires both parts to be made accurately and is wasteful where the thread only has to hold two parts together.

Loose classes are for difficult conditions, not for convenience. Class 1 and 7H exist for threads that will be heavily coated, assembled in the field, or tapped into gummy material. Specifying them because a thread is hard to tap is treating the symptom — the usual cause is a dull tap, the wrong tap geometry, or a pilot hole that is too small.

One practical point about coating: a plated 2A or 6g external thread gains the coating thickness on both flanks, effectively growing the thread. That is exactly what the allowance in the class is for, and it is why the class must be chosen before plating is specified rather than after.

Frequently Asked Questions

What is the difference between 2A and 2B threads?: A is an external thread — a bolt or stud — and B is internal, a nut or tapped hole. Both are the standard class. A 2A bolt is designed to assemble with a 2B nut, and that pairing covers the great majority of commercial fasteners.
What does 6g mean on a metric thread?: It is the standard metric external thread class. The g indicates a fundamental deviation below nominal, which provides an allowance so that a plated thread still assembles. It pairs with a 6H internal thread, which is the metric default for nuts and tapped holes.
Should I specify class 3 threads?: Only where the thread is a locating feature or a controlling drawing requires it. Class 3 has no allowance, so both parts must be made near the middle of their tolerance zones — more expensive for both, and no functional benefit for a joint that only has to hold two parts together.
Why are metric classes shown as 6g rather than 2A?: Because the two systems name their classes differently. Unified uses numbers 1 to 3 with letters A and B; metric uses numbers 4 to 8 with letters for the fundamental deviation and case for internal or external. They express the same idea — a class number for tightness and a letter for which part — in different notation.
What happens if I plate a 6h bolt?: It may not fit its nut. A 6h external thread has no allowance, so the coating thickness adds directly to the thread size. Where a fastener will be plated after threading, 6g — or a purpose-made allowance — is the correct class.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	standard	ASME B1.1-2019
[2]	ISO 965-1 — ISO general purpose metric screw threads, tolerances, principles and basic data	standard	ISO 965-1:2013
[3]	ISO 965-2 — Limits of sizes for general purpose external and internal screw threads	standard	ISO 965-2:2024

Data Sources

Standard	Revision	What it covers on this page
ASME B1.1 — Unified Inch Screw Threads	ASME B1.1-2019	the 1A to 3B Unified classes
ISO 965-1 — Metric screw thread tolerances: principles and basic data	ISO 965-1:2013	the metric class notation and fundamental deviations
ISO 965-2 — Limits of sizes for general purpose external and internal screw threads	ISO 965-2:2024	the limit dimensions for each metric class

Cross-checked against:

Every class checked against the ASME B1.1 and ISO 965 class definitions
The case convention verified for both systems — A/B uppercase in Unified, g/H case-coded in metric
The coating allowance relationship between 6g and 6h cross-checked against plating practice

Classes describe the thread fit only. The limit dimensions for a specific class and size come from ASME B1.1 or ISO 965-2 tables and are not given here. Where a fastener is plated after threading, the class must be chosen to leave room for the coating.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Thread Tolerance Classes." https://refmanual.com/charts/thread-tolerance-classes/ (accessed 2026-09-29). Data from ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019.

BibTeX

@misc{thread-tolerance-classes,
  title        = {Thread Tolerance Classes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/thread-tolerance-classes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Thread Tolerance Classes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/thread-tolerance-classes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B1.1 — Unified Inch Screw Threads, ASME B1.1-2019
ER  -

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