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Unified and Metric Thread Class and Tolerance Reference (2A, 3A, 6g, 6H)

Thread class tells you how tight the fit is and whether there is an allowance for plating. This reference decodes Unified classes and metric tolerance classes in one place, so machinists and inspectors read a drawing the same way.

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System	Callout	External or internal	Allowance	Fit and tolerance	Typical use
Unified (ASME B1.1)	1A	External	Yes	Loosest, widest tolerance	Quick assembly, dirty or plated environments
Unified (ASME B1.1)	2A	External	Yes	Medium, most common	General commercial and mechanical fasteners
Unified (ASME B1.1)	3A	External	No	Tightest tolerance	Precision, aerospace and critical assemblies
Unified (ASME B1.1)	1B	Internal	No	Loosest, widest tolerance	Quick assembly, mates with 1A
Unified (ASME B1.1)	2B	Internal	No	Medium, most common	General tapped holes and nuts, mates with 2A
Unified (ASME B1.1)	3B	Internal	No	Tightest tolerance	Precision tapped holes, mates with 3A
Unified (ASME B1.1)	2A/2B pair	Both	External only	Free running fit with room for plating	Default callout when drawing is silent in the US
Unified (ASME B1.1)	UNJ (3A/3B style)	Both	No	Controlled root radius, tighter than UN	Aerospace fasteners, higher fatigue strength
Metric (ISO 965)	6g	External	Small fundamental deviation g	Medium tolerance grade 6	General purpose bolts, the usual external callout
Metric (ISO 965)	6e	External	Larger deviation e	Medium tolerance grade 6	Heavier coatings, room for thick plating
Metric (ISO 965)	6h	External	None	Medium tolerance grade 6	Parts with a close fit, light or no plating
Metric (ISO 965)	4h	External	None	Fine tolerance grade 4	Precision parts, tight fit
Metric (ISO 965)	6H	Internal	None	Medium tolerance grade 6	General tapped holes and nuts, mates with 6g
Metric (ISO 965)	5H	Internal	None	Finer tolerance grade 5	Close fit tapped holes
Metric (ISO 965)	7H	Internal	None	Looser tolerance grade 7	Coarse fit, deep or blind holes
Metric (ISO 965)	6H/6g	Both	External only	Standard medium fit pair	Default metric callout for commercial parts

Allowance is the intentional gap between mating pitch diameters, used for plating, lubrication and ease of assembly. Exact pitch diameter limits depend on size and pitch and are listed in ASME B1.1 and ISO 965 tables or in Machinery's Handbook. This sheet explains the callouts and does not replace those tables.

Link thread class to your inspection plan

Optional: we can show how to add thread class and gauge type to inspection characteristics in your ERP.

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How to read a thread callout

In the Unified system a callout such as 1/4-20 UNC-2A has the nominal size, threads per inch, the series, and a class. The number is the tolerance class: 1 is loose, 2 is general, 3 is close. The letter is the thread type: A for external and B for internal. In the metric system, M10 x 1.5 6g means a medium tolerance grade 6 on an external thread with fundamental deviation g, while 6H is the usual internal pair. Search the table for your callout and use the Fit and Allowance columns.

A means external thread, B means internal thread.
Lowercase g, e, h are external; uppercase H, G are internal.
Only classes 1A and 2A, and metric g and e, include allowance.

Why allowance exists

Allowance lets a plated or coated bolt still go into a nut. A 2A thread has allowance to leave room for a thin coating, but the plated thread must still pass the maximum material limit of the unplated class. A 3A thread has no allowance, so a plated 3A part must be machined undersize before plating, or the coating thickness controlled to a very low value. If your drawing says dimensions apply after plating, the machined size must be calculated back from that.

Choosing a class

Class 2A/2B and metric 6g/6H cover most commercial work and are the best default when a drawing is silent. Class 3A/3B and metric 4h/5H cost more because they need better tooling, tighter process control and often a go/no-go plus functional gauging for every lot. Pick a looser class when the thread is in a hostile environment or needs to assemble quickly. Pick a tighter class when vibration, sealing or fatigue life matters and the customer demands it.

Common mistakes

Machinists often cut a 3B tapped hole with a 2B tap and rely on measurement to pass. That works only if the tap is ground for the class and the material cooperates. Another mistake is assuming 6g and 6H mate with each other in the same pitch diameter limits: there is an allowance on the external side, so the nominal thread is not the same as the limits. Always gauge with the right go/no-go class, and record the gauge in your inspection plan.

Frequently Asked Questions

What is the difference between 2A and 3A threads?

2A is the common general purpose class with an allowance and medium tolerance. 3A has no allowance and a tighter tolerance, so it is used in precision and aerospace work. 3A parts cost more to make and inspect.

What thread class should I use if the drawing is silent?

In US practice the default is 2A for external and 2B for internal Unified threads. In metric practice the usual default is 6g external and 6H internal. Confirm with the customer when the part is critical.

Does the B in 2B mean anything about material?

No. A and B only mean external and internal. B is internal threads such as a tapped hole or nut. It has nothing to do with the material or grade of the part.

What does 6H mean on a metric thread?

6 is the tolerance grade for a medium fit and H is the position of the tolerance for an internal thread, which starts at the basic size with no allowance. It normally mates with a 6g external thread.

If your ERP quality module cannot store thread class on the operation or inspection characteristic, Netray can show you how to add it without touching core code.

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