Thread Class and Fit: 2A/2B and 6g/6H Explained

What tolerance is applied to

Tolerance classes are applied primarily to the pitch diameter, and that is the number worth learning to look up. The pitch diameter is where the flanks of the mating threads actually bear, so it governs both whether the parts assemble and how much play they have once assembled. Crest and root diameters carry tolerances too, but they are secondary - the pitch diameter is the controlling dimension in every thread standard.

For a 60° thread the basic pitch diameter is the major diameter less 0.6495 times the pitch, straight out of ISO 68-1 and ASME B1.1. Every ThreadRef size page lists it: an M10 × 1.5 has a basic pitch diameter of 9.026 mm, a 1/4-20 UNC has 0.2175 inch, a 1/2-13 UNC has 0.4500 inch. Those are the reference points that a tolerance class brackets.

A class of fit then specifies two things about that bracket. The first is the allowance - a deliberate offset that moves the external thread's tolerance zone away from the basic size, guaranteeing clearance even when both parts are made to their most generous limit. The second is the tolerance itself, the width of the permitted band. Loosening the fit can mean either a larger allowance or a wider tolerance, and the two classes systems package them differently.

The Unified classes: 1A/1B, 2A/2B, 3A/3B

ASME B1.1 uses a number for the fit and a letter for which part it applies to. A always means the external thread - the bolt or screw - and B always means the internal thread, the nut or tapped hole. The number runs 1 to 3, from loosest to tightest.

Class 1A/1B is a loose fit, intended for quick assembly, for threads that will be handled in dirty conditions, or for parts that must go together without any risk of binding. It is uncommon in general production.

Class 2A/2B is the general-purpose fit and accounts for the overwhelming majority of commercial fasteners. Unless a drawing says otherwise, a standard bolt is 2A and a standard tapped hole is 2B. Class 2A carries an allowance, which is why it tolerates plating and light corrosion so well: the clearance was designed in.

Class 3A/3B is the close fit, with a tighter tolerance and - importantly - no allowance on the external thread. Its tolerance zone starts at the basic size. That gives minimal play, which is what you want for a precision adjustment screw or a highly loaded joint, and it also means a 3A thread has no room for a plating build-up. Specify 3A on a part that will be zinc-plated afterwards and you may find the finished bolts no longer gauge.

Mixed callouts are legal and quite common. A 2A bolt in a 3B hole is perfectly valid; the classes on the two halves do not have to match.

• A = external thread (bolt), B = internal thread (nut or tapped hole).
• 1A/1B loose, 2A/2B general purpose, 3A/3B close.
• 2A and 1A carry an allowance; 3A does not.
• Unmarked commercial fasteners are effectively 2A/2B.

The metric classes: 6g, 6H and the rest

ISO 965-1 splits the two ideas explicitly. A metric tolerance class is a number followed by a letter. The number is the tolerance grade, running from 3 to 9, with smaller meaning tighter. The letter is the fundamental deviation - the position of the tolerance zone relative to the basic size - and it is where the allowance lives.

Case carries meaning and is not optional. Lowercase letters describe external threads: e, f and g place the zone below the basic size with progressively smaller clearance, while h places it exactly at the basic size with no allowance at all. Uppercase letters describe internal threads: G places the zone above the basic size with clearance, and H places it at the basic size. Writing 6h when you meant 6H specifies a completely different part.

The default general-purpose combination is 6g for external threads and 6H for internal, which is roughly the metric equivalent of 2A/2B - the g allowance on the bolt does the same job as the Unified 2A allowance, leaving room for coating and handling damage. A plain M10 × 1.5 bolt with no class stated is 6g. Tighter work uses 4h6h or 5g6g; looser assembly work uses 7H or 8g.

A full callout can carry two classes, because the pitch diameter and the crest diameter can be toleranced separately. In M10 × 1.5-5g6g the first pair applies to the pitch diameter and the second to the major diameter. Where only one class appears, it applies to both.

Choosing a class in practice

For most work, do not choose. The default classes exist because they are right almost all of the time, and specifying something tighter without a reason simply raises cost and scrap rate while making the parts harder to assemble.

There are four situations where the class genuinely matters. The first is plating and coating: an external thread with no allowance - 3A or 6h - has nowhere for the coating to go, so anything that will be zinc-plated, phosphated or anodised after threading needs the allowance that 2A or 6g provides. The second is precision motion, where a lead screw, adjuster or instrument thread needs the reduced play of a close class to avoid backlash.

The third is high-temperature or galling-prone assemblies. Stainless and nickel alloys gall readily, and a tight fit makes it worse; a looser class plus anti-seize is a common and effective answer. The fourth is field assembly in dirt, where a loose class means a thread that still goes together after being dropped in gravel.

Whatever class you specify, it only means something if it is verified. Threads are gauged rather than measured in production: a go ring gauge or plug gauge must run on freely and a no-go gauge must not. Thread micrometers and three-wire measurement give an actual pitch diameter reading where a number is needed rather than a pass or fail.

What a class does not do

A tolerance class says nothing whatsoever about strength. It is a geometry specification. A class 3A bolt is not stronger than a class 2A bolt of the same size and grade; strength comes from the property class or SAE grade and from the tensile stress area, which is a function of diameter and pitch, not of fit.

Nor does a class make different thread systems compatible. Tolerance is measured in hundredths of a millimetre; the gap between a metric and a Unified thread of similar size is measured in tenths. No class of fit bridges an M8 to a 5/16-18 UNC, and none makes a coarse thread run in a fine hole.

Finally, a class is not a substitute for cleanliness. The allowance in a 2A or 6g thread is there to accommodate plating and minor handling damage, not grit, thread-locking compound residue or a bruised crest. A thread that will not start by hand is telling you something a tolerance table cannot fix.