What Are Metric Thread Tolerance Classes?

Metric thread tolerance classes are a standardized way to specify how tightly or loosely a bolt and nut will fit together. If you’ve seen designations like “M10 x 1.5 – 6H/6g”, those last characters (6H/6g) are the tolerance classes for the internal thread (nut) and external thread (bolt). In simple terms:

• Tolerance classes ensure compatibility: They control the allowed variation in thread size so that any bolt of a given class will fit any nut of the matching class.
• Prevent jamming or wobbling: By defining acceptable limits, tolerance classes prevent threads from being too tight (causing jamming) or too loose (causing wobble).
• Combination of a number and letter: Each class is denoted by a number and a letter (for example, 6H or 6g). The number indicates the tolerance grade (size of the allowed tolerance range), and the letter indicates the position of that tolerance range relative to an ideal reference (which affects the snugness or clearance).

In short, metric tolerance classes are a shorthand that tells manufacturers and engineers how precisely the threads are made and how a bolt and nut will fit together.

Internal vs External Threads: Understanding “H” vs “g”

It’s important to distinguish between internal threads (the threads inside a nut or a tapped hole) and external threads (the threads on a bolt or screw). The tolerance letter in the class designation is capitalized for internal threads and lowercase for external threads:

• Internal Threads (Nut or tapped hole): Denoted by capital letters like H or G. For example, a nut might be classified as 6H.
• External Threads (Bolt or screw): Denoted by lowercase letters like g, h, e, or f. For example, a bolt might be 6g.

What do these letters mean in practice? They indicate the allowance (intentional gap or overlap) built into the thread:

• “H” for internal threads: Think of H as the standard for a nut. 6H (the most common nut class) means the nut’s thread is made with essentially no additional allowance beyond basic size. In other words, the hole is not made extra large – it’s right at the basic dimension plus a standard tolerance. This usually provides a good tight fit without being too tight.
• “G” for internal threads: A class like 6G (less common for nuts) would mean the internal thread is slightly larger (G provides a small allowance). This can be used if extra clearance is needed (for example, if a coating will be applied inside the thread or if a very long engagement length needs a bit more room).
• “g” for external threads: The lowercase g (as in 6g for a bolt) indicates the bolt’s threads are made slightly smaller than the basic reference size. This gives the bolt a bit of clearance when screwing into a standard nut. It’s the most common external thread class for general bolts because it guarantees easy assembly (the bolt isn’t too thick for the nut) and even allows for a little plating on the bolt.
• “h” for external threads: Lowercase h (e.g., 6h) means the external thread is made with no allowance, essentially at the basic maximum material size. A 6h bolt would be a snug fit in a 6H nut, because both are right at the basic size limits. 6h is typically used if you want a slightly tighter fit or when no plating or coating will be applied on the bolt. Standard bolts without any coating might sometimes use 6h for a closer fit, but most commercial fasteners use 6g.
• (Other letters like e or f for external threads indicate even more allowance, meaning the bolt is made much smaller than the nut’s basic size – these are usually reserved for special cases like thick coatings or unusually loose fits.)

Key point: By convention, nuts (internal threads) generally use “H” and bolts (external threads) use “g” for a normal fit. The letters ensure that one part is slightly larger or smaller as needed so the threads can slide together easily.

Decoding the Numbers and Letters (e.g., 6g, 6H)

Now let’s break down the notation itself. A metric tolerance class has two parts: 1. A number (Tolerance Grade) – This is typically 3 through 8 for metric threads, with 6 being the standard in most cases. 2. A letter (Tolerance Position) – As discussed, uppercase (like H or G) for internal threads, lowercase (like g or h) for external threads.

What the Number Means (Tolerance Grade): The number indicates how large or small the tolerance range is: – A lower number (such as 4 or 5) means a tighter tolerance (smaller allowed variation). This results in a more precise thread but requires higher manufacturing accuracy. – A higher number (such as 7 or 8) means a looser tolerance (larger variation allowed). This can be useful for easy assembly or for very large threads, but the fit will be a bit more slack.

The most common grade is 6, which is considered a “medium” tolerance – a good balance of precision and ease of manufacturing. In fact, if you don’t specify a class on a drawing, the assumption for metric threads is usually “6H/6g” (internal/external) as a default.

What the Letter Means (Tolerance Position/Allowance): The letter mainly describes the allowance – the intentional clearance or overlap relative to the basic thread size: – “H” / “h”: Zero allowance. H (for internal) means the hole is not oversized; h (for external) means the bolt is not undersized. They are right at the basic size limit. For example, a 6H nut has no extra space beyond standard tolerance, and a 6h bolt has no intentional reduction in diameter. – “G” / “g”: Small allowance. G (internal) makes the hole a bit larger; g (external) makes the bolt a bit smaller. 6g is by far the most common for bolts because it gives just enough clearance to ensure a smooth fit and to accommodate things like plating thickness. – Other letters (E, F, e, f): These provide larger allowances (E/F for internal threads, e/f for external threads) but are rarely used in everyday fasteners. They might appear in special applications (for example, a galvanized bolt might use a 6e thread class because galvanizing adds a thick coating – the bolt needs to be much smaller before coating so that after plating it isn’t too tight).

In summary: When you see a thread callout like M8 × 1.25 – 6H or M8 × 1.25 – 6g, you can interpret it as follows: – M8 × 1.25: The thread’s basic size (8 mm diameter, 1.25 mm pitch). – 6H (if it’s an internal thread like a tapped hole or nut): Tolerance grade 6 (standard medium tolerance) and position H (no allowance on internal thread – a normal nut fit). – 6g (if it’s an external thread like a bolt): Tolerance grade 6 and position g (small allowance on external thread – standard bolt fit).

Both pieces – the nut and bolt – need their tolerance classes specified, and together they ensure a compatible fit.

Common Thread Fit Pairings (e.g., 6g Bolt with 6H Nut)

For metric fasteners, certain combinations of internal/external tolerance classes are considered standard fits. The most common pairing by far is:

• 6H internal thread (nut) with 6g external thread (bolt).

This pairing (often written as 6H/6g) is the go-to “medium fit” for most general-purpose screws, bolts, and nuts. It provides: – Reliable assembly: A 6g bolt will always fit into a 6H nut without special effort. – Standard clearance: Enough room to account for manufacturing tolerances and minor dirt/debris, but not so much that the joint is wobbly. – Versatility: Suitable for typical machine threads where you want a balance of ease-of-assembly and decent tightness.

Other pairings or variations you might encounter: – 6H/6h: If someone used a 6h bolt with a 6H nut, the fit would be a bit tighter than usual because the bolt has no allowance (it’s slightly larger than a 6g bolt). This isn’t the usual combination, but it could be used if a designer wants less play (for example, to improve alignment). Assembly might be a touch snug but still workable. – 6G/6g or 6H/6g with modifications: In special cases like very long thread engagements or certain materials, an internal thread might be cut slightly larger (using a G instead of H) to avoid binding over length. For example, a long nut or deep tapped hole might be designated as 6G to give extra clearance along a long engagement. This prevents a bolt from seizing if threads aren’t perfectly aligned over a long distance. – Coarse allowances for coatings: If a bolt is going to be heavily coated (like hot-dip galvanized), a class like 6g might still be too tight after the thick zinc coating is applied. In these cases, you might see an external thread specified as 6e or 6f, which means the bolt is made significantly undersized before coating. After coating, it will roughly end up around 6g or 6h size. Similarly, nuts for galvanizing are often tapped oversize (sometimes notated differently, since internal thread classes beyond H aren’t in common labels but effectively a larger H or special designation). – Precision fits: If a highly precise and minimal-clearance fit is needed (for example in measuring equipment or aerospace), one might specify a lower grade like 4H/4h or 5H/5g. These are uncommon in everyday hardware because they require tight manufacturing control. A 4H/5g for instance might ensure almost no wiggle, but the cost and difficulty to produce are higher and you must assemble carefully.

Takeaway: For most engineering and design needs, 6H/6g is the standard choice. Straying from this common pairing is only done with good reason (like accounting for special coatings, very tight tolerance needs, or unusual lengths of thread engagement). When in doubt, stick to 6H nuts and 6g bolts for a reliable medium fit.

Metric vs Imperial Thread Classes (Comparison with UNC, UNF, etc.)

If you’re familiar with imperial (inch-based) threads, you might know terms like UNC (Unified National Coarse), UNF (Unified National Fine), or the classes 2A/2B, 3A/3B, etc. Here’s how metric tolerance classes compare to those concepts:

• Thread series (Coarse vs Fine): In the imperial system, UNC vs UNF refers to the coarseness of the thread (UNC has fewer threads per inch – a larger pitch – compared to UNF). In the metric system, we simply specify the pitch in the thread callout (for example, M10×1.5 is coarse, while M10×1.0 would be a finer pitch). So, UNC/UNF is not a tolerance class but a pitch specification. The metric equivalent is just saying the pitch value or calling it “coarse” or “fine” thread in words.
• Tolerance/Fit classes (Metric vs Unified): The imperial Unified thread standard uses classes like 1A, 2A, 3A for external threads (A = external) and 1B, 2B, 3B for internal threads (B = internal). These define how tight the thread fit is:
• Class 1 (1A/1B): Very loose fit, lots of clearance for easy starting and assembly even if threads are dirty or slightly damaged. Rarely used unless you need quick assembly in non-critical situations.
• Class 2 (2A/2B): Standard fit for most inch-series fasteners – a good balance of snugness and ease, similar to the metric 6H/6g fit.
• Class 3 (3A/3B): A tight, high-precision fit with minimal clearance, used for high-quality, high-strength, or precise applications.

Comparing to Metric: The metric 6H/6g fit roughly corresponds to the imperial Class 2B/2A fit in terms of typical clearance and usage. Both are the default for general-purpose screws and nuts.

• A metric “6g” external thread is similar to an imperial Class 2A external thread. Both have a bit of allowance for coating and easy assembly.
• A metric “6H” internal thread is similar to an imperial Class 2B internal thread (the standard nut tolerance).
• If you see a metric thread class like 4H/4h (very tight), that would be somewhat akin to a Class 3 fit in imperial – used when you need minimal play.
• On the other end, a metric class 8H/8g (if ever specified) would be looser, somewhat comparable to a Class 1 fit (more play for easy assembly or dirty environments).

Example comparison: – A bolt labeled M10 × 1.5 – 6g and a nut M10 × 1.5 – 6H is the metric standard. The imperial equivalent might be a 3/8-16 UNC Class 2A bolt with a Class 2B nut (3/8-16 UNC is roughly similar size and pitch, and Class 2 is standard fit). – If we had a M10 – 4g6g bolt with a M10 – 4H nut (very tight metric fit), an imperial analog could be a 3/8-16 UNF Class 3A bolt with Class 3B nut (fine pitch and tight class – though note UNF is fine thread, I’m mixing pitch and fit for analogy).

Important distinction: UNC/UNF define the pitch (coarse/fine), while the classes 1, 2, 3 define the tolerance fit. In metric, the tolerance class (like 6g) is independent of the pitch specification. You could have coarse or fine metric threads both using 6H/6g fits. Always consider both the pitch and the tolerance class when comparing systems.

Design Tips for Choosing the Right Tolerance Class

Selecting the proper thread tolerance class for your application can impact manufacturability, assembly, and performance. Here are some practical tips to guide your choice:

• Use the default (6H/6g) for general applications: In most cases, a 6H internal / 6g external thread fit will be appropriate. It’s the industry standard for a reason – it provides a reliable medium-clearance fit that works for typical bolts, screws, and nuts. If you’re unsure, default to this class unless you have specific requirements to change it.
• Choose tighter classes (lower numbers) for precision, but beware of assembly difficulty: If your design requires a very tight fit with minimal play – for example, in precision instruments or where alignment is critical – you might consider using a tolerance grade like 5 or 4 for the threads. This will make the threads more precise. However, tighter tolerances mean higher manufacturing cost and potentially trickier assembly. A 4H/4g fit will screw together with very little slop, but any small error or dirt can cause binding. Use tight fits only when necessary, and ensure parts are kept clean and made with high precision.
• Use looser classes (higher numbers or more allowance) for ease and challenging environments: If you need easy, quick assembly or are dealing with harsh conditions, a looser fit might help. For example, threads that will be assembled in the field (where dust or damage might be present) could benefit from extra clearance. While you typically won’t call out something like 7H/7g for standard screws, certain large-diameter or long-engagement threads might effectively use a class 7 or 8 tolerance to avoid seizing. Class 1 (inch) or grade 8 (metric) fits are very loose – use them when functionality (easy spin by hand, resistance to dirt) matters more than precision.
• Account for coatings and platings: Plating, paint, or coatings will make external threads thicker and internal threads tighter (smaller). If you plan to plate a bolt (common coatings are zinc, nickel, etc.), stick with the standard 6g class for the bolt, which already includes a small allowance for typical plating thickness. For very thick coatings like hot-dip galvanizing, you may need a special thread class (often the bolt is made significantly undersized, like using 6e or even custom allowances, and the nut may be tapped oversize). The rule of thumb is to ensure that after coating, the threads end up roughly in a 6H/6g relationship. Never assume a tight class (like 6h or 3A) will work after coating – always provide allowance or specify “before plating” class.
• Consider corrosion and wear: If the environment might cause corrosion (rust, etc.) or if the threads will wear over time due to frequent assembly, a little extra initial clearance can prevent problems. Rust can build up on a bolt or inside a nut; a fit that’s too tight (little clearance) might seize up if rust or debris forms. In designs prone to corrosion, it’s wise not to push for the tightest possible fit. Similarly, if a fastener will be repeatedly removed and reinstalled, a standard or slightly looser tolerance will make it easier to thread on each time without galling or jamming.
• Long engagement lengths need more clearance: As mentioned earlier, if you have a nut that is very deep or a bolt screwing into a thick tapped section, manufacturing variations in pitch over a long length can cause binding. In such cases, designers sometimes specify the internal thread with a slightly larger tolerance position (for example, using G instead of H for the internal thread). This effectively loosens the fit just a touch, making assembly smoother in deep holes. The general rule: for thick or long thread engagements, avoid the tightest fits – give a bit more room so the parts don’t bind before fully assembled.
• When in doubt, ask or test: If you’re unsure which class to use for a critical application, don’t hesitate to consult threading charts or fastener experts. It can also be helpful to make prototypes: try a standard 6H/6g fit first, and if it’s too loose or too tight for your needs, adjust accordingly (maybe a 6H/5g for a bit tighter, or 6H/7g for looser). The right tolerance class can depend on the specific combination of materials, manufacturing process, and functional requirements.

By understanding these tolerance classes and following these tips, you can ensure that your bolts and nuts not only fit together properly but are optimized for your application – whether that’s a tight precision instrument or a rough-and-ready field assembly.