Fits: H7 Fit Table and ISO 286

Two parts are meant to go together: a shaft in a bore, a bearing on a seat, a pin in a hole. Whether these parts later assemble easily, grip, or only go together under a press is no accident. It is decided by a note such as "Ø50 H7/g6" on the drawing. For buyers sourcing mechanical components from international markets, that short string of letters and numbers is a key: it governs function, assembly and complaint risk in equal measure.

In brief: A fit describes how the tolerances of a hole and a shaft interact. In the hole-basis system the hole always carries the fundamental deviation H. H7 is by far the most common hole tolerance in mechanical engineering. The matching shaft (g6, h6, k6, n6, p6, s6 …) then determines whether you get a clearance fit, a transition fit or an interference fit. The tolerance values are standardised in ISO 286. The H7 fit table below lists the limit deviations by nominal size range.

What is a fit, and why does it concern buyers?

A fit is the dimensional relationship between two mating parts before assembly, classically between a hole (the "containing" part) and a shaft (the "contained" part). Because no part can be machined to an exact nominal size, a permissible tolerance zone is defined for each. Their relative position determines whether clearance or interference results after assembly.

For technical procurement this is more than a design detail. The fit specification dictates how precisely a manufacturing partner must work. It therefore drives how expensive the part becomes and how high the risk is that a delivery will not function. A fit chosen too tight raises manufacturing cost with no functional benefit; one chosen too loose causes bearing play, noise or component failure. A buyer who can read fits reviews drawings more critically and qualifies suppliers more deliberately.

Fits build directly on the same tolerance system that underpins general tolerances per ISO 2768. Both govern permissible dimensional deviation, only at different levels of precision. In practice, fit specifications are complemented by geometric tolerances per ISO 1101, which cover shape requirements such as straightness and perpendicularity.

The ISO fit system: what does the "H" in H7 mean?

A tolerance designation such as H7 or g6 has two parts: a letter for the position of the tolerance zone relative to the zero line (the fundamental deviation), and a number for the size of the tolerance zone (the standard tolerance grade, IT). Capital letters always denote holes, lower-case letters denote shafts.

The "H" carries a special meaning: it designates a hole whose lower deviation lies exactly on the zero line. An H hole is therefore never smaller than the nominal size, only larger. This is precisely why it forms the basis of the hole-basis system: the hole stays constant (always H), and the desired fit is set through the shaft alone. This is the dominant system in practice, because holes are harder to machine and inspect than shafts, so you keep the more difficult part constant.

The number after the letter, the 7 in H7, is the standard tolerance grade. The smaller the number, the tighter the tolerance: IT5 is finer than IT7, IT11 considerably coarser. H7 hits the practical middle ground between accuracy and economy, which makes it the standard hole tolerance for fits in general mechanical engineering.

What are the three types of fit, and how do you recognise them?

The relative position of the hole and shaft tolerance zones produces three fundamental characters. They are the most important distinction when selecting a fit for a function.

• Clearance fit: The hole tolerance zone lies entirely above that of the shaft. Clearance always results, so the parts assemble freely or with a sliding motion. Typical for rotating or sliding connections such as plain bearings or guides.

• Transition fit: The tolerance zones overlap. Depending on the actual sizes of the mating parts, a small clearance or a small interference can result. It serves accurate centring in connections that can still be dismantled without damage.

• Interference fit: The shaft tolerance zone lies above that of the hole. Interference always results. The parts are pressed, shrunk or expanded together and transmit forces by friction. Typical for fixed seats of gears or bearing rings.

The chart below shows, for common H7 pairings at a nominal size of Ø50 mm, how the character shifts from clearance to interference:

H7 fit table: which hole limit deviations apply by nominal size range?

The heart of any work with fits is the ISO fit table. Since H7 is the reference hole tolerance, its limit deviations deserve a close look. The lower deviation is by definition 0 (the zero line); the upper deviation equals the standard tolerance grade IT7 and grows with the nominal size. All values in micrometres (µm); 1 µm = 0.001 mm.

Example: A Ø50 H7 hole may measure between 50.000 mm and 50.025 mm, never smaller than the nominal size. The 25 µm span is the standard tolerance grade IT7 in this range.

Practical note: from nominal sizes above 120 mm, the upper deviation rises to 40–46 µm. Whether a manufacturing partner can reliably verify this span depends heavily on the inspection equipment available. For bores in this size range, it is worth specifying the measurement method and calibration standard in the procurement specification before series production begins.

Adding the shaft: common pairings at Ø50 mm

It is the shaft that determines which fit results. The following table shows, for a nominal size of Ø50 mm, the character produced by each shaft tolerance combined with an H7 hole:

Negative result values mean interference, positive values mean clearance. The figures apply to the nominal range over 30 to 50 mm; in other ranges the magnitudes shift, but the character of the pairing is preserved.

Which fit for which application?

The most common question in practice is not "What does H7 mean?" but "Which fit do I need here?". The answer follows the function: should something rotate, should it centre, or should it sit firmly? The overview below summarises proven H7 pairings.

An important note on rolling bearings: a bearing has two separate fit locations. The shaft at the inner-ring seat carries tolerance k6, m6 or n6; the housing bore for the outer ring typically receives H7. Both tolerances are specified independently, and the final recommendation depends on bearing type, load and speed; the SKF fitting recommendations or the respective bearing catalogue provide the specific values.

What risks do fits carry in international procurement?

Fits are most risky where design and manufacturing are separated in space and language, which is exactly the situation in international sourcing.

In our procurement experience, most fit problems arise not from a lack of manufacturing capability, but from unclear specification and differing measurement and inspection habits. A pattern we see frequently: manufacturing partners receive only the tolerance code on the drawing, without the concrete limit deviations in µm. The code "H7/g6" is internationally understood, but the actual micrometre values vary by nominal size range and are not always looked up independently by every supplier. Including the limit deviations from the fit table directly in the procurement specification closes this interpretation gap.

Three points are decisive in practice. First, unambiguous specification: the fit designation belongs clearly on the drawing, ideally supplemented by the actual limit deviations in µm, not just the code. Second, agreement on inspection equipment: a dimension in the µm range is only as reliable as the gauge or instrument used to verify it, so aligning measurement method and calibration before the production ramp-up pays off. Third, initial sample inspection: fit dimensions are among the critical characteristics to secure before series release as part of product sampling in procurement.

In ongoing operation, a targeted incoming goods inspection safeguards the fit dimensions, because an interference that only shows up during assembly is the most expensive kind of complaint. It is precisely at this interface between technical requirement and supplier reality that the work of Line Up begins.

Frequently asked questions

What does H7 mean for a fit?

H7 denotes a hole tolerance: the capital letter H places the lower deviation on the zero line (0), and the number 7 stands for standard tolerance grade IT7. An H7 hole is therefore never smaller than the nominal size and may be larger by the IT7 amount, reaching 50.025 mm at Ø50 mm. Capital letters always mark the hole, lower-case letters (g, h, k, p …) the shaft. Because H sits as a fixed reference on the zero line, it forms the basis of the hole-basis system, in which the desired fit is set through the shaft alone.

What is a fit table and how do you read it?

A fit table lists the limit deviations of holes and shafts by nominal size range in micrometres (µm). You first locate the relevant nominal size range (for example "over 30 to 50 mm"), then read the upper and lower deviation from the column of the relevant tolerance (such as H7 or g6) and combine both values into the finished fit. The H7 fit table above in this article shows the hole values, while the matching pairing table gives the corresponding shaft side. Practical example: nominal size Ø50 mm → size range "over 30 to 50 mm" → H7 hole: EI = 0 µm, ES = +25 µm. The part may therefore measure between 50.000 mm and 50.025 mm.

What is the difference between clearance, transition and interference fits?

A clearance fit always produces clearance, so the parts assemble freely (e.g. H7/g6). A transition fit can give a small clearance or interference depending on the actual sizes and serves accurate centring (e.g. H7/k6). An interference fit always produces interference, so the parts are pressed or shrunk together and transmit forces by friction (e.g. H7/p6).

Which fit do you choose for a rolling bearing?

A rolling bearing has two separate fit locations. At the inner-ring shaft seat, the shaft is typically toleranced k6, m6 or n6 so that the inner ring does not creep under load. The housing bore for the outer ring typically receives H7, which is the H7 fit in the sense of the hole-basis system. The precise specification for both locations depends on bearing type, load and speed and is given in the bearing catalogue or on the drawing. For plain bearings, a clearance fit such as H7/g6 is usual at the shaft.

Where are fits standardised?

The limit deviations and the tolerance system for fits are defined internationally in ISO 286: ISO 286-1 governs the system of standard tolerance grades and deviations, while ISO 286-2 contains the table values for holes and shafts. For dimensions that are not individually toleranced, the general tolerances per ISO 2768 apply in addition.

Conclusion: fit knowledge prevents costly complaints

A fit is only as good as its execution in the finished part. The H7 fit table and the hole-basis system set the framework, but function and complaint risk are decided by whether the tolerance is understood, cleanly manufactured and reliably inspected across the entire supply chain. In international procurement in particular, this is exactly where the leverage lies.

Line Up supports the procurement of mechanical components as a process specialist: from drawing and specification review through supplier qualification to initial-sample and incoming-goods inspection. With our own branch office in China and more than 30 years of experience in international procurement, we make sure fit dimensions are verified at the manufacturer before any freight costs arise.

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