# Transition fits

Fits permitting either clearance or interference: limit calculations, assembly consequences and inspection requirements.

Reviewed: 2026-11-01

Tags: parts-and-design, fits, tolerances, assembly

## What transition means

A transition fit permits either clearance or interference, depending on the accepted pair of actual sizes. [MISUMI shaft/hole fit guide, updated 20 February 2025](https://us.misumi-ec.com/blog/shaft-hole-tolerances-for-clearance-interference-fits/)

**Original limit arithmetic for the cited 20 mm H7/m6 fit:** minimum clearance is `20.000 − 20.021 = −0.021 mm`, or 21 µm interference. Maximum clearance is `20.021 − 20.008 = +0.013 mm`, or 13 µm clearance. The full spread is 34 µm, matching the 21 µm hole tolerance plus 13 µm shaft tolerance.

ISO 286-1:2010 provides the linear-size tolerance code system and fit terminology for cylinders and two opposite parallel surfaces, without constraints of orientation or location. A fit designation therefore addresses part of the interface definition; it does not replace axis, form or surface requirements needed for the assembly. ISO's listing identifies edition 2, April 2010, confirmed in 2026, with a 2013 corrigendum separately listed. [ISO 286-1 scope and edition](https://www.iso.org/standard/45975.html)

## Calculate both extremes

Use one explicit sign convention. For diametral clearance `C = hole diameter − shaft diameter`:

| Quantity | Worst-case calculation |
| --- | --- |
| Minimum clearance | `Cmin = hole_min − shaft_max` |
| Maximum clearance | `Cmax = hole_max − shaft_min` |
| Clearance-range width | `Cmax − Cmin = hole tolerance + shaft tolerance` |

A negative C means interference of magnitude `−C`. A range spanning negative and positive values permits both assembly conditions. Do not change the sign convention between a drawing, calculation and inspection report.

These equations combine permitted extremes without assuming a production distribution. A transition range does not mean half of production will have interference, nor does the proportion of its negative range establish that probability. Such claims require actual manufacturing distributions and a pairing rule.

## Bounded example: nominal 20 mm H7/m6

Ganter's supplier extract of ISO 286-2 fundamental tolerances gives, in the **over 18 through 30 mm** band, H7 hole deviations of 0/+21 µm and m6 shaft deviations of +8/+21 µm. No technical issue date is verified on this extract; use the contracted standard and edition for acceptance. [Ganter ISO 286-2 extract](https://live-katalog.ganternorm.com/pdf/ganter/en/286_2.pdf?dispositiontype=attachment)

| Feature | Limit sizes |
| --- | --- |
| 20 H7 hole | 20.000–20.021 mm |
| 20 m6 shaft | 20.008–20.021 mm |

This is a dimensional example, not a recommended fit for an unspecified application. An actual 20.010 mm hole paired with a 20.012 mm shaft has 2 µm dimensional interference; another accepted pair can have clearance. Neither the class nor this arithmetic establishes insertion force, torque capacity or freedom from damage.

## Decide whether both outcomes are acceptable

Define the intended function before choosing the classes. If every accepted assembly must slide freely at the reference condition, a range permitting interference needs further design review. If every assembly must retain torque or axial load by interference, a range permitting clearance does not establish that retention.

Document any independent retention feature, assembly process and service condition. Selective assembly can control paired sizes, but then identify the pairing rule and matched components; do not describe selected pairs as unrestricted interchangeability. Keep the permitted assembly force and final functional acceptance separate from the dimensional fit.

For rolling bearings, NSK shows why the actual application matters: inadequate fit can permit ring creep, while excessive interference can crack an inner ring or reduce internal clearance and generate heat. This is bearing-specific guidance, not proof that an arbitrary transition fit is appropriate for a rotating bearing ring. [NSK fits and internal clearance](https://www.nsk.com/tools-resources/abc-bearings/fits-and-internal-clearance/)

## Processing and operating conditions

State whether sizes apply before or after coating, heat treatment and final finishing. Account for the actual temperature and material response of both members. A small room-temperature clearance or interference should not be assumed to persist when the two parts operate at different temperatures.

NSK describes reduced effective interference from rough-surface deformation during pressing, applied load and rising bearing temperature. These effects reinforce the need to evaluate the finished joint rather than treating nominal size difference as an operating rating. [NSK application explanation](https://www.nsk.com/tools-resources/abc-bearings/fits-and-internal-clearance/)

Review engagement length, lead-in geometry, burrs, cleanliness, surface texture, alignment and required form. A long shaft can bind in a distorted bore despite a favorable local size result. Conversely, a successful trial insertion does not establish every required size, position or surface characteristic.

## Inspection and handoff

Preserve the following in a manufacturing or agent-readable record:

1. Nominal size, complete hole/shaft classes, units and governing standard/edition.
2. Explicit limit sizes and the sign convention used for clearance or interference.
3. Datums, required form, engagement and final processing/inspection condition.
4. Assembly method, pairing restrictions and independent retention requirements.
5. Measured dimensional results and the agreed functional acceptance.

Choose inspection methods for the actual characteristics. Local diameter readings, functional gauging and an assembled performance test provide different evidence. Resolve contradictory numeric limits and class designations before production, rather than letting the machinist infer which requirement controls.

## Related reference

[Part features](index.md)

## Source scope

Reviewed on the date above. MISUMI prints its February 2025 update; ISO metadata supports the edition and scope stated above. Ganter's extract and NSK's live page have no verified technical revision date. Source-specific application guidance, original calculations and proposed release checks are distinguished from contractual requirements.