What the ISO 286 Tolerance / Fit System Is, and How to Read It
Why tolerances are needed, how to read a code like Ø20 H7/g6, and what clearance, transition, and interference fits mean.
Why tolerances are needed
No manufacturing process can produce a part at a perfectly exact size — there is always a small deviation. ISO 286 describes how large that deviation may be in a standard language, so a shaft made in one factory and a hole made in another still fit together correctly.
How to read the code: Ø20 H7/g6
This code describes two parts:
H7 → hole tolerance. g6 → shaft tolerance.
Letter — position of the tolerance: uppercase = hole, lowercase = shaft. H means the hole's lower limit is exactly the nominal size (20 mm) — so the hole can only be larger, never smaller. g means the shaft's upper limit is slightly below nominal — so the shaft is always a bit smaller than nominal.
Number — IT grade (precision): the smaller the number, the narrower the tolerance band, meaning more precise and more expensive manufacturing. IT6 is a tighter tolerance than IT7.
What the fit type means
| Code | Fit type | Meaning |
|---|---|---|
| H7/g6 | Clearance fit | Shaft is always smaller than the hole; parts assemble by hand or light push and can rotate/slide freely. Used in bearing-mounted shafts and sliding parts. |
| H7/k6 | Transition fit | Sometimes slight clearance, sometimes slight interference, depending on actual sizes. Used where location matters but the joint is detachable. |
| H7/p6 | Interference fit | Shaft is always larger than the hole; parts are joined by pressing or thermal expansion/contraction and are hard to disassemble. Used for permanent joints like bearing housings. |