ISO 286 Fits Explained: H7/g6, Hole-Basis, and How to Actually Choose
ISO 286 is the international limits-and-fits system: one letter, one number, and a shaft/hole pair acquires a guaranteed relationship — slip, tap, or press — before either part is made. It is the vocabulary underneath almost every rotating or locating joint in mechanical design, and it repays an hour of understanding many times over.
What the callout is saying
A designation like Ø40 H7/g6 packs four facts into two codes. Each code is a letter plus a grade:
- Letter — position of the tolerance zone. For shafts, letters a–h sit below nominal (progressively less clearance), j–n straddle or start just above it, and p onward guarantee interference. For holes the letters are capitalized; H is the special case whose lower deviation is exactly zero, so an H hole never goes undersize.
- Number — width of the tolerance zone. This is the IT grade: IT5 is fine, IT11 is coarse. The grade number alone never tells you where the zone sits — that is the letter's job — only how wide it is.
- Case — hole or shaft. Uppercase applies to the hole (internal feature), lowercase to the shaft (external feature). H7 is a hole; g6 is a shaft.
So H7/g6 reads: a hole sized nominal-to-plus-IT7, against a shaft that starts below nominal by the g deviation and spans IT6. The result is always clearance — the only question is how much.
The IT grades are computed, not arbitrary
IT grades come from a standard tolerance unit that scales with size: i = 0.45·√³D + 0.001·D in micrometres, with D the geometric mean of the diameter step in millimetres. Grades are fixed multiples — IT6 = 10i, IT7 = 16i, IT8 = 25i, and so on — evaluated on stepped size ranges. The ranges designers use most:
| Nominal step (mm) | IT6 (µm) | IT7 (µm) | IT8 (µm) | IT9 (µm) | IT11 (µm) |
|---|---|---|---|---|---|
| >18–30 | 13 | 21 | 33 | 52 | 130 |
| >30–50 | 16 | 25 | 39 | 62 | 160 |
| >50–80 | 19 | 30 | 46 | 74 | 190 |
| >80–120 | 22 | 35 | 54 | 87 | 220 |
That is why the same fit code means different micrometres at different diameters — the table lookup is size-step dependent, and a few letters (c, d, e and the deep-interference letters from t onward) additionally change between the 30–40 and 40–50 sub-steps.
Hole-basis vs shaft-basis
Two conventions anchor the system. Hole-basis keeps the hole at H (lower deviation zero) and picks the shaft letter to set the fit. It is the default choice because holes are made with fixed-size tooling — drills, reamers, boring bars — so standardizing the hole and varying the turned or ground shaft is cheaper. Shaft-basis is the mirror image (h shaft, chosen hole letter) and survives where the shaft is already fixed: bought-in ground bar, standard bearings pressed onto a mandrel, or one shaft carrying several different fits.
The fits you will actually use
Limit clearances below are computed for the >30–40 mm step (hole always H7 = +25/0 µm; positive = clearance, negative = interference):
| Fit | Class | Limits (µm) | Typical use |
|---|---|---|---|
| H11/c11 | clearance | +120 / +440 | loose running: hinges, exposed shafts, dirty environments |
| H9/d9 | clearance | +80 / +204 | free running: general shafts with generous lubrication |
| H8/f7 | clearance | +25 / +89 | normal running: journal boxes, moderate-speed rotation |
| H7/g6 | clearance | +9 / +50 | precision sliding: accurate location that still moves freely |
| H7/h6 | clearance | 0 / +41 | locational clearance: spigots, hand-assembled parts, no perceptible play |
| H7/js6 | transition | +33 / −8 | slight clearance bias: dowels, precise location with easy assembly |
| H7/j6 | transition | +30 / −11 | similar to js6, marginally more likely to clear |
| H7/k6 | transition | +23 / −18 | true transition: gear hubs, couplings — snug, assembled with light taps |
| H7/m6 | transition | +16 / −25 | tight transition: accurate location, press possible |
| H7/n6 | transition | +8 / −33 | mostly interference: semi-permanent assembly |
| H7/p6 | interference | −1 / −42 | light press: bearings, bushings; can just touch at worst corner |
| H7/r6 | interference | −9 / −50 | medium press: gears and couplings transmitting torque |
| H7/s6 | interference | −18 / −59 | heavy press: permanent joints, significant assembly force |
| H7/t6 | interference | −23 / −64 | very heavy press or shrink fit territory |
| H7/u7 | interference | −35 / −85 | shrink-fit class: maximum transmissible torque without keys |
Check the sub-step. In the >40–50 mm sub-step the c, t and u shaft deviations shift (c to −130, t to +54, u to +70 µm), moving those three rows to roughly +130/+450, −29/−70 and −45/−95 µm. Every other row shown is identical across the whole 30–50 step. For any diameter outside 30–40 mm, recompute from the standard tables — do not extrapolate.
How to choose
- Start from function, not habit. Needs to move? Clearance — pick from how freely (h6→c11 is an increasing-play ladder). Needs to locate without fasteners? Transition. Needs to transmit load on its own? Interference — and then check the hub stresses, because a heavy press fit can crack a thin boss.
- Default to hole-basis H7. It is the cheapest accurate hole and every shaft letter pairs with it cleanly.
- Spend grades where they pay. Tightening a hole from IT8 to IT7 or a shaft from IT7 to IT6 is a process step change in cost. Put the fine grade on the feature that controls the fit; leave the other coarse.
- Remember the assembly. A press fit is not just a fit — it is a dimension in a stack-up that can close gaps elsewhere. Fits chosen in isolation surprise you at assembly.
Common mistakes
- Reading the grade as position: 6 is not "tighter than 7 in the same place" — f7 and f6 start at the same deviation and differ only in width.
- Specifying H7/n6 where you meant "definitely presses" — n6 can still clear by up to 8 µm at this size. If assembly must never slide, move to p6 or beyond.
- Copying a fit from a different diameter without re-reading the tables.
- Ignoring temperature: aluminium housing, steel shaft, hot service — differential expansion can eat a designed clearance whole.
These tables are what SuperNX applies automatically: fits declared on the model are read as the published ISO 286 zones rather than re-keyed numbers, so the analysis chain inherits the same values your supplier will measure against.