fits and tolerances
Fits and Tolerances: Clearance, Interference and H7/h6
A fit is a decision about how two parts behave together — how much they can move, whether they need a press, whether they can be taken apart. The ISO hole-basis system turns that decision into two letters and two numbers, and once you know which line of the table you want, the chart does the rest.
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- H7/h6
- Sliding to close-running fit, small clearance
- H7/g6
- Sliding fit, location without play
- H7/p6
- Light press fit
- H7/s6
- Heavy press fit
A fit is a specification of behaviour, not a number. Once the behaviour is decided, ISO 286 supplies the numbers, and the part stops being an argument at inspection.
Specifications
| Standard | ISO 286 (limits and fits) · ANSI B4.2 equivalent |
|---|---|
| Hole basis | H tolerance on the hole, shaft letter varies |
| General location | H7/h6 |
| Sliding | H7/g6 |
| Running in a bushing | H7/f7 |
| Press fits | H7/p6 (light) · H7/s6 (heavy) |
Frequently asked questions
What does H7/h6 mean?
A hole with an H7 tolerance and a shaft with an h6 tolerance, in the ISO hole-basis system. At 20 mm nominal it gives a fit ranging from 0.034 mm clearance to 0.013 mm interference — a close running or light press fit.
What is the difference between a clearance and an interference fit?
A clearance fit always leaves a gap, so the parts move or rotate and assemble by hand. An interference fit always overlaps, so the parts must be pressed or shrunk together and transmit torque through friction.
Which fit for a bearing?
Follow the bearing manufacturer's recommendation: usually an H7 bore and an h6 or k6 shaft for a rotating inner ring. The bearing maker's catalogue takes precedence over a general table.
Why is the hole always H in hole-basis fits?
Because holes are made with standard reamers and gauges, and varying the shaft is cheaper. Fixing the hole at H (lower deviation zero) and moving the shaft letter produces the whole range of fits from one set of tools.
Do I need a coded fit on every part?
No. Most parts are fine with a symmetric tolerance on the diameter. Coded fits belong where the function depends on the fit — bearings, dowels, press-fit inserts — and they cost more to make.
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