Reference · Released
Basic Fit Reference
A beginner's guide to ISO fits (hole basis). Understand clearance, transition, and interference fits, and when to use common designations like H7/g6, H7/h6, H7/k6, and H7/p6. Select a fit class below to see typical applications.
Select a fit type
Example designation
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Fit type
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Typical application
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Mating method
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Understanding ISO fits
Fits are written as hole / shaft, e.g. H7/g6. The capital letter (H) is the hole tolerance; lowercase (g) is the shaft. The number is the tolerance grade (IT5–IT12). Lower grade = tighter tolerance.
- Hole basis (H): the hole is held to its basic size (H). The shaft letter determines the fit. This is the most common system.
- Clearance fits: shaft is always smaller than the hole. Gears on shafts, sliding parts, bearings in housings that must slide.
- Transition fits: may have slight clearance or interference. Precision location, keyed assemblies, pulleys that can be disassembled.
- Interference fits: shaft is larger than the hole. Requires press, heat, or cold assembly. Permanent or semi-permanent joints, gears on shafts, bearing outer rings.
| Fit | Type | Typical use |
|---|---|---|
| H7/g6 | Clearance | Precise sliding, bearings in bushes |
| H7/h6 | Clearance | Running fit, removable guides |
| H7/js6 | Transition | Precise location, light loading |
| H7/k6 | Transition | Location with key/press, gears |
| H7/n6 | Transition / Interference | Semi-permanent, light press |
| H7/p6 | Interference | Press fit, permanent joints |
| H7/s6 | Interference | Heavy press, steel on steel |