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ISO 286 Limits and Fits

ISO 286 is the international standard defining the system of tolerance grades (IT) and fundamental deviations for holes and shafts, used to specify precision fits on engineering drawings.

ISO 286 uses a letter-number code: the letter indicates the fundamental deviation (position of the tolerance zone relative to nominal), and the number indicates the tolerance grade (width of the zone). Uppercase letters are for holes (H7, G6), lowercase for shafts (h6, g6). H7 is the most common hole tolerance in aerospace (lower deviation = 0, tolerance width = IT7).

In practice, a fit class callout has to be expanded to actual limit dimensions. "8.0 H7" means an upper limit of 8.015mm and a lower limit of 8.000mm (for the 6-10mm range, where IT7 = 15 micrometers and the H fundamental deviation = 0). Whoever machines or checks the bore needs these numbers, not just the fit code.

Common aerospace fit classes: H7 (standard precision bore), H6 (high precision bore), G6 (close running fit), G7 (sliding fit). The tolerance band tightens with lower IT grade numbers and varies with nominal size per ISO 286 tables.

Related Guides

ISO H7 Tolerance Chart G6 Fit Class Tolerance Limits

Frequently Asked Questions

What does H7 mean on a drawing?
H7 is an ISO 286 hole tolerance where H means the lower deviation is zero (hole at or above nominal) and 7 is the tolerance grade (IT7). For an 8mm bore, H7 means 8.000 to 8.015mm.

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