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H7/h6 is one of the most common fit combinations in mechanical engineering. It describes a sliding fit (clearance fit) in the hole-basis system:

• H7 — hole with fundamental deviation H (lower limit = nominal, EI = 0) and IT grade 7
• h6 — shaft with fundamental deviation h (upper limit = nominal, es = 0) and IT grade 6

This combination always produces zero to positive clearance, making it ideal for locating fits, pistons, and spigots.

The three fit types are:

• Clearance fit — the shaft is always smaller than the hole, leaving a gap. Assembly and disassembly are easy. Used for running and sliding fits.
• Interference fit — the shaft is always larger than the hole. Assembly requires pressing or heating. Creates a permanent joint relying on friction.
• Transition fit — may result in either slight clearance or slight interference, depending on actual manufactured dimensions within tolerance. Used for accurate location.

ISO 286 notation uses letter + number format:

• Uppercase letters (H, G, F...) = holes
• Lowercase letters (h, g, f...) = shafts
• The letter defines the fundamental deviation (position of tolerance zone)
• The number is the IT grade (tolerance zone width)

Example: H7/g6 = hole with H deviation and IT7 tolerance, mated with shaft with g deviation and IT6 tolerance.

A fit is written as Hole/Shaft, e.g. H7/h6, H7/p6.

Most common combinations in the hole-basis system (H hole):

• H7/f7 — free running fit (bearings, long shafts)
• H7/g6 — close running fit (precision slideways)
• H7/h6 — sliding fit (pistons, spigots)
• H7/k6 — transition fit (keyed shafts)
• H7/n6 — tight transition (light press for location)
• H7/p6 — press fit (bearing bushings)
• H7/s6 — heavy press fit (permanent coupling)

IT (International Tolerance) grade defines the width of the tolerance zone. The standard grades are IT01 through IT18:

• IT5 — precision (gauges, fine instruments)
• IT6 — high precision (precision fits)
• IT7 — general engineering (most common for fits)
• IT8–IT9 — semi-precision (free-running fits)
• IT11 — rough work (clearance fits, non-critical)

Each step roughly multiplies the tolerance by 1.6×. The actual width depends on the nominal dimension: larger parts have wider tolerances for the same IT grade.