ISO 286 · inch shafts

Shaft fits and tolerances

A shaft fit is a diameter tolerance chosen so the mating part goes on the way you want it to: slides off by hand, taps on with a mallet, or presses on for keeps. These are the ISO 286 shaft limits we cut to, computed straight from the standard.

ISO 286 describes a fit with a letter and a number. The letter sets where the tolerance band sits relative to the exact nominal size (the fundamental deviation); the number sets how wide the band is (the IT grade). Lowercase letters are shaft tolerances, uppercase are hole tolerances. On a turned shaft you are almost always specifying the lowercase shaft class, and the mating bore is bored or bought to the matching hole class.

The three things a fit can do: a clearance fit (letters a through h) always leaves room, so the part slides on. An interference fit (p and up) is always tighter than the shaft, so it has to be pressed or shrunk on. A transition fit (j, k, m, n) straddles the line, snug either way, and is what you want when a part must run true but still come apart for service.

Every number below is computed from the ISO 286 fundamental-deviation and IT-grade tables for the exact diameter, then converted to inches. These are the same limits the configurator applies when you pick a fit class on a bearing seat, so the reference and the cut part agree by construction.

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Clearance and location fits

Upper and lower deviation from nominal, in inches (0.0001 in resolution). All values are zero-or-negative: the shaft is at or below nominal.

Transition and interference fits

Upper and lower deviation from nominal, in inches. Positive values sit above nominal, which is what creates the grip in a press fit.

Picking a shaft class

g6 is the workhorse clearance fit for a shaft that turns in a bushing or slides through a bore, close but free. h6 and h7 sit right at nominal on the high side and are the default for a locating diameter that assembles by hand.

k5 and k6 are the standard transition fit for a rolling-element bearing inner ring on a rotating shaft: tight enough that the ring does not creep, loose enough to still press on and off with an arbor. n6 and p6 step up to a light and then a true interference fit for gears, couplings, and pulleys that must not move at all.

Tolerance is not finish

A tight fit is only as good as the surface it rides on. A k5 bearing seat cut to a rough finish will not hold its size in service. Fit sets the diameter; surface finish sets how that diameter behaves under load. See the surface finish guide for the Ra we hold on bearing and seal diameters.

Related: Pump shafts · Motor shafts · Surface finish reference.

Frequently Asked Questions

These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard and confirmed against your part before machining. See all guides on the guides page.