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ISO 286 Fit Selection: H7/g6 vs H7/h6 and Friends

October 7, 2026

Pick the fit by function, not by habit — that is the entire method. In the ISO 286 system the shaft letter sets where the shaft's tolerance zone sits relative to the hole: letters a through h progressively reduce guaranteed clearance, j through n straddle the line into transition territory, and p onward guarantees interference. Once you know what the joint must do — move freely, locate precisely, separate by hand, or carry torque on friction alone — the fit code chooses itself from a short list of preferred pairs. This is that list, ordered loosest to tightest, with the honest guidance about when each one is wrong.

The preferred hole-basis fits, loosest to tightest

Hole-basis is the default convention: the hole is always H (its lower deviation is zero, so it never goes undersize) and the shaft letter sets the character. Limit clearances below are computed for the >30–40 mm step — positive is clearance, negative is interference — and the numbers are size-step dependent, so recompute for other diameters rather than extrapolating.

Reading one row as a drawing check: Ø35 H7/k6 is a hole of 35.000–35.025 against a shaft of 35.002–35.018 — the pair can arrive with 23 µm of clearance or 18 µm of interference, which is precisely what "transition" means: you will not know which you got until the parts exist. The ISO 286 fit calculator computes limit clearances for any designation and diameter, the same tables ship as machine-readable JSON, and the mechanics of letter-plus-grade are in ISO 286 fits explained. This article is about the pick.

How to choose — three questions

• Does it move? Then you need guaranteed clearance. How much follows the conditions: contamination and thermal growth point to the loose end (c11, d9); a lubricated running journal sits at e8/f7; precise sliding guidance is g6.
• Must it locate precisely AND come apart? That is transition territory. H7/h6 assembles by hand with no perceptible play — the default for mating faces and covers. H7/k6 is snug, goes together with light taps, and still separates — the classic for keyed hubs and pins that must not rock. H7/n6 is mostly interference and effectively semi-permanent.
• Must it carry torque or axial load on friction alone? Interference — and the row follows the load and the hub stiffness. p6 is a light press that still disassembles; s6 is a genuine press fit for permanent joints and moderate torque; u6 is heavy-press or shrink territory where you also check that the hub survives the hoop stress.

If the honest answer is a mix — locate precisely, come apart easily, and carry torque — that is what keys, dowels and bolts are for. Give the fit the location job and let a mechanical element carry the load.

Bearing seats: the one place the rules get specific

Bearing fits obey a logic of their own, because creep is the failure you are designing against: a ring that is loose and rotating relative to the load direction will walk around its seat, fretting the surfaces until the fit — and eventually the shaft — is destroyed. The rule of thumb: the ring that rotates relative to the load gets the interference seat, and the ring that is stationary relative to the load gets the clearance or transition seat. For the common case — shaft rotating in a fixed housing — the inner-ring shaft seat wants slight interference (k5–m6 class shafts, heavier as load grows) while the outer-ring housing seat stays at H7 or a mild transition. Flip the load path — rotating housing, stationary shaft — and the assignment flips with it.

Do not finalize this from a generic table. The bearing maker's own fit recommendations account for load magnitude, speed, temperature and the bearing's internal clearance — and a heavy press on a ring can consume the radial clearance the bearing was built with. The ISO 286 designation is the vocabulary; the bearing catalog owns the callout.

Things that change the pick after you have picked it

• Temperature. Differential expansion is arithmetic, not opinion: an aluminium shaft at Ø40 (CTE about 23×10⁻⁶/K) in a steel housing (about 12×10⁻⁶/K) gaining 60 K eats roughly 26 µm of clearance — enough to turn a marginal running fit into a seizure, and equally able to open a light press fit at temperature. Design the fit at operating temperature, not at the inspection bench.
• Surface finish. A press fit's real interference is less than the measured one — surface peaks crush during assembly. Rough bores surrender a meaningful slice of the designed grip, which is why heavy interference fits specify finish alongside the fit code.
• Lubrication. Running fits assume a film. A g6 or f7 journal running dry, or at a speed its film cannot support, wears into clearance it was never given — the fit is a system property, not a shaft property.
• Assembly method. p6 goes together with an arbor press and comes apart; s6 wants a real press and a plan; u-class generally means shrinking the hole or cooling the shaft. If the joint must be serviced, a u-class fit is the wrong answer however good the torque math looks.

The short version

• Moves → clearance, c11 through g6 by how freely; locates and separates → h6/k6; carries load on friction → p6/s6/u6 by torque and hub strength.
• H7 hole plus shaft letter is the default — the letter chooses the behavior, the grade numbers spend the money.
• Bearing seats follow rotation-relative-to-load, and the bearing maker's table owns the final designation.
• Temperature, finish, lubrication and assembly method all modify the pick — the fit code is the start of the decision, not the end.