Reference

Engineering Fits Chart

Clearance, transition, and interference fits per ISO 286, with common hole-basis fit designations and a plain-English use for each, so you can spec how two parts go together.

An engineering fit describes how tightly a shaft sits in a hole. ISO 286 defines three families: clearance fits (the shaft is always looser than the hole, so parts slide), interference fits (the shaft is always larger, so parts must be pressed or shrunk together), and transition fits (the result can be slightly loose or slightly tight depending on where each part falls in its tolerance). Most drawings use the hole-basis system, where the hole tolerance stays at H and the shaft letter/grade changes to set the fit.

The three fit types (ISO 286)

The fit is the relationship between the hole and shaft tolerance zones. In the hole-basis system the hole is held constant (H) and the shaft is varied.

Fit type	What happens	Assembly	Example (hole basis)
Clearance	Shaft is always smaller than the hole; there is always a gap	Slides together by hand	H7/g6, H7/h6
Transition	Result can be a small clearance or a small interference within tolerance	Light tap or hand press; locates precisely	H7/k6
Interference	Shaft is always larger than the hole	Pressed, or heat the hub / chill the shaft	H7/p6, H7/s6

Hole-basis is the common default because standard reamers and drills produce an H hole. Shaft-basis (constant h shaft) is used when off-the-shelf shafting sets the datum.

Common hole-basis fits and what to use them for

These are the preferred hole-basis fits from ISO 286. Reading them: the capital letter and grade (H7, H8...) is the hole; the lowercase letter and grade (g6, k6...) is the shaft.

Fit	Category	Common name	Use for
H11/c11	Clearance	Loose running	Wide clearance where accuracy is not critical: agricultural linkages, coarse pins, parts with dirt or thermal swing
H9/d9	Clearance	Free running	Freely rotating assemblies with generous clearance: idler pulleys, loose bushings, long shafts
H8/f7	Clearance	Close running	Running fits with good accuracy and moderate speed: sleeve and journal bearings, guided shafts
H7/g6	Clearance	Sliding	Parts that slide or index freely but locate accurately: spigots, gauge and dowel pins in bushings
H7/h6	Clearance	Locational clearance	Precise location with no movement, still assembled and disassembled by hand: register diameters, covers, spigots
H7/k6	Transition	Locational transition	Accurate location where a light interference is acceptable: dowels and pins tapped in, hubs keyed against rotation
H7/p6	Interference	Locational interference	Parts that must stay put without keys: small bearing rings on shafts, press-fit dowels and locating pins
H7/s6	Interference	Medium drive	Semi-permanent, torque-carrying joints: bearing inner rings, gears and collars pressed or shrunk onto shafts

Categories per ISO 286. The actual clearance or interference in micrometres depends on the nominal diameter, so read the exact limits from an ISO 286 table for your size before release.

How to call a fit out on a drawing

Give the nominal size then the tolerance class for each mating feature, e.g. the hole as "25 H7" and the shaft as "25 g6", or write the pair as 25 H7/g6. You can also list the resulting limit dimensions in parentheses. Match the ISO tolerance grade to the function: tighter grades (IT6, IT7) cost more to hold, so reserve them for features that actually mate.

Fit types and designations reflect ISO 286-1 and the preferred hole-basis fits in common practice. Exact clearance and interference values vary with nominal diameter; confirm the limits and what a shop can hold on your material and size before release.

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Frequently Asked Questions

What is the difference between clearance, transition, and interference fits?

A clearance fit always leaves a gap so the parts slide or rotate. An interference fit makes the shaft larger than the hole so the parts must be pressed or shrunk together and stay put. A transition fit sits in between: depending on where each part lands in its tolerance, the result can be a slight clearance or a slight interference, which gives accurate location.

What does H7/g6 mean?

It is a hole-basis fit designation. H7 is the hole tolerance class and g6 is the shaft tolerance class, both per ISO 286. H7/g6 is a sliding clearance fit: the parts locate accurately but still slide, so it suits index pins, spigots, and gauge pins in bushings.

Which fit should I use for a press-fit bearing or dowel?

For parts that must stay put without a key, an interference fit such as H7/p6 (locational interference, for small bearing rings and press-fit dowels) or H7/s6 (medium drive, for torque-carrying gears and collars) is typical. Press fits are assembled with a press or by heating the hub and chilling the shaft.

What is the hole-basis system?

In the hole-basis system the hole tolerance is held constant at H and the shaft class is varied to create the fit. It is the common default because standard drills and reamers naturally produce an H hole, so it is cheaper to change the shaft. The shaft-basis system does the reverse and is used when standard shafting sets the size.

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