Engineering Fits & Tolerances: Clearance, Transition and Interference
Understand engineering fits and tolerances - clearance, transition and interference - and how to read ISO hole-basis callouts like H7/g6 to specify joints that assemble and perform.
Two parts that have to go together — a shaft in a bore, a pin in a hole, a bearing on a journal — only work if the gap between them is right. Too loose and the joint rattles or leaks; too tight and it cannot be assembled, or it cracks the part. The engineering language for controlling that gap is the fit, defined by the tolerances on the two mating features. This guide explains clearance, transition and interference fits, the ISO hole-basis system (the familiar H7/g6 style callouts), and how to choose and specify a fit that assembles and performs the way you intend.
Tolerance, Allowance and Fit
A tolerance is the permitted variation on a single dimension — a hole specified as ∅20 +0.021/0 may finish anywhere from 20.000 to 20.021 mm. A fit is the relationship between two toleranced features once assembled: the range of possible gaps (or overlaps) between, say, that hole and the shaft going into it. Because both parts vary, the fit is itself a range — from the tightest combination (largest shaft in smallest hole) to the loosest (smallest shaft in largest hole). Good design controls that whole range, not just the nominal size.
The Three Families of Fit
| Fit type | What happens | Typical use |
|---|---|---|
| Clearance | Shaft always smaller than hole — there is always a gap | Rotating/sliding parts, easy assembly, lubricated joints |
| Transition | May end up slightly loose or slightly tight | Accurate location where the part is still removable |
| Interference (press) | Shaft always larger than hole — parts are forced together | Permanent joints: bearings, bushings, gears on shafts |
The choice flows directly from function. A part that must rotate or slide needs a clearance fit. A part that must locate precisely but still come apart for service needs a transition fit. A part that must never move relative to its mate — a bearing race pressed onto a shaft — needs an interference fit, where the elastic squeeze holds it in place.
Reading H7/g6 Callouts
The ISO system describes a fit with two short codes, one for each part. The letter sets where the tolerance band sits relative to the nominal size (uppercase for holes, lowercase for shafts), and the number sets the IT grade — how wide the band is (a smaller number is tighter). Most design uses the hole-basis system: the hole is held at a standard H band and the shaft letter is varied to create the fit. A few common ones:
- H7/g6 — a close running clearance fit. Good for shafts that rotate or slide with a small, controlled gap.
- H7/h6 — a location/sliding fit with minimal clearance; parts go together by hand and locate well.
- H7/k6 — a transition fit for accurate location that can still be pressed apart.
- H7/p6 — a light interference (press) fit for semi-permanent assembly.
Hole-basis is preferred because holes are made with fixed-size tools (reamers, bored to gauge) while shafts are easy to turn or grind to any diameter — so it is cheaper to standardise the hole and adjust the shaft. For the IT-grade values and general tolerance bands, see our engineering reference tables.
Choosing the Right Fit
- Start from motion. Rotating/sliding → clearance. Located but removable → transition. Permanent → interference.
- Tighten only where you must. Each IT grade tighter costs more to machine and inspect. Specify a close fit on the one feature that needs it, not the whole part.
- Mind the material. Plastics expand and creep far more than metals, so a press fit sized for steel may be too tight or relax over time — see our materials guide.
- Account for temperature. A fit set at room temperature changes when the assembly runs hot; for dissimilar metals, check differential expansion.
- State the surface too. A clean fit needs the right surface finish on both faces — a rough bore behaves looser than its size suggests.
The Bottom Line
A fit is a decision, not an accident. Decide whether the joint must slide, locate or lock, pick the matching family — clearance, transition or interference — and specify it with a standard hole-basis callout like H7/g6 so any shop reads it the same way. Tighten only the features that earn it, allow for material and temperature, and your parts will go together the first time and stay together for the life of the product.
MechPart Pro machines to ISO fits and GD&T callouts, holding the bores, shafts and surface finishes your assembly needs and inspecting them to your drawing. Share your model and tolerances and our engineers will confirm the fits and flag any that are tighter (or looser) than the function requires, as part of our free design-for-manufacturability review. For the alloys behind these parts, see our tool steel and stainless steel pages.
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