Engineering Fits: Clearance, Transition & Interference
How a shaft and hole are meant to fit together — the three fit types, hole-basis vs shaft-basis, common designations like H7/g6 and H7/p6, and how to read the callout.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 7 min read
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Quick Answer
An engineering fit describes how a shaft and hole are meant to mate — clearance (a gap, they slide), transition (a whisper of clearance or interference, they locate), or interference (the shaft is bigger, they lock). It is written as two tolerances combined, like 25 H7/g6: capital letter for the hole, lowercase for the shaft, number for the IT grade. H7/g6 is a sliding clearance; H7/p6 is a press fit.
How a Shaft and Hole Are Meant to Mate
A bare dimension says "25 mm." A fit says what that 25 mm is supposed to do — slide, locate, or lock. An engineering fit is two tolerances combined: the hole tolerance and the shaft tolerance, and it is the relationship between their zones that determines clearance, transition, or interference. Getting the fit right is what separates a shaft that spins freely from one that seizes, and a pin that locates from one that falls out. This guide covers the three fit types and how to read the callout on a drawing.
The Three Fit Types
Clearance fit — the shaft is always smaller than the hole, leaving a gap. The parts slide or rotate freely; used for running fits, guides, and spigots.
Transition fit — the result may be a small clearance or a small interference. Assembly is a light tap or hand press, and location is precise; used for locating pins and hubs.
Interference fit — the shaft is always larger than the hole. Assembly requires pressing, heating, or cooling; used for rigid joints like press-fit dowels and bushings. Our press fit guide covers this in detail. The two borderline families — where locating stops and locking starts — are compared band by band in our transition fit vs interference fit guide.
Hole-Basis vs Shaft-Basis
Fits are set up one of two ways. In the hole-basis system, the hole is held at the basic size and the shaft tolerance is varied — the ISO default, with the hole designated H (lower deviation zero). In the shaft-basis system, the shaft is held at the basic size and the hole tolerance is varied, with the shaft designated h (upper deviation zero). Hole-basis is far more common, because it is cheaper to vary the shaft (turning) than to hold multiple hole sizes (reaming).
Common Fit Designations
The hole-basis fits that show up most on machining drawings:
| Designation | Fit type | Typical use |
|---|---|---|
| H7/g6 | Clearance (sliding/running) | Shafts that slide or rotate |
| H7/h6 | Clearance (snug) | Hand-assembled locating fits |
| H7/k6 | Transition | Accurate location, light tap |
| H7/p6 | Interference (press) | Press-fit dowels, hubs, bushings |
These four cover the vast majority of machined fits — sliding, snug, transition, and press. The exact numerical tolerances come from the ISO 286 table for the specific diameter range; the designation tells you the type at a glance.
How to Read a Fit Callout
A fit callout like 25 H7/g6 reads as: 25 mm basic size, an H7 hole, and a g6 shaft. The capital letter is the hole, the lowercase letter is the shaft, and the number is the IT grade (the tolerance width). The letter sets where the tolerance zone sits, the number sets how wide it is. For the tolerance side of the same equation, see our tolerance guide, and for the holes these fits land in, our reaming guide.
FAQ
What are the three types of engineering fits?
Clearance, transition, and interference. In a clearance fit, the shaft is always smaller than the hole, so there is a gap and the parts slide or rotate freely. In a transition fit, the result may be either a small clearance or a small interference, so assembly is a light tap or hand press and location is precise. In an interference fit, the shaft is always larger than the hole, so assembly requires pressing, heating, or cooling — the parts are locked together. Our press fit guide covers the interference case in detail.
What is hole-basis vs shaft-basis?
Two ways to set up the system. In the hole-basis system, the hole is held at the basic size and the shaft tolerance is varied — this is the ISO default, and the hole is designated H (lower deviation zero). In the shaft-basis system, the shaft is held at the basic size and the hole tolerance is varied, with the shaft designated h (upper deviation zero). Hole-basis is far more common in practice, because it is cheaper to vary the shaft than to keep multiple reamer sizes.
What does H7/g6 mean on a drawing?
It is a fit callout: H7 is the hole tolerance (an H7 hole), g6 is the shaft tolerance (a g6 shaft), and together they define a clearance fit. The capital letter is the hole, the lowercase letter is the shaft, and the number is the IT grade — the tolerance width. So 25 H7/g6 means a 25 mm nominal size with an H7 hole and a g6 shaft, giving a small sliding clearance.
What are IT tolerance grades?
The IT grade is the number in a fit or tolerance callout — IT6, IT7, IT8, and so on. The number sets the tolerance width (how much variation is allowed), while the letter sets the zone position (where that band sits relative to nominal). Lower numbers are tighter; IT6–IT11 are the grades most machined fits use. The exact numerical tolerance for each grade depends on the nominal size and must be read from the ISO 286 table.
Which fit should I use for a bearing or pin?
It depends on whether the parts must move or lock. A running or sliding shaft uses a clearance fit (H7/g6 or H7/h6). A locating pin that must not move uses an interference fit (H7/p6), or a transition fit (H7/k6) if light assembly and accurate location matter. Bearings usually combine both: clearance or transition on the rotating ring, interference on the stationary ring. Our press fit guide covers the interference specifics.
How is a fit related to tolerance?
A fit is two tolerances combined — the hole tolerance and the shaft tolerance — and it is the relationship between their zones that produces clearance, transition, or interference. The tolerance (IT grade) sets how much each part can vary; the fit designation sets how those two variation bands sit relative to each other. Our unilateral vs bilateral tolerance guide covers the tolerance side of the same equation.
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Fits Machined to the Callout
IsoCNC holds clearance, transition, and interference fits to the H7/g6, H7/k6, and H7/p6 callouts on your drawing, verified on a CMM. Send the drawing; an engineer reviews every RFQ, typically within 1–2 business days.