Fit Types

A fit is the relationship between a hole and a shaft at their nominal size. Get it wrong and parts either won't assemble or will be too loose. This page covers which fit to use, what the ISO designations mean, how assembly method affects cost, and the mistakes we see on drawings every day.

Which Fit Type Do You Need?

Start here. Match your application to the correct fit category and designation.

Clearance, Transition, Interference — At a Glance

ISO Hole Basis System

The ISO system (ISO 286) defines fits using letter+number designations. The system is built on the hole basis: the hole's lower deviation is always zero (designated by "H"), and the shaft tolerance is varied to create different fit types.

Fundamental deviation is the letter part. It defines how far the tolerance zone sits from the nominal size. For holes: A–H zones sit above nominal (clearance), JS–N straddle nominal (transition), P–ZC sit below nominal (interference with a shaft). For shafts: it's the mirror image.

IT grade is the number part. IT6 through IT11 are the most common in general machining:

Common Fit Designations

These 15 fits cover the vast majority of engineering applications. Tolerance values shown are for nominal 25mm diameter.

Values shown for nominal diameter 25mm. Positive range = clearance. Negative range = interference. Consult ISO 286 tables for other diameters — tolerance values scale with size.

Clearance Fits Deep-Dive

Clearance fits are the most common and cheapest. The shaft is always smaller than the hole. Parts assemble by hand and can move relative to each other.

H7/g6 — Sliding Fit

The tightest common clearance fit. Clearance is 7–41 μm at 25mm. The shaft slides smoothly but has minimal play. Used for guide shafts, sliding bearings, and spool valves where accurate positioning matters but some motion is needed.

Limitation: Requires good surface finish (Ra 0.8–1.6 μm). Any burrs or debris will cause binding. Not suitable for dirty environments.

H7/h6 — Locational Clearance Fit

Near-zero clearance: 0–34 μm at 25mm. The shaft fits snugly but can still be pushed in by hand. This is the standard locational fit — used when parts need to be precisely located relative to each other but also removable. Typical: locating rings, alignment pins, exchangeable inserts.

Practical note: At the tight end of the tolerance band, this fit can feel like a very light interference. Apply a thin film of oil before assembly.

H8/f7 — Running Fit

Moderate clearance: 20–74 μm at 25mm. Designed for continuous rotation with lubrication. This is the standard choice for shafts running in plain bearings (bushings). The clearance is large enough to maintain an oil film but small enough to prevent vibration.

Speed consideration: For higher speeds, move to H8/e7 or H7/e8. The larger clearance accommodates thermal expansion and reduces viscous drag in the oil film.

H7/e8 — Loose Running Fit

Larger clearance: 40–97 μm at 25mm. Used for high-speed shafts, pistons in cylinders, and applications with significant thermal expansion. Easy assembly — no alignment issues.

H11/c11 — Very Loose Fit

Large clearance: 110–370 μm at 25mm. Non-critical applications: cover plates, dust caps, parts in dirty environments where debris tolerance matters more than precision. Cheapest to manufacture — wide tolerance bands on both hole and shaft.

Transition Fits Deep-Dive

Transition fits may result in either a small clearance or a small interference, depending on where the actual hole and shaft fall within their tolerance bands. They provide precise location with some ability to transmit torque.

H7/js6 — Symmetric Transition Fit

The shaft tolerance is symmetric about the nominal (±6.5 μm at 25mm). Statistically, about 50% of assemblies will have slight clearance and 50% slight interference. Used for parts that need precise location but may need disassembly: pulleys, handwheels, light-duty gears.

Assembly: Usually goes in with hand pressure or a light tap with a mallet. No press needed in most cases.

H7/k6 — Standard Transition Fit

The most common transition fit. Range: −15 to +19 μm at 25mm. Most assemblies end up with slight interference, but some will have slight clearance. Used for gears, sprockets, and couplings mounted on shafts where the connection needs to be secure but the part should be removable with an arbor press.

Keyway note: Transition fits alone do not reliably transmit torque. Add a keyway and key for torque transmission. The fit handles radial location; the key handles the torque.

H7/n6 — Push Fit (Near-Interference)

Range: −28 to +6 μm at 25mm. Almost always produces interference. Used where the part should not come loose during service but disassembly is still possible with a press: spindle mounts, precision wheels, permanent coupling flanges.

Assembly: Requires an arbor press. Don't hammer — uneven force can damage the shaft or misalign the part.

Interference Fits Deep-Dive

Interference fits permanently join parts. The shaft is always larger than the hole. Assembly requires force or thermal methods. The interference creates radial pressure at the interface, which transmits torque and axial loads through friction.

H7/p6 — Light Interference Fit

Interference: 1–35 μm at 25mm. The lightest interference that's still considered permanent. Used for bearing outer races pressed into housings, thin-walled sleeves, and permanent gear mounts. Can be assembled with a standard arbor press.

Stress check: Even light interference generates hoop stress in the outer member. For thin-walled housings (wall thickness < 0.5x diameter), verify the hoop stress doesn't exceed the material's yield strength.

H7/r6 — Medium Interference Fit

Interference: 7–41 μm at 25mm. Standard for bearing inner races pressed onto shafts, hub assemblies, and permanent mechanical joints. Requires a hydraulic press or heavy arbor press (5–20 tons depending on size).

Practical tip: Apply a thin coat of anti-seize compound or oil before pressing. Never use grease on bearing press fits — it can work into the bearing and contaminate the lubricant.

H7/s6 — Heavy Interference Fit

Interference: 14–48 μm at 25mm. At this level, force-pressing becomes risky — the press forces are high enough to damage parts or gall the surfaces. Thermal shrink fitting (heating the hole member, freezing the shaft) is the preferred assembly method.

Typical uses: Railroad wheel presses, large gear hubs, permanent structural connections. Not used in small precision assemblies.

H7/u6 — Very Heavy Interference Fit

Interference: 27–61 μm at 25mm. The heaviest standard interference fit. Assembly by force pressing is not recommended — thermal methods are mandatory. Used for extremely high-torque connections and structural joints that must never separate.

Disassembly: Not practical without heat. Heating the outer member expands it enough to release the shaft. Expect potential damage to one or both parts on disassembly.

Shrink / Expansion Fit Assembly

Thermal assembly avoids the high forces and surface damage of press fitting. The principle is simple: heat the outer member (hole) to expand it, and/or cool the inner member (shaft) to shrink it, until the shaft slides in freely. When the parts return to room temperature, the interference is achieved.

Temperature Differential Needed

The required temperature change depends on the interference value and the coefficient of thermal expansion (CTE):

ΔT = δ / (α × d)

Where δ = diametral interference (mm), α = CTE (°C&supmin;¹), d = nominal diameter (mm). Add a safety factor of 2–3x to ensure easy assembly.

Practical Example

H7/s6 fit on a 100mm steel shaft in a steel hub. Maximum interference = 48 μm (at 25mm), scales to approximately 86 μm at 100mm.

ΔT = 0.086 / (12 × 10&supmin;&sup6; × 100) = 72°C
With 3x safety: ΔT = 216°C
Heat hub to 216°C above room temperature (≈ 240°C)

Cooling the Shaft (Alternative)

Instead of (or in addition to) heating the hub, you can shrink the shaft:

Surface Finish Impact on Fits

Surface roughness directly affects the actual fit. The measured dimension on a drawing is the average of peaks and valleys. When two surfaces are pressed together, the peaks get crushed — the effective interference is less than the dimensional interference suggests.

For clearance fits, the effect is reversed: peaks reduce the effective clearance, making the fit tighter than calculated. This is usually tolerable, but worth noting for H7/g6 sliding fits where any tightness causes binding.

Common Mistakes