Fit tolerance determines how two mating parts — typically a shaft and a hole — go together: with play, with a firm grip, or somewhere in between. Choosing the right fit decides whether a pin slides smoothly, a bearing sits securely or a gear transmits torque without slipping.
Tolerance is the acceptable deviation from a specified dimension. It ensures that parts fit together correctly, function as intended and meet quality standards. Fit tolerance is a specific type of tolerance — alongside general tolerance — that focuses on the relationship between mating parts and defines the permissible clearance or interference between them. This article explains what fit tolerance is, how fits are specified, which types of fits exist and which ones to use for custom parts.
What is tolerance?
In mechanical engineering, tolerance is the permissible variation in the dimensions or geometry of a part. Every manufacturing process has some natural variation, so a design never specifies just one ideal value — it specifies limits. As long as the actual dimension lies between the maximum and minimum limit, the part can be assembled, functions correctly and remains interchangeable with other parts made to the same drawing or 3D model.
What is fit tolerance?
Fit tolerance defines the degree of clearance or interference between mating parts such as shafts and holes — in other words, how snugly or loosely they fit together. Depending on the fit, a shaft slides easily into the hole with some play, sits tightly with interference, or lies in between.
Getting the fit right is crucial for the function, performance and service life of an assembly. A fit that is too tight can lead to binding, excessive friction or even damaged parts. A fit that is too loose can cause instability, misalignment or premature wear. That is why fit tolerances need to be understood and specified precisely.
How fits are specified: H7/p6 explained
Fits for holes and shafts are specified with a standardised system of letters and numbers defined in ISO 286. Holes use capital letters, such as H7; shafts use lowercase letters, such as p6. A fit is written as a pair, for example H7/p6.
- The letter defines the position of the tolerance zone relative to the nominal dimension (the fundamental deviation). For a hole with H, the lower limit equals the nominal size.
- The number is the IT tolerance grade and defines the width of the tolerance zone. The lower the number, the narrower — and the more precise — the tolerance.
A concrete example at a nominal diameter of 25 mm: an H7 hole may measure between 25.000 and 25.021 mm, and a p6 shaft between 25.022 and 25.035 mm. The shaft is always larger than the hole, so H7/p6 is an interference fit. For small diameters the values shrink accordingly: a 4 mm H7 hole has a tolerance of +0.012/0 mm.
Most designs use the hole-basis system: the hole is fixed as H, and the desired fit is achieved by choosing the shaft tolerance. This is practical because holes are made with fixed-size tools such as reamers and checked with standard gauges. The shaft-basis system works the other way round, with the shaft fixed as h — useful when one shaft carries several components with different fits.
This standardised notation makes communication between designers, manufacturers and suppliers simple and unambiguous, and keeps fit tolerances consistent across components and assemblies.
Types of fits
There are three types of fits: clearance fit, transition fit and interference fit. Each one serves a different purpose.
Clearance fit
A clearance fit always leaves intentional space between the mating parts: the shaft is smaller than the hole across the entire tolerance range. Parts can be assembled and disassembled easily, with minimal force. Clearance fits are used wherever easy assembly, rotation or sliding movement, alignment flexibility or thermal expansion matter — for example shafts running in plain bearings, sliding guides or pivots.
Transition fit
A transition fit lies between clearance and interference fits and balances ease of assembly with accurate location. Depending on where the actual dimensions fall within their tolerances, the result is either a small clearance or a slight interference. Transition fits are used where parts must be located precisely but still be assembled with little force — for example dowel pins in fixtures, or gears, pulleys and couplings mounted on shafts with a key.
Interference fit (press fit)
With an interference fit, the shaft is always larger than the hole. The parts are joined with force — pressed together or assembled with a temperature difference by heating the outer part or cooling the inner part (shrink fit). The resulting press fit holds the parts securely in place and offers high load-bearing capacity, torque transmission and resistance to vibration or axial movement.
Press fits are used where a strong, permanent connection is required, such as bearing bushings, gears and pulleys fixed on shafts. Design the interference carefully: it must be large enough for a secure connection without causing excessive stress or deformation. Proper lubrication and assembly technique are also essential for a successful press fit and a long service life.
Common ISO fits at a glance
The table lists frequently used fits in the hole-basis system according to ISO 286, from loose to tight. Use it as a starting point and always check the actual limits for your nominal diameter.
| Fit | Type | Character | Typical applications |
|---|---|---|---|
| H11/c11 | Clearance | Loose running fit with large clearance | Loose pivots and hinges, parts with wide tolerances |
| H8/f7 | Clearance | Close running fit | Shafts in plain bearings, gearbox shafts at moderate speeds |
| H7/g6 | Clearance | Sliding fit, small clearance | Sliding gears, guide bushings, precise spigots |
| H7/h6 | Clearance | Locational clearance fit, assembled by hand | Covers, centring spigots, parts that are regularly disassembled |
| H7/k6 | Transition | Accurate location, light assembly force | Gears, pulleys and couplings on keyed shafts |
| H7/n6 | Transition | Tighter location, slight interference likely | Bushings and gears that require precise location |
| H7/p6 | Interference | Light press fit | Pressed-in bushings, locating pins, parts needing rigidity and alignment |
| H7/s6 | Interference | Medium drive fit | Gears and couplings permanently fixed on shafts |
Fit tolerances for custom parts from meviy
meviy manufactures individually designed custom parts from your own 3D CAD data — not standard or catalogue parts. You define the fit-relevant features directly on your model, and the price and delivery date update instantly when you change a tolerance.
- Precision holes with fit tolerances such as H7 in parts made by CNC milling — H7 holes are finished by reaming
- Shafts, pins and bushings for your fits, produced by CNC turning
- Hole tolerances set on screen: select the hole on the 3D model and choose the tolerance field — see how to edit hole specifications in meviy
- Instant price and lead time for every change — no 2D drawing required
Whether you need a single prototype or a small batch, CNC machining with meviy lets you check the cost impact of a tighter or looser fit before you order.
Frequently asked questions about fit tolerance
What does H7 mean?
H7 is an ISO 286 tolerance class for holes. "H" means the lower limit of the hole equals the nominal size, so the hole can only be the same size or slightly larger. "7" is the IT tolerance grade, which defines how wide the tolerance zone is. For a 25 mm hole, H7 allows 0 to +0.021 mm.
What is the difference between an interference fit and a transition fit?
With an interference fit, the shaft is always larger than the hole, so the parts must be pressed or shrunk together. With a transition fit, the result can be either a small clearance or a slight interference, depending on the actual dimensions — it locates parts precisely but needs less assembly force.
Which fit is used for bearings?
It depends on the load direction. The bearing ring that rotates relative to the load usually needs a tight fit — shafts are often toleranced k6 or m6 — while the stationary ring gets a looser fit, with housings often toleranced H7. Always follow the bearing manufacturer's recommendations.
What is the difference between fit tolerance and general tolerance?
General tolerances, for example according to ISO 2768, apply to all dimensions that have no individual tolerance. Fit tolerances are specified individually for mating features such as shafts and holes, using ISO 286 tolerance classes like H7 or g6, to define exactly how the parts fit together.
Choosing the right fit for your design
Understanding fit tolerance is crucial for the function and assembly of mechanical components. By selecting the right fit, you achieve exactly the clearance or interference your mating parts need — and with it, the performance and reliability of the whole assembly. Consider the intended application, the operating conditions and the manufacturing process when you choose your fits.