ISO 286 Limits and Fits: From Fit Selection to Tolerance Stack-Up
ISO 286 defines standardized hole and shaft tolerances that determine how parts fit together. Understanding clearance, transition, and interference fits — and how they feed into stack-up analysis — is essential for reliable mechanical assemblies.
Clearance Fit
Shaft is always smaller than hole. Parts slide freely.
Transition Fit
May have clearance or interference depending on actual sizes.
Interference Fit
Shaft is always larger than hole. Press fit required.
How ISO 286 Works
ISO 286 uses a letter-number code to specify tolerances:
- Letter — Defines the deviation (position relative to nominal)
- Number — Defines the IT grade (tolerance width)
For holes, uppercase letters (A–Z) are used. For shafts, lowercase letters (a–z) are used. The letter indicates where the tolerance zone sits relative to the nominal diameter.
H7/g6 = H7 hole + g6 shaft
H = hole at zero line (minimum = nominal) | 7 = IT7 tolerance
g = shaft below zero line (negative deviation) | 6 = IT6 tolerance
Key Letters Explained
Hole Letters (Uppercase)
| Letter | Position | Typical Use |
|---|---|---|
| H | Zero line (min = nominal) | Standard hole basis — most common |
| G | Slightly above zero | Close running fit |
| F | Above zero | Running fit with clearance |
| P, R, S | Below zero | Interference fits (shaft basis) |
Shaft Letters (Lowercase)
| Letter | Position | Typical Use |
|---|---|---|
| c, d | Well below zero | Loose running fit |
| f | Below zero | Easy running fit |
| g | Slightly below zero | Close running / sliding fit |
| h | Zero line (max = nominal) | Sliding / locating fit |
| j, k | Straddles zero | Transition fit |
| n, p | Above zero | Light interference |
| r, s, t | Well above zero | Medium/heavy press fit |
Common Hole-Basis Fits
In hole-basis systems (most common), the hole is held constant at H and the shaft letter varies:
| Fit | Type | Application |
|---|---|---|
| H11/c11 | Clearance (loose) | Agricultural, mining — dirt tolerance |
| H9/d9 | Clearance (free) | Gearbox shafts, pulley bores |
| H8/f7 | Clearance (easy running) | Journal bearings, general rotation |
| H7/g6 | Clearance (close running) | Precision sliding, spigots |
| H7/h6 | Clearance (sliding) | Locating fits, easy assembly |
| H7/k6 | Transition | Keyed shafts, location without interference |
| H7/n6 | Transition (tight) | Light press, hub on shaft |
| H7/p6 | Interference (light) | Press fit requiring light force |
| H7/s6 | Interference (medium) | Permanent assembly, gears on shafts |
| H7/u6 | Interference (heavy) | Shrink fit, high torque transmission |
Calculating Fit Clearance
For any fit, clearance (or interference) is the difference between hole and shaft sizes:
Clearance = Hole size − Shaft size
Positive = clearance (gap) | Negative = interference (overlap)
Example: Ø25 H7/g6
For a 25mm nominal diameter:
| Feature | Min (mm) | Max (mm) |
|---|---|---|
| H7 Hole | 25.000 | 25.021 |
| g6 Shaft | 24.980 | 24.993 |
| Min Clearance | 25.000 − 24.993 = +0.007 | |
| Max Clearance | 25.021 − 24.980 = +0.041 | |
This H7/g6 fit guarantees clearance between 0.007 and 0.041 mm — the shaft will always slide in the hole.
Fits in Tolerance Stack-Ups
When a fit interface appears in a stack-up, you need to account for the clearance (or interference) range:
Clearance Fits in Stack-Ups
Add the clearance as a dimension with its own tolerance. For H7/g6 above: nominal = 0.024mm (midpoint), tolerance = ±0.017mm.
Transition Fits
The clearance can be positive or negative. Use the full range: if clearance varies from −0.005 to +0.015, model as nominal = +0.005, tolerance = +0.010/−0.010.
Interference Fits
After pressing, the effective dimension is the shaft (larger). The interference is absorbed by material deflection, not the stack. Use shaft size as the effective dimension.
Stack-Up Example with Fit
Consider a simple bearing assembly with a clearance fit:
| Dimension | Nominal | Tolerance | Dir |
|---|---|---|---|
| Housing bore to datum | 50.000 | ±0.025 | + |
| H7/g6 clearance (Ø50) | 0.025 | +0.016/−0.009 | + |
| Bearing width | 15.000 | ±0.050 | − |
The fit clearance (0.025 ±0.0125 mm) becomes a contributor to the total stack variation. It's often overlooked — leaving it out underestimates the assembly tolerance range.
⚠️ Common Mistake: Ignoring Fit Clearance
A sliding fit can shift the shaft position within the clearance range. If your stack depends on shaft position, you must include the clearance as a tolerance contributor.
IT Grades Reference
The number in a tolerance code (H7, g6) refers to the IT grade — the tolerance width:
| IT Grade | Typical Tolerance (Ø25) | Application |
|---|---|---|
| IT5 | 9 µm | Precision bearings, gauges |
| IT6 | 13 µm | Precision fits, shafts |
| IT7 | 21 µm | Standard holes, fits |
| IT8 | 33 µm | General machining |
| IT9 | 52 µm | Fits with clearance |
| IT11 | 130 µm | Rough machining, casting |
Hole Basis vs. Shaft Basis
Most designs use hole basis (H hole + variable shaft) because:
- Holes are harder to adjust — standard reamers come in fixed sizes
- Shafts are easier to turn to any diameter
- One hole size works with multiple shaft fits
Shaft basis (h shaft + variable hole) is used when:
- Standard bar stock is used as-is
- Multiple bearings/bushings fit on one shaft diameter
- The shaft is hardened and cannot be machined
Common Mistakes
Using Fit Codes Without Looking Up Values
"H7/g6" means different absolute tolerances at Ø10 vs Ø100. Always look up the actual deviation and tolerance for your specific nominal diameter.
Forgetting Clearance in Stack-Ups
A loose-running fit like H9/d9 can have 0.1mm+ of clearance. If the stack depends on shaft position within the bore, this clearance is a real tolerance contributor.
Mixing Hole and Shaft Basis
Pick one system and stick with it. Mixing H holes with h shafts (both at zero line) gives a fit that varies from line-to-line clearance to slight interference — usually not what you want.
Summary
- Select fit type — Clearance, transition, or interference based on function
- Choose fit code — H7/g6 for precision sliding, H7/p6 for press fit, etc.
- Look up values — Get actual deviations for your nominal diameter
- Calculate clearance range — Min and max clearance (or interference)
- Add to stack-up — Clearance becomes a dimension with its own tolerance