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Short answer: Start from function. Choose H7/g6 for a precise sliding or locating clearance fit, H7/h6 for a close locating fit that may be zero clearance at the limits, H7/k6 (transition) for an accurate location that can be assembled with light force, and H7/p6 for a locational interference fit: rigid, accurately aligned location pressed in without high bore pressure (not intended to carry torque by friction alone; use H7/s6 or tighter for that). These are ISO 286 hole-basis fits; the set listed here matches the preferred hole-basis fits of ANSI/ASME B4.2. Always look up the actual limits for the nominal size in ISO 286-2, because the microns change with diameter.
How ISO 286 fit names work
- The number is the IT grade (tolerance width). IT7 is wider than IT6. Holes are usually one grade coarser than shafts because bores are harder to make accurately.
- The letter is the fundamental deviation (position of the zone relative to nominal).
- H hole: the lower limit equals the nominal size, and the zone sits above it. This is "hole basis."
- Shaft letters a–h sit at or below nominal and give clearance (h touches nominal).
- js, k, m, n straddle nominal and give transition fits.
- p, r, s and beyond sit above the hole zone and give interference.
Hole basis (H) is preferred in most shops because standard reamers and gauges are made to H7.
Steps to select a fit
1. Define function. Does the part slide or rotate on assembly? Must it locate precisely? Must it transmit torque or axial load through friction alone? Must it be disassembled?
2. Pick the fit family.
- Clearance for sliding, rotating or easy assembly.
- Transition for accurate location with removable parts, often keyed or bolted.
- Interference for permanent location or friction drive.
3. Choose the specific class from the ISO 286-1 preferred-fit guidance (ISO 286-1 lists preferred hole-basis combinations).
4. Look up the limits in ISO 286-2 for your nominal size, then compute minimum and maximum clearance or interference.
5. Check assembly method and environment. Interference needs a press, or heating or cooling. Different materials (for example, an aluminum housing on a steel shaft) change the fit with temperature.
6. Follow supplier tables for bearings, bushings and dowel pins. They specify their own shaft and housing tolerances, which overrides generic choices.
Worked example
A ⌀20 mm locating pin on a fixture plate has to:
- locate a removable workpiece accurately (the workpiece bore fits over the pin), and
- stay permanently fixed in the plate.
Pin in plate: a permanent fit is needed, so choose interference: plate bore H7, pin p6 (H7/p6). Look up both limits in ISO 286-2 for the 18–30 mm range and confirm that minimum interference is greater than zero and maximum interference is acceptable for pressing. Note that commercial dowel pins come with a fixed tolerance (often m6 or similar). In that case, choose the bore to suit the pin's catalog tolerance.
Workpiece on pin: the workpiece must slide on by hand with minimal play, so choose H7/g6 on the workpiece bore against the pin's locating diameter. If the pin is a single diameter at p6, the workpiece bore needs its own clearance relative to p6. Designers often use a stepped pin for exactly this reason: one diameter for the press fit and one for location.
The example shows a general point: one shaft diameter rarely serves two fits. Give each function its own diameter.
Common mistakes
- Writing "H7" with no shaft class and assuming the machinist will "make it fit."
- Copying fits from an old drawing at a very different diameter. The micron values differ by size.
- Ignoring temperature when housing and shaft materials differ.
- Specifying interference on thin-walled hubs without checking hoop stress and distortion.
- Over-specifying. H6/g5 where H7/g6 would work raises cost with no functional gain.
- Leaving out surface finish. A rough bore changes the effective fit after the first assembly.
Quick selection guide (qualitative)
| Function | Typical hole-basis choice | Assembly |
|---|---|---|
| Loose running, large clearance | H11/c11 or H9/d9 | By hand, generous play |
| Running fit for shafts in plain bearings | H8/f7 or H7/f7 | By hand |
| Precise sliding or locating, small clearance | H7/g6 | By hand, minimal play |
| Close locating, may reach zero clearance | H7/h6 | By hand, can be snug |
| Accurate location, removable | H7/k6 (or H7/js6) | Light mallet or press |
| Locational interference, rigid aligned location | H7/p6 | Press |
| Medium or heavy drive, permanent | H7/s6 and beyond | Press or thermal |
These pairings follow the preferred hole-basis fits of ANSI/ASME B4.2 (which uses ISO 286 tolerance zones) and similar tables in engineering handbooks; H7/f7 is a common but non-preferred alternative. Confirm every case against the limits for the actual size, and against supplier data for bearings and pins. Write the fit on the drawing as the hole and shaft classes (for example ⌀20 H7 on the bore and ⌀20 p6 on the pin), or add the computed limits in brackets, so the shop doesn't have to look them up.
FAQ
What's the difference between H7/h6 and H7/g6?
Both are clearance fits. g6 always leaves a small positive clearance. h6 can reach zero clearance at the limits, so it locates more tightly but can bind.
Is H7/k6 removable?
Usually yes, with light force (mallet or light press). That is why it is used for gears and couplings that are keyed and occasionally serviced.
Shaft basis or hole basis?
Hole basis is the default. Shaft basis (h shaft) suits cases like one drawn bar stock carrying several parts with different fits.
Where do I find the numbers?
ISO 286-2 tables, or the equivalent tables in Machinery's Handbook. ANSI B4.2 is the metric ASME equivalent.