Reading the code
- H: fundamental deviation of the hole. H means the lower limit equals the nominal size (deviation zero). This is the hole-basis system: the hole is the fixed reference and the shaft is varied.
- 7: tolerance grade IT7 for the hole. At 18 to 30 mm, IT7 is 21 µm.
- g: fundamental deviation of the shaft. Lower-case g places the shaft just below nominal (7 µm below at 18 to 30 mm).
- 6: tolerance grade IT6 for the shaft, which is tighter than the hole (13 µm at 18 to 30 mm). Shafts are made one grade finer because they are easier to machine accurately.
Worked example: 25 mm H7/g6
| Upper (mm) | Lower (mm) | |
|---|---|---|
| Hole 25 H7 | 25.021 | 25.000 |
| Shaft 25 g6 | 24.993 | 24.980 |
| Clearance | 0.041 (max) | 0.007 (min) |
Maximum clearance is the largest hole minus the smallest shaft (25.021 − 24.980 = 0.041 mm). Minimum clearance is the smallest hole minus the largest shaft (25.000 − 24.993 = 0.007 mm). The sheet is computed from the ISO 286-1 tolerance grades and fundamental deviations by the limits and fits calculator, which you can run for any other size.
H7/g6 compared with its neighbours
Same 25 mm size, all hole-basis with an H7 hole. The numbers are in µm; a negative value means interference.
| Fit | Type | Result at 25 mm |
|---|---|---|
| H7/f7 | Free running | 20 to 62 µm clear |
| H7/g6 | Close running, precision sliding | 7 to 41 µm clear |
| H7/h6 | Sliding, locating | 0 to 34 µm clear |
| H7/k6 | Transition, light location | 15 µm interference to 19 µm clearance |
| H7/n6 | Transition to light press | 28 µm interference to 6 µm clearance |
| H7/p6 | Locational press fit | 1 to 35 µm interference |
| H7/s6 | Medium drive press fit | 14 to 48 µm interference |
Moving down the table the shaft grows relative to the hole: clearance fits (f7, g6, h6) first, then transition fits (k6, n6) where the result depends on the parts you pick, then interference fits (p6, s6) that need force or heat to assemble. For a press fit, continue to the interference fit calculator for contact pressure, assembly force and torque capacity.
When to choose H7/g6
- Sliding or turning locating fits with lubrication: spindle sleeves, slide guides, pivots, pilot diameters.
- Parts that must be taken apart by hand and relocate to within a few microns.
- Where H7/h6 could bind from thermal growth or form errors, because h6 allows zero clearance.
For free-running rotating shafts with oil, use a looser fit such as H7/f7 or H8/f7. For fixed assemblies, use a transition or press fit.
Check the limits yourself
Enter the size and H7/g6 in the ISO 286 limits and fits calculator. Combine fits with the tolerance stack-up calculator to see how they affect a whole assembly.
Questions
What does H7/g6 mean?
H7/g6 is a hole-basis ISO 286 fit. H7 is the hole: fundamental deviation H (lower deviation zero, so the hole is never smaller than nominal) with tolerance grade IT7. g6 is the shaft: fundamental deviation g (slightly below nominal) with grade IT6. The result is a close running fit with a guaranteed small clearance. For a 25 mm size the hole is 25.000 to 25.021 mm, the shaft 24.980 to 24.993 mm, and the clearance 0.007 to 0.041 mm.
What is the clearance of an H7/g6 fit?
It depends on size. At 10 mm the clearance is 5 to 29 µm, at 25 mm it is 7 to 41 µm and at 50 mm it is 9 to 50 µm (1 µm = 0.001 mm). Use the limits and fits calculator for any size up to 500 mm.
Is H7/g6 a clearance, transition or interference fit?
Clearance. The largest g6 shaft is always smaller than the smallest H7 hole, so the parts always assemble freely and never bind at the limits.
What is the difference between H7/g6 and H7/h6?
H7/h6 has a shaft whose upper deviation is zero, so the minimum clearance is zero (a sliding fit, parts can touch at the limits). H7/g6 shifts the shaft down by the g deviation (7 µm at 25 mm), which guarantees a running clearance suitable for a light oil film.
When should I use H7/p6 instead?
H7/p6 is a locational interference (press) fit: at 25 mm the shaft is 22 to 35 µm over the nominal and the hole 0 to 21 µm, so the parts always need force or heat to assemble. Use the interference fit calculator to get the pressure, assembly force and torque capacity.
Is H7/g6 the same in ANSI B4.2?
For metric sizes yes. ANSI B4.2 adopts ISO 286, so the same letters and grades give the same limits.
Engineering aid for guidance only; values follow ISO 286 and are shown to help you check a design. Verify critical designs against the current standard.