ISO 286 fits explained: H7/g6, H7/k6 and H7/p6 in practice
ISO 286 fits combine a hole tolerance and a shaft tolerance to get a clearance, transition or interference fit. Most industrial drawings use the hole-basis system with an H7 hole and choose the shaft letter by function: g6 or f7 for parts that must slide or turn, h6 or js6 for location, k6 or m6 for light press or bearing seats, and p6 or s6 for permanent press fits.
Guilherme Rodrigues ItinoseUpdated 2 min read
Key takeaways
- Hole-basis (H) is standard because reamed or bored holes are harder to adjust than shafts.
- Clearance fits slide, transition fits locate, interference fits transmit load.
- Write the fit on the drawing (e.g. Ø20 H7/g6) and, if useful, the limits.
- Check bearing manufacturer recommendations for housing and shaft fits.
- Tight fits need the right process: reaming, boring or grinding.
Many assembly problems come down to a single line on a drawing: the fit. A bearing that spins in its housing or a pin that will not go in usually had the wrong fit, or none at all.
The three families of fit
| Fit | What happens | Typical examples (hole basis) |
|---|---|---|
| Clearance | Always a gap; parts slide or rotate | H7/g6, H8/f7, H11/c11 |
| Transition | Small gap or small interference | H7/h6, H7/js6, H7/k6 |
| Interference | Always interference; parts are pressed | H7/p6, H7/s6 |
Why hole basis
Holes are made with reamers, boring bars and standard tools; changing their size is harder than turning a shaft to a different diameter. That is why most drawings fix the hole at H and choose the shaft letter to get the fit.
Reading H7
For a 20 mm nominal size, ISO 286 gives an H7 hole of 20.000 to 20.021 mm. A g6 shaft is 19.980 to 19.993 mm, so the H7/g6 pair always has clearance between 0.007 and 0.041 mm. Tables in ISO 286-2 give these values for every size range.
Practical choices
- Gears or pulleys sliding on shafts: H7/g6.
- Locating parts that must be assembled by hand: H7/h6.
- Bearing seats: follow the bearing manufacturer, often k6 or m6 shafts for rotating inner rings.
- Permanent bushings: H7/p6 or tighter, pressed in.
Call out the fit on the drawing, add limits when useful and make sure the process can achieve it. Our CAD engineering and documentation work includes fit selection and tolerance checks.
When to outsource this
- Parts do not assemble, or assemble too loose
- You are detailing bearing seats, pins and bushings
When not to
- Non-functional features where a general tolerance is enough
FAQ
What does H7 mean?
H is the position of the tolerance zone (lower limit at nominal size) and 7 is the tolerance grade. For a 20 mm hole, H7 means 20.000 to 20.021 mm.
Which fit for a sliding shaft?
H7/g6 is a common running or sliding fit with small clearance. For freer running with more clearance, H8/f7 is often used.
Which fit for a dowel pin?
Dowel pins usually press into one part (interference) and slide into the other with a location or light clearance fit, following the pin standard.
Should I write limits or the fit code?
The code is standard and compact. Adding the limit dimensions helps shops that do not have the tables at hand.
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