What does H7/g6 mean?

It is an ISO 286 fit. The capital H7 is the hole: H puts its smallest size at the nominal size, 7 is the grade. The lowercase g6 is the shaft, which sits just under the nominal size. Together they make a sliding clearance fit.

What is the difference between clearance, transition and interference fits?

A clearance fit always leaves a gap. An interference fit always needs force or heat to assemble. A transition fit can come out either way, depending on the actual sizes.

How do I work out the maximum clearance?

Take the largest hole and subtract the smallest shaft. The minimum clearance is the smallest hole minus the largest shaft. If that is negative, it is interference.

Mechanical prints: lesson 3 of 7

Limits and fits

Read the hole and shaft limits, then answer. Get 10 right to finish. ISO practice uses H7 codes with the deviations shown.

What kind of fit is this?

How to read a fit

A fit is two toleranced sizes that go together: a hole and a shaft. Compare the worst cases. The smallest hole against the largest shaft gives the tightest fit. The largest hole against the smallest shaft gives the loosest fit.

Clearance fit: the smallest hole is at least as big as the largest shaft. There is always a gap.
Interference fit: the smallest shaft is bigger than the largest hole. It is pressed or shrunk together.
Transition fit: the zones overlap. Some pairs slide, others need a light press.

ISO fit codes

ISO 286 writes a fit as the size, the hole class and the shaft class, such as Ø25 H7/g6. Capital letters are holes, lowercase are shafts. Most fits are hole based: the hole is H, with its smallest size at the nominal size, and the shaft letter sets the fit.

Fit	Type	Ø25 deviations (mm)	Typical use
H7/g6	Clearance	Hole +0.021/0, shaft -0.007/-0.020	Sliding
H7/h6	Clearance	Hole +0.021/0, shaft 0/-0.013	Location, parts assembled by hand
H7/k6	Transition	Hole +0.021/0, shaft +0.015/+0.002	Accurate location
H7/n6	Transition	Hole +0.021/0, shaft +0.028/+0.015	Tighter location
H7/p6	Interference	Hole +0.021/0, shaft +0.035/+0.022	Light press fit
H7/s6	Interference	Hole +0.021/0, shaft +0.048/+0.035	Medium drive fit

Values for sizes over 18 up to 30 mm, from ISO 286-2. Other size ranges have other values.

Common mistakes

Mixing up H and h. Capital H is a hole, lowercase h is a shaft.
Pairing the wrong limits. Maximum clearance uses the largest hole and the smallest shaft.

Questions

What does H7/g6 mean?: It is an ISO 286 fit. The capital H7 is the hole: H puts its smallest size at the nominal size, 7 is the grade. The lowercase g6 is the shaft, which sits just under the nominal size. Together they make a sliding clearance fit.
What is the difference between clearance, transition and interference fits?: A clearance fit always leaves a gap. An interference fit always needs force or heat to assemble. A transition fit can come out either way, depending on the actual sizes.
How do I work out the maximum clearance?: Take the largest hole and subtract the smallest shaft. The minimum clearance is the smallest hole minus the largest shaft. If that is negative, it is interference.

Explained in our own words and checked against several textbooks and published tables. For the full rules, see ASME Y14.5, ASME Y14.36, ISO 286, ISO 965, ISO 1302 and ISO 21920-1.