The short answer
Hole and shaft fits are a function callout disguised as letters and numbers. Once you know what motion or retention you need, the fit class becomes much easier to choose.
What the symbols mean
In the ISO system, the letter identifies the fundamental deviation and the number identifies the tolerance grade. H7 on the hole side is common because the lower deviation is zero in the basic hole system. Pairing it with different shaft classes creates clearance, transition, or interference fits.
That matters because the same nominal size can behave very differently in assembly depending on the fit pair.
How to think about common fit families
Clearance fits are for sliding or easy assembly. Transition fits are for accurate location with slight clearance or slight interference depending on actual limits. Interference fits are for press or shrink assembly where relative movement is not allowed.
H7/g6 is commonly treated as a sliding clearance fit. H7/h6 is a locational clearance fit. H7/p6 moves into press-fit territory. The exact limit values depend on basic size range, so the class matters more than a single memorized number.
What drives cost
The tighter the fit class, the more the process may shift toward reaming, grinding, honing, selective assembly, temperature-controlled fitting, or more inspection. That is normal. Precision mating parts should cost more than a loose bolt hole.
The cheapest mistake in design is often picking the loosest fit that still works.
How to avoid RFQ confusion
A nominal diameter and a plus-minus tolerance is not the same thing as a fit designation. If the part mates with another shaft or bore, say the intended fit class or the functional intent. That saves a lot of back-and-forth.
Also specify whether the supplier is responsible for both members of the fit or only one.
What an experienced buyer does next
Related reading: How to call out surface finish on engineering drawings: Ra, Rz, and N-grades and Common DFM mistakes that increase your machining quote by 30%+.
The right move is usually to define the real functional requirement, remove the decorative requirements, and let the supplier build a route around what actually matters.
Comparison table where relevant
| Fit example | General intent | Typical use |
|---|---|---|
| H7/g6 | Sliding clearance | Guided movement |
| H7/h6 | Locational clearance | Accurate location with assembly ease |
| H7/k6 or n6 | Transition | Accurate location with tighter feel |
| H7/p6 | Interference / press | Permanent or high-retention assembly |
How to specify this in your RFQ
Use fit designations on the drawing when function depends on mating behavior. State whether assembly is by hand, light press, or thermal method. If the mating part is supplied by someone else, include its nominal class or measured limits in the RFQ.
A clean RFQ does not just list requirements. It separates must-haves from preferences so the supplier can optimize where it is safe.
Have a part that needs quoting? Email your drawings to rfq@precisionmachining.co -
we return a competitive quote within 24 hours. Phone: +1 312-579-0808.