When two parts mate (like a shaft in a hole), the fit describes how loosely or tightly they go together, and it’s created by the tolerances on each part. There are three families.
| Fit | Result | Example use |
|---|---|---|
| Clearance | Always a gap — slides freely | Rotating shafts, sliding parts |
| Transition | May be slightly loose or tight | Locating parts that must be removable |
| Interference (press) | Always tight — must be pressed | Permanent joints, bearing races |
How tolerances create fits
Each part has an allowed size range. If the hole is always larger than the shaft, you get clearance; if the shaft is always larger, interference; overlap of the ranges gives a transition fit. Standard systems (like ISO hole-basis fits) define these with code letters and numbers so designers specify a fit consistently. Match the fit to function, and don’t over-specify — tighter fits cost more to machine.
Frequently asked questions
What’s a clearance fit? One with a guaranteed gap, so parts slide or rotate freely.
What’s an interference fit? A tight fit that must be pressed together and holds by friction.
How do tolerances set the fit? The overlap (or gap) between the hole and shaft size ranges determines it.
Hole-Basis vs. Shaft-Basis Systems
Most shops standardize on a hole-basis system, where the hole’s tolerance is held at a fixed baseline (commonly H7) and the shaft’s tolerance is varied to produce the desired fit — this is practical because reamers and other hole-finishing tools come in fixed sizes, while a shaft is easy to turn to any diameter. A shaft-basis system works the other way around and shows up more in industries where the shaft size is standardized, like certain bearing applications.
Temperature changes the fit. A fit measured at room temperature can tighten or loosen once a part reaches operating temperature, especially when the shaft and bore are different materials with different thermal expansion rates. Press fits assembled by heating the outer part (or freezing the inner one) rely on this directly — the temporary size change from thermal expansion is what lets parts that would otherwise interfere slide together.