Tolerance is not always better when higher. IT6 and IT8 can differ in cost by a factor of two; the key is the fit requirement.
How we control it
- Surfaces mating with bearings or seals use IT6–IT7 for running accuracy;
- Installation and locating surfaces use IT8–IT9, sufficient and lower cost;
- Non-mating surfaces use free tolerances, do not mark high precision across the whole drawing.
It is also why the Precision series has stable lead times.