How Fits and Tolerances Work
Every hole-shaft pair needs a fit. The fit decides how tight or loose the parts go together. ANSI B4.1 defines five groups of fits. Each group serves a different purpose.
Running and Sliding Fits (RC)
RC fits always have clearance. The shaft is always smaller than the hole. RC1 is the tightest. It works for precision slides and gauges. RC9 is the loosest. It works for parts that expand with heat or collect dirt.
Locational Clearance Fits (LC)
LC fits hold parts in place with a small gap. The parts can be taken apart by hand. Use LC fits for pins and bearings that need to come out for service.
Locational Transition Fits (LT)
LT fits sit between clearance and interference. The actual parts may have a small gap or a small press. Use LT fits when you need accurate alignment but still want to take parts apart.
Pro tip: When in doubt, start with RC4 for running parts or LC2 for stationary parts. These cover most common needs. Use our drill bit size chart to find the right starting hole size for your fit.
Locational Interference Fits (LN)
LN fits always have interference. The shaft is bigger than the hole. You press the parts together. They stay put without pins or screws. Use LN fits for gears on shafts or bearings in housings.
Force and Shrink Fits (FN)
FN fits have heavy interference. They need a press or heat to assemble. FN1 is a light drive fit. FN5 is a shrink fit that needs the hub heated to 300-400 degrees. These fits carry heavy loads and torque.