Fits and Limits
|Dial Indicator used to measure Form Limits|
L and T is a Fit Tolerance
Fits are a fit tolerance or how much clearance between two mating parts. If there is no clearance the fit is 0.000. If you have negative clearance (tight fit) then the clearance might be stated as 0.003T If you have a loose fit then the clearance might be stated as 0.003L
Common examples of fits are the fit of the piston into the cylinder. Example, IO-520 Piston to Cylinder is 0.012L or 12 thousandths of an inch maximum clearance between the cylinder wall and piston. A tight fit is a bushing pressed into a hole or a guide or seat in a boss. Sometimes a tight fit is called a "shrink fit" as the parts (such as guides and seats) are installed chilled to shrink their size while the cylinder is heated to expand the hole.
Limits are a size or form tolerance or how much a part can deviate in size or shape. For example, a part might be 1.000 +-.003. Form Limits include such things as "run-out" on shafts and flanges.
As a side note. I get a laugh when I hear someone state that they overhauled an engine to "factory new limits." What they meant to say is: "Factory new Fits and Limits". There are several areas where Lycoming publishes wear limits or fit limits but not new limits. A new limit is more of a manufacturing tolerance and not always useful to an overhaul process. But a fit limit is always important.
See my article on "Factory New Limits and Other Nonsense"