I am overhauling an A-65. The table of limits give figures, example 0.002L or 0.003T. Explain what T & L mean.

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john.schwaner's picture
I am overhauling an A-65. The table of limits give figures, example 0.002L or 0.003T. Explain what T & L mean.

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"



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AskBob's picture

Steve340 on Thu, 12/13/2012 - 09:29
Checking on this one. UNofficial guesstimate is length and thickness. But this is an off the cuff guesstimate, I have put it up for discussion. Where's a machinist when you need one @

john.schwaner's picture

Class: Fit tolerance

Loose Fit (positive clearance)
Tight Fit (negative clearance)

Steve340's picture

Good thank you, I had put the question up on our site, and my guys found this, and determined the following


Clearly shows loose and tight fit

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