George, Running clearance is the clearance between the item in mention (camshaft, crankshaft journal) and the bearing material. Or, the oil clearance. .002 inches total(.001 per side), aprox, is considered normal.
This allows room for the "oil cushion" to exist. If the clearance is too small, not enough oil cushion will exist and scuffing may occur.
If the clearance is too big, in the case of a rod or main bearing, it can make noise. In the case of too much clearance in a cam bearing, it usually just leads to lower than desireable oil pressure.
Many people check for proper oil clearance by using something called "plastic gage", a special plastic string you smash between the bearing material and crankshaft journal, then measure the width of the smashed string using a scale provided.
The resulting width is in direct proportion to the clearance (supposedly). Myself, I like to use micrometers and measure the inside diameter of the bearing as assembled (assemble the rod with the bearing shells, torque the nuts and measure the ID with a inside mic or telescoping gage), then subtract the OD of the journal (measure with a micrometer).
In the case of cam bearings, using inside/outside micrometers is the only way to properly measure the clearance.... for obvious reasons.
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