Wiktionary
bolt circle
n. (context engineering English) A theoretical circle on which the centerpoints of bolt holes lie when the bolt holes are positioned as equally spaced in a circle. Common examples are the bolt circle for the lugs on a car's wheel, or the bolts on a http://en.wikipedia.org/wiki/bolted%20joint between pipe flanges. Often abbreviated "BC" or "B.C." on engineering drawings. Often treated as a basic dimension, when true position for the centers is specified using http://en.wikipedia.org/wiki/geometric%20dimensioning%20and%20tolerancing.