"bcd" term in chainring
 

what does the term "bcd" refer to when talking about chainrings?

Bolt Center Diameter. Literally, if you were to draw a circle through the center of the chainring bolts, the diameter of that circle is the measure referred to. Usually specific to a manufacturer. Easier to research what goes with your crank than to try and measure yourself.

Sheldon has a nice table for various old and new BCD standards for different manufacturers HERE.

Four 4-bolt chainrings the BCD is easy to measure, just measure the distance across two opposing bolts. For 5-bolt cranks it's not so straight forward so the usual method is to measure the distance between two adjacent bolts and look up a conversion chart such as Sheldon Brown's.

ok-thanks for the info