To determine the bcd of your chainrings, measure the distance between two adjacent chainring bolt holes from center to center. It is critical to know the bcd of your crankset when you are selecting a new chainring for your bike. BBSHD Chainring Spacer (With images) Spacer On bicycle chainrings, this dimension is usually measured in millimeters. How to measure chainring bcd . Figuring out what size chainring you have is pretty easy, it's just a matter of knowing what size bolt circle diameter (or bcd) you have. All you need to do to calculate the bcd is measure the hole spacing a between adjacent holes, count the number of holes n and plug them into the equation above to calculate the bcd. On some cranks, like 4 bolt cranks, it’s fairly easy to measure, but for road and cyclocross with 5 bolts, it’s not so easy. When ordering a chainring make sure you get one which has the same bolt spacing pattern, although from this page i'd say you have evenly spaced bolts. With...