By Brad Quartuccio

The interface by which a chainring is fastened to a crank arm is referred to by a measurement and pattern, namely the Bolt Circle Diameter (BCD) and the number of bolt holes. While the number of bolts is obvious, the BCD can be hard to ascertain without a known ring handy for comparison. To determine the BCD of your chainring, place one bolt hole centered over “Home” and then pivot the ring until a labeled hole is visible through another bolt hole on your chainring. This gauge can be used to determine the BCD of most common chainring interfaces —except for the smallest ring of mountain bike cranksets. If your chainring doesn’t match up, it likely won’t be easy to replace.