The CDG-2 easily and accurately measures bolt circle diameter (BCD) in millimeters for a wide variety of four- and five-bolt chainring patterns. Made of anodized aluminum and permanently laser engraved, the tool centers itself in chainring holes, chainring bolts, or crank arm holes. The CDG-2 also includes a center-to-center distance scale and a handy reference chart listing additional mounting pattern measurements.
In the old school BMX world, most chainrings are 110mm bcd. Some were borrowed from the road bike world and are 130mm bcd. Every once in a while you find a 144mm bcd ring being used. But there are even stranger things out there, such as Taiwan made bikes in the early to mid 1980's with 108mm bcd. This tool tells you for sure what bcd your spider or chainring is so you get the right parts first time, every time.