Park Tool

Chainring Diameter Gauge

When it comes time to replace a worn out chainring or if you need to change up your gearing, knowing the chainring’s bolt circle diameter, or BCD, is key to finding the right ring for your crank's spider. The BCD is the diameter of the circle formed by the bolt holes. The CDG-2 quickly and easily measures the BCD accurately in millimeters for a wide variety of common four and five bolt chainring patterns. The CDG-2 also includes a center to center measuring scale and a quick reference chart for determining the BCD of less common chainring bolt patterns. For quickly identifying common 4 and 5-Hole BCD patterns, simply position the tips of the CDG-2 in the center of adjacent bolt holes and read the BCD in the appropriate window. For uncommon BCD patterns, position the tips of the CDG-2 in the center of the adjacent bolts and read the center-to-center measurement at the tip of the pointer. Locate the number of bolt holes on the chart and the closest C-C measurement determines the BCD. The CDG-2 is made of anodized aluminum and all markings are permanently laser etched for longevity.