Park Tool Chainring Diameter Gauge

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.