The park tool cdg 2 chainring diameter gauge 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 park tool cdg 2 chainri