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If you are a home or shop mechanic, or just a cyclist who has ridden enough over the years to build-up a collection of chainrings, you will likely find the Park Tool CDG-2 Chainring Diameter Gauge exceptionally helpful. With this gauge, you can easily and precisely measure bolt circle diameter (BCD) in millimeters for a wide variety of four and five-bolt chainring patterns. You'd be surprised how many sizes and options have been made and used, especially with triple chainring set-ups and proprietary products, such as with the ever-changing many generations of XTR mountain cranksets.

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. Nothing we have seen is easier or more accurate than the CDG-2, and if you actually need one, accuracy in determining BCD is crucial.