How to Measure BCD on a Chain Ring
A chain ring's bolt circle diameter determines what crank sets it is compatible with, and is the key measurement needed when shopping for new chain rings. The BCD is a measurement of the distance between the bolts that hold the chain rings together. All of a crank set's chain rings must have the same BCD, or else the stack bolt cannot secure the rings together.
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Instructions
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Measure the distance between adjacent bolt holes. The BCD measurement is technically a center-to-center measurement, but since the bolt hols are circular, measure from the left edge of one to the left edge of the adjacent hole. This ensures accuracy in your measurement.
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Count the number of bolt holes in your chain ring.
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Establish the BCD by multiplying your measurement times the BCD multiplier. For three-bolt chain rings, multiply the distance by 1.155. Multiply the measured distance by 1.414 to establish the BCD of a four-bold chain ring, or by 1.701 to establish the BCD of a five-bolt chain ring. Six-bolt chain rings require you to multiply the measured distance by 2.000 to determine the BCD.
Example: You measure an 88.8 mm bolt-to-bolt distance on a five-bolt chain ring, so you multiply 88.8 mm by 1.701, resulting in a BCD measurement of 151 mm.
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