Senior Member (Retired)
Join Date: Sep 2001
Location: Great North Woods
Bikes: Vittorio, Centaur triple; Casati Laser Piu, Chorus Triple.
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From Branfordbike (without permission)
Chainrings are most often measured in reference to their Bolt Circle Diameter (BCD) depicted as B in the above picture. BCD is literally the diameter of an imaginary circle that connects the centers of all the chainring bolts.
It is easier to measure the distance between adjoining chainring bolts as depicted by A in the above picture. Use the chart below to determine what size rings you have.
74 BCD Rings: A = 43.5 mm, B = 74 mm
130 BCD Rings: A = 76.4 mm, B = 130 mm
135 BCD Rings: A = 79.5 mm, B = 135 mm
144 BCD Rings: A = 84.6 mm, B = 144 mm