How To Measure Crank/Chainrings Size BCD

Posted by on in Cranksets


I show how to measure a chainring or crank to be able to replace a chain ring. The main things you need to know are the number of teeth and the BCD (Bolt Center Diameter). Well, and the number of holes. I show what to measure and how to calculate or look up the BCD.

As referenced in the video, this is Sheldon Brown’s BCD crib sheet:

The multipliers I used to calculate BCD by measuring the distance between adjacent bolt holes are:
3 bolt x 1.155
4 bolt x 1.414
5 bolt x 1.699
6 bolt x 2.000

Some of the most common BCDs:
4 hole: 64mm, 80mm, 104mm, 120mm
5 hole: 74mm, 110mm, 130mm, 135mm


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