How To Measure Chainring Bcd. On bicycle chainrings, this dimension is usually measured in millimeters. Alternatively you can measure the distance between two adjacent bolts and use the table below to determine the bcd.
This will give you the bcd for a 4 hole. Bcd = colt circle diameter, the 4 or 5 bolts are arranged in a circle, the diameter of which is the bcd. Threaded for standard m8 chainring bolts.
On a chainring with 4 bolts the bcd is the distance between two bolts across from each other.
Figuring out what size chainring you have is pretty easy, it's just a matter of knowing what size bolt circle diameter (or bcd) you have. Here are instructions on how to measure your bcd. Since the bcd probably isn't stamped on your crankarm, you need to take a measurement and do a calculation. Figuring out what size chainring you have is pretty easy, it's just a matter of knowing what size bolt circle diameter (or bcd) you have.