How To Measure Chainring Bcd. For the xx1 spider, we recommend the 6mm bolts (or any short m8 chainring bolt). On a 4 bolt chainring you can simply measure the distance between the centres of opposite bolts.
On bicycle chainrings, this dimension is usually measured in millimeters. On a chainring with 4 bolts the bcd is the distance between two bolts across from each other. On a chainring with 4 bolts the bcd is the distance between two bolts across from each other.
Use the measurement (x) to find your bcd on the chart below.
With this measurement (b) you can refer to the table below to identify the corresponding bcd, or use one of the following equations: On a chainring with 4 bolts the bcd is the distance between two bolts across from each other. On a 4 bolt chainring you can simply measure the distance between the centres of opposite bolts. You could measure the distance from the bb spindle to the center of one bolt hole (gives you the colt circle radius) and double it.