BCD Bolt Circle Diameter
How to measure your BCD (Bolt Circle Diameter)
Bolt Circle Diameter or BCD is the diameter of the circle that goes through the center of all of the bolts on your chainring. On bicycle chainring this dimension is usually measured in millimeters. It is critical to know the BCD of your crankset when you are selecting a new chainring for your bike. In many cases the BCD is printed right on the chainring. Sometimes it is stamped or engraved on the back side of the chainring.
If it is not labeled on your chainring you will need to measure it. On a chainring with 4 bolts the BCD is the distance between two bolts across from each other. It can be somewhat difficult to make this measurement if you don't have a caliper since the crankarm gets in the way.
Alternatively you can measure the distance between two adjacent bolts and use the table below to determine the BCD. This is the easiest method to use for chainrings with five bolts. Note that there are a few cranksets that have non-standard arm spacing. If this is the case the distance between adjacent bolts will be different as you move around the chainring. If this is the case out standard chainrings will not fit for your application.
4 Bolt Chainrings
|BCD||Distance (mm)||Distance (inches)|
4 Hole Examples
5 Bolt Chainrings
|BCD;||Distance (mm)||Distance (inches)|
5 Hole Examples