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How to measure Bolt Circle Diameter (BCD)
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 like in the example below. 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. The image below shows this measurement on a 104mm BCD chainring.
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. The image below demonstrates this measurement on a 104 BCD chainring.
4 Bolt Chainrings
| BCD | Distance (mm) | Distance (inches) |
| 64mm | 45.3mm | 1.78in |
| 88mm | 62.2mm | 2.45in |
| 94mm | 66.5mm | 2.62in |
| 96mm | 67.9mm | 2.67in |
| 102mm | 72.1mm | 2.84in |
| 104mm | 73.5mm | 2.90in |
| 120m | 84.9mm | 3.34in |
5 Bolt Chainrings
| BCD | Distance (mm) | Distance (inches) |
| 94mm | 55.2mm | 2.18in |
| 110mm | 64.7mm | 2.55in |
| 130mm | 76.4mm | 3.01in |
The equation for the BCD for any symmetric bolt pattern is given by:
BCD = d / sin(180/n)
where d is the center to center distance between adjacent bolts, n is the number of bolts, and the angle is in degrees.
BCD Guide
The last method is to remove the chainring from your bike and place it on a printout of the BCD guide in the link below. Make sure to print it out in the actual size with no scaling. The just find the circle pattern that lines up with the holes on your chainring.
Exceptions
There are always exceptions. Here are a few specific examples to be aware of.
- For SRAM X7 (new version), X9, XO, XO1, XX1 our solution is the direct mount rings. These rings on these cranksets are 94 BCD. With our direct mount solution you remove the original chainring and spider and replace it with our ring that interfaces directly with the splines on the crankset. You will need a T25 Torx L-wrench to remove the three bolts that secure the chainring to the crankarm.
- SRAM Red Exogram has a hidden bolt that screws into the crank arm. The chainring bolt may be too long to firmly secure the chainring so you will need to add a washer (see photo) or carefully file down a bolt to the proper length.
