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Bolt Circle Diameter (BCD)

Chainrings: Understanding BCD

Bolt Circle Diameter (BCD) is the diameter of an imaginary circle running through the center of the chainring mounting holes. It is always defined in millimeters. See measurement “A” in the below diagram.

Measuring along this imaginary line can sometimes be tricky. Luckily, it’s possible to calculate the BCD of a chainring by simply measuring the center-to-center distance of two adjacent bolt holes. With this measurement (“B”) you can refer to the table below to identify the corresponding BCD, or use one of the following equations:

•For 3-bolt chainrings: “A” = “B” x 1.155
•For 4-bolt chainrings: “A” = “B” x 1.414
•For 5-bolt chainrings: “A” = “B” x 1.701

“A” = BCD (mm)
Number of bolts
“B” = Bolt-to-bolt measurement (mm)
Common Application

58 5 34.3 Compact MTB triple – inner
64 4 45.3 Standard MTB – inner
74 5 43.5 Compact Road double/triple – inner
80 4 56.6 SRAM MTB double – inner
86 3 74.5 FSA 3-bolt
88 4 62.2 XTR M985 – inner/outer
92 5 53.3 Dura Ace 7800/Ultegra 6700 triple – inner
94 5 55.4 Compact MTB triple – middle/outer
102 4 72.1 XTR M960 – inner
104 4 73.6 Standard MTB – middle/outer
110 5 64.7 Compact Road double/triple – middle/outer
110 (Campy) * 5 64.7 Campagnolo Compact/CX – inner/outer
120 4 84.9 SRAM MTB double – outer
130 5 76.4 Standard Road double – inner/outer
135 5 79.5 Campagnolo Road – inner/outer
144 5 84.6 Track
146 4 103.3 XTR M960 – outer

* Campy 110 BCD rings have a single offset bolt. They are not interchangeable with other 110 BCD rings and cranks.

Thanks QBP for the info and picture!

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