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Understanding Chainring Measurements

Understanding Chainring Measurements
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 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