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Bolt Circle Diameter (B.C.D.)
On a crankset, the diameter of the circle formed by the stack bolts.
Common 5-bolt double-chainwheel sets use 130 mm or 110 mm diameters.
Modern triple chainwheel sets have two diameters, a large one for the two outer chainrings, and a smaller diameter, with a separate set of bolts, for the granny ring.
Full-size triples usually use 110 mm/74 mm, or 130 mm/74 mm for newer "road triples." Campagnolo uses 135 mm/74 mm. Compact triples commonly use 94 mm/58 mm.
Some crank sizes:
"C-C" is the distance between adjacent stack bolts.
This is usually easier to measure directly than the actual Bolt Circle Diameter.
Measuring BCD: This 110 mm BCD ring measures 64.7 mm center-to-center
(or, from left side to left side, or right side to right side --easier to measure).Multiplying this dimension by 1.701 will give you the actual BCD for a 5-bolt chainring. Use 1.155 for 3-bolt chainrings.
If there is an even number of holes, diameter can be measured directly unless something is in the way -- usually, the crank. Then measure at two adjacent bolt holes, and multiply by 1.414 for 4-bolt chainrings and 2.000 for 6-bolt chainrings.
For dimensions of specific models, see my Bicycle Crank/Chainring Bolt Circle Diameter Crib Sheet.
