Bolt Circle Adapter

Sometimes, your preferred chainring doesn't work with your preferred crankset. Maybe an 8-speed chainring isn't available for a 12-speed crank, or vice versa. This adapter changes one bolt circle diameter into another.

I designed this particular adapter for SRAM's short-lived 107 BCD. It replicates the shape of a SRAM chainring, and bolts on with one set of chainring bolts. Another set attaches it to a chainring of a different BCD; in this case, symmetric 104 BCD.

The design does add a ~3mm inboard offset to the chainring. This can be fixed with a different chainring-adapter-crank order, or it might create the chainline you want, as it did for me.

I made versions for 107 -> 110 and 104 -> 110 as well. Since the design is so simple—it's concentric bolt holes stamped or lasercut out of sheet metal—it can be easily redesigned for another pair of BCDs.