What is 386? 386 is short for 3 chainring bolts in an 86mm bcd. 386 is proprietary to Full Speed Ahead 2 speed mountain bike cranksets. Having an 86mm bcd allows us to have a smaller inside chainring (27 tooth instead of the traditional 29t on 104 bcd cranksets) for a gear range that is more suitable for all types of mountain bike riding.