These mongo Shimano 7800 chainrings are designed for TT use on 10 speed drivetrains and with cranksets that have a 130mm bolt diameter. works perfectly with a 42T(or larger) 7800 inner ring. It will work with a 39T inner, but decreased shifting performce will occur due to the amount of tooth differential between the rings. This is probably the best TT ring we sell. Pinned and ramped ring with profiled teeth for perfect shifting and polished to a pefect finish. Choose from 54T, 55T or a monster 56T ring.