Shimano Di2 SM-JC40/41 Rear Junction Box

The Shimano SM-JC40/41 Rear Junction connects all of the Dura-Ace 9070 and Ultegra 6770 Di2 electronic components to the battery unit. It also serves as the interface between the Front Wire/Junction, which transmits data from the controls, and the Di2 Derailleurs.

This piece is also called Junction B and is available in two versions: SM-JC40 for externally routed wires, which mounts under the bottom bracket shell, and SM-JC41 for internally routed wires, which mounts inside the down tube just above the bottom bracket shell.

The external version (SM-JC40) also provides several alternate wire routing options which allow the user to take up varying amounts of slack (up to 1.2cm) for electronic wires that are slightly too long. All electronic wires (EW-SD50) are sold separately.

Note: Neither the SM-JC40 or SM-JC41 Rear Junction is compatible with older 7970-series Dura-Ace Di2 components.