This small part is really the heart of the Ultegra Di2 system.
The Shimano SM-JC40/41 Rear Junction, also known as Junction B, connects all the Dura-Ace 9070 and Ultegra 6770 Di2 electronic components to the battery unit. In addition to this important task, it serves as the interface between the Front Wire/Junction (Junction A), which transmits data from the controls, and the Di2 Derailleurs.
This unit available is available in two versions, one for internal and one external wires. The external model, SM-JC40, mounts under the bottom bracket shell. This Junction 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. The internal model, SM-JC41, mounts inside the down tube just above the bottom bracket shell. 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.
B-Stock - This product has one or more B-Stock units available. These units can be purchased at a discount (see option select). B-Stock units were returned from other customers and may have missing or damaged packaging materials. These units are otherwise as new. The full manufacturer warranty applies. Click Here for more information.
Have been riding Di2 Ultergra for over 6000 miles and just love it. Never a malfunction or a problem-UNTIL there was a complete FAILURE! Shimano has designed and engineered this system very well but this internal junction box can and has come unplugged! Hence complete failure of system. In order to fix have to pull the bottom bracket. I was lucky to be in an area where there was a LBS that knew what to do and could do it on short notice. Also had a private SAG close by.
This is a very poor design- these connections should be twist lock or some other method so that is failure cannot happen. I am having LBS pull the BB and secure the connections with electric tape.