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Shimano Di2 SM-JC40/41 Rear Junction Box

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MSRP:$30.99
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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.

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Reviewed 9/19/2017
by Anonymous
Cycling Enthusiast
Dover Plains, NY
Value Rating
Overall Rating
1 of 2 customers found this review helpful.
Di2 is wonderful -BUT!

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.

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