Shimano SM-JC41 Di2 Internal Junction Box

Shimano’s Di2 system is made up entirely of plug-and-play components. Their SM-JC40 and SM-JC41 Di2 Junction Box is what goes by the bottom bracket. Specifically, these are both four-port boxes. One cable comes from Junction A, aka the stem box, either internally or externally. One comes from the battery. One goes to the front derailleur. One goes to the rear derailleur. The SM-JC40 is the external box, for people who are wiring their Di2 externally. It affixes to the bike via the bolt that typically holds the under the bottom bracket cable guide to the bottom bracket shell. It comes with two different-sized bolts for this purpose The SM-JC41 is the internal box, for people who are wiring their Di2 internally. It typically rests in the downtube or seat tube. Many let it be, as it’s light and compact; some hold it in place with double-sided tape. The Shimano SM-JC40 and SM-JC41 are your choices for plugging in your Shimano Di2 derailleurs and battery.

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