When converting to Di2 from a standard cable based setup, holes are left in the down tube, the bottom bracket, and the rear derailleur cable exit. The down tube hole and the bottom bracket hole can let water and debris enter the bottom bracket, contaminating the bearings and flooding the Di2 cable connections.
Note: Shimano cable reliefs (not included) are also required for a Di2 conversion: one for the wire entering the down tube from the junction, and one for the wire connected to the front derailleur. You will need one SM-GM01 6 x 6 grommet (down tube), and one SM-GM02 7 x 8 grommet (front derailleur). These grommets are sold in packs of four and can be found on Amazon and other cycling supply shops.
The bottom bracket cap replaces the plastic cable guide that falls out when the shift cables are removed. The rear derailleur grommet replaces the cable grommet in the rear to prevent the wire from resting directly on bare metal. The down tube plugs and bottom bracket cap form a water tight seal for the bottom bracket that will prevent contaminants from entering the inside of the frame.
Weight: < 3g
When installing the bottom bracket cap, always attach it from the rear of the bike first (hook back, clip front). Gently clip in the front using your thumb as shown in the pictures. If not installed correctly, the tab can break. This is due to the angle the hole is cut in the frame.
To remove the BB cap, use the small ridge in the rear center of the cap, to pop off the rear of the cap first. This is the opposite direction used in the installation. The front of the cap faces the front of the bike and is indicated by arrows on the inside.
When installing the rear strain grommet, spread it enough to slip it over the wire and use the wire to help pull it snugly into the hole while pushing the plug in. Use the reverse process to remove it.