Internally routed Di2 systems have a small but real vulnerability at the frame entry point—that's the spot where the EW-SD50 wire disappears into the downtube or chainstay, and it's also where road spray, grit, and moisture want to follow. Shimano's SM-GM01 grommet is the finishing piece that seals that transition. It's a 6mm circular rubber insert that seats into the frame's cable port and grips the wire as it passes through, and the pack ships with four pieces—usually enough for a full build with a spare or two left in the parts drawer.

The job is straightforward but worth doing right. Without a grommet, you're relying on friction between the wire and the frame hole to keep things quiet and positioned, and you're leaving an open channel for water to work its way inside the frame. With one properly seated, the wire holds its position when you turn the bars, the frame opening gets a weather seal, and you stop hearing that faint tick of a loose wire tapping the inside of the downtube on rough pavement.

Sizing is the one thing to get right before ordering. Frame manufacturers spec either a 6mm round hole (which is what the SM-GM01 fits) or a 7x8mm oval (which needs the SM-GM02 instead). If you're not sure which your frame uses, check with the framebuilder or measure the opening—the wrong shape won't seat properly and doesn't seal. Most road frames designed around Di2 use the 6mm round standard, but it's worth confirming rather than assuming.