Losing the small rubber cover on a Di2 rear derailleur's charging port is the kind of thing that happens on the workstand — it gets flipped open to plug in the charger, then it disappears into the shop floor or the parts bin and nobody notices until the next charge cycle. The Y3GK89400 is the factory replacement for that exact piece on the RD-R9250, Shimano's 12-speed Dura-Ace Di2 wireless rear derailleur. It's the same cover that shipped installed on the derailleur from the factory, so fit and finish match what was already there.

Compatibility here is narrower than it might look. This part number is specific to the R9250 Dura-Ace derailleur — the Ultegra R8150, 105 R7150, and GRX RX825 rear derailleurs use a different charger cover (Y3J145000) because the charging port geometry on the R9200-series flagship differs from the lower-tier R9200 platform variants. Grab the wrong one and it won't seat correctly, so match the part number to the derailleur.

The cover's job is straightforward: it seals the charging interface on the derailleur body against water, road grit, and the general abuse a rear derailleur takes sitting inches off the pavement. Riding without it isn't catastrophic — the port itself is designed to survive some exposure — but leaving the interface open long-term invites corrosion and dirt intrusion that can eventually cause charging issues. Replacing a lost cover is a few-dollar fix versus dealing with a fouled charging port later.... Read More