Bosch Chainring Lockring O-Ring Ring Ring Ring If you have a Bosch mid-drive motor and need a new lockring O-ring for your chainring ring ring, then you're in the right place. This Bosch O-Ring is designed to work with BDU4XX, BDU37YY, BDU31YY, BDU34YY, and BDU38YY models. Features Compatible with Bosch e Bike System 2 and The Smart System Works with BDU4XX, BDU37YY, BDU31YY, BDU34YY, and BDU38YY models Bosch part number: 1270. 016. 119 | Bosch | Chainring Lockring O-Ring For Bdu4Xx, Bdu37Yy, Bdu31Yy