The Birzman Bottom Bracket Socket Adapter fits inside the Birzman Shimano Hollowtech II BB socket, and adapts it for use with Shimano BB9000 and BBR60 bottom bracket cups, which have a different shape than typical Hollowtech II bottom brackets. It's made of hardened steel for durability, and then precision-machined for a precise fit. Birzman backs it with their limited lifetime tool warranty.