Shimano Boss Free Type Generally, 6-8 speed models with the model number starting with MF-TZ. This is a standard widely used in children's mountain bikes and 6-7 speed city bikes. Equivalent to Shimano TL-FW30. A 24mm monkey wrench or spanner is required. The fourth photo is an example of a sprocket and is not the item for sale, but please check carefully as the necessary tools will differ for HG cassette types. Actual measurements: Outer diameter of the splines: 23.5mm Inner diameter of the splines: 21.2mm Number of splines: 12 For a smooth transaction, please purchase without comments!