Shimano Boss-Free Type Generally, 6-8 speed models with the model number starting with MF-TZ. This standard is used in a high percentage of 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 change for HG cassette sprockets. Actual Measurements: Outer diameter of the splines: 23.5mm Root diameter of the splines: 21.2mm Number of splines: 12 For a smooth transaction, please purchase without commenting!