SRAM X01 DH Rear Derailleur - 7-Speed Short Cage Black with Clutch A3 Model-Forerunner 255 If you have a round

If you have a round seat tube with no braze-on mount

Free Chain in bottom cog (unsupported chain - shorter is better and will increase shifting precision):

}KMC Reversible Chain Tool

Sizing: 128

SRAM X01 DH Rear Derailleur - 7-Speed Short Cage Black with Clutch A3 Model-Forerunner 255 If you have a roundSRAM X01 DH Rear Derailleur 7 Speed Short Cage Black with Clutch A3. The 7 speed X01 DH rear derailleur makes shifting smooth and reliable no matter where you ride. Engineered to maintain consistent chain gap across all seven gears it eliminates vertical movement for faster shifting reduced shift force and an end to ghost shifting. The clutch features a smoother torque curve for a quieter more consistent operation and feel. The 12 tooth X SYNC pulley