Shimano XTR M971/980 Front Derailleur Clamp Shim 31.8mm Derailleur = the size of the clamp on the front derailleur you have Frame = the size of the seat tube the derailleur plus shim will fit Front Derailleur Clamp: 34.9 (1-3/8") Shimano Front Derailleur Seat Tube Shims