Shimano AD17-M Front Derailleur Clamp Shim, reduces 34.9mm to 31.8mm

Derailleur = the size of the clamp on the front derailleur you have. Shimano Front Derailleur Seat Tube Shims. Front Derailleur Clamp: 31.8 (1-1/4"). Front Derailleur Clamp: 34.9 (1-3/8"). Our motto is 'All Riders, No Attitude!'.