This is a pack of three optional Mugen Seiki 1.10mm Clutch Springs. These springs use 1.10mm thick wire, making them hard.

You can fine tune the way the clutch engages by using different tension clutch springs. The bigger the spring, the stiffer it is and the later in the RPM range the clutch will engage. The smaller the spring, the softer it is and the earlier in the RPM range the clutch will engage.