This is a set of stock 1.0mm clutch springs. This is the most common clutch spring for the Mugen MBX5 buggies. The stiffer the spring, the later the clutch will engage allowing the engine to spool up to a higher RPM range before the clutch comes on. The later the clutch engages, the more the clutch will slip causing the clutch shoes to wear out more quickly. If you need more bottom end from your engine, consider using 1.1mm springs with the aluminum shoes.