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.
Put these on my Savage X. They work great with a Mugen Nut (MUGC0704) as the springs rest perfectly. Initially I tried to install these with the HPI clutch/pilot nut and the recessed groove sat so far back (towards flywheel instead of clutch side) that to get the springs to sit I had to almost bend the tip of the spring at a crazy angle. I was not comfortable with keeping the stress of the mount area on the stock nut. So if you get these springs make sure you also buy the mugen nut as well.