Answer-RC 3 Shot Clutch Spring set - Complete including 0.9mm Gold springs, 1.0mm Silver springs and 1.1mm Black springs.
A lighter spring allows the clutch to engange earlier with more progeression. A thicker spring allows the engine to rev higher prior to engagement of the shoe coming in with more bite and more aggression. If driving on a low grip surface it is generally considered best to run a lighter spring making the car easier to drive out the corners.
Due the generic nature of 3-shoe clutches these springs should be suitable for the majority of 1/8 buggys and truggys on the market.