Compression springs do a couple of things. First they supply the pressure to engage the belt. They also determine the rate at which the CVT transmission changes from low to high gear. A 1000rpm spring will go into a higher gear than a 2000rpm spring. This NCY 1000 rpm compression spring is specifically for the GY6 50.

Measurements:

Wire Diameter: 4.0mm Inner DIameter:49.0mm Length: 106.0mm