With this tuning set 4 x spacer rings (inside 20.2 mm diameter) 2 mm - 3 mm - 4 mm - 5 mm (made of steel), you can easily regulate any speed from 25 - 60 km/h. Suitable for: REX LONGJIA YIYING YY50QT QM50QT-6 JMSTAR ZNEN HUATIAN CHINA scooter 4-stroke with crankshaft 20 mm diameter.