Yes, I've geared-up 2 Kymcos using gy6 gear kits. One of the stock gears in the set has to be swapped in the standard stock Kymco gear set. The up-gear set is 2 replacement gears, and there's 4 gears in a complete installed set. In the Kymco, one of the 2 remaining Kymco gears has to replaced with a GY6 gear. The Kymco gear appears to be identical to the GY6 gear, but it has a different inside diameter. If your Kymco is fitted with a QMB/GY6 engine instead of the SD10 engine, you can simply swap in the entire GY6 gear set. I figured it out by experimenting using gears from the local GY6 recycling-bin. It's not a big deal since the gears are cheap. Any well-stocked Chinese scooter dealer will have the odd-ball gear in stock, and it's less than $10. Show the mis-fit Kymco gear to the parts man, and they know exactly which gear you need. Both 50cc and 150cc scooters have the same issue.