Problems with those china FD kits, clutch bells breaking the welds, holes not centered for correct installation/drive roller shaft operation, bearings going kaput way too early. If it were me I'd buy a staton or Dax kit. Dax is pricey but it's a good kit and he'll back it up with good warranty service, Staton will too. With FD the way to get the fastest speed is with a bigger diameter roller vs a smaller roller. With a HS 142f 49cc motor I like the Staton 1-1/8th in roller. It gives me a good balance for torque and speed, about 22 mph. It will climb steep hills w/o pedaling witch I like and 22 mph is fast enough. These are my choices you'll need to find what makes you content..........service after the sale is worth something in my book. I can't put a $ on it but this I do know. Cheapest isn't always the best value. The big vendors in this business only answer the ph when you want to buy something. When you need warranty service they won't take care of it. Not all vendors are that way, just the biggies who sell at rock bottom prices.
