Re: Researching for the best motorized bicycle kits
IMHO the bottom line on these china 2-strokes is "they're cheap", both in cost and in quality. The kits dont cost much, and the QC isn't very good, but replacement parts are cheap when you need to repair.
In China there are "factories" where the castings are made, and then there are numerous "assembly lines" were the products are finished and assembled. The kits dont come out of one "factory" completely assembled like a car in Detroit... The quality of finish put on the parts depends on the people doing the finishing work, they get paid by piece work so turning out more parts per day is what they are aiming for. For this reason the quality on any given item can vary from kit to kit, even from the same company.
What I have done is simply come to this realization and accept the situation for what it is worth. This means basically if you want to be able to ride your bike reliably you should probably build up a spare motor (its cheap enough) and have it ready and waiting in case the motor you are running fails. At that point you just swap motors and get your bike rolling right away, then rip apart the blown up one and repair it and hold it as a spare ASAP. Otherwise you may have some down time while you repair your motor if it does fail, which is pretty likely, depending on how hard/much you ride, how well your bike is built, and the quality of parts you got to start with...