I believe any of the kits you buy have the potential to last and run well for many years if they or good out of the box and you make the needed upgrades for better fasteners and in my case I like to replace the wrist pin bearing with a better one.
I will have to admit that I dont always replace the bearing before running the engine for a while, my oldest BGF 66cc has been running strong now since fall of 2009, it has ZERO mods to the internals, only replaced the weak soft mounting studs, proper maintenance and use of a quality 2 smoker oil all running through a well tuned carb makes a huge difference.
I was at my local bike shop yesterday picking up some tubes, the owner knows I have motorized bikes and thinks they are nice, he told me I was the only person around here that he knows of that has been able to keep mine going, he said he has had three people come in with them asking him to work on them, of coarse he doesn't have a clue about the engines so he always turns them away, he was hinting around about referring these people to me but I dont have the time to mess with other peoples cobbled up mess as he had described them to me...LOL!
I told him that many people have bad luck because they just dont know how to fix the simple things and do the simple upgrades to the common problem areas of these china engines. I would be happy to help them if I were to see them having trouble or I will tell them what they need to do, but no time to run a motorized bike repair shop....LOL!
As it has been mentioned many times on this forum by many people, buy from a seller who has a rep. for taking care of customer service issues and answering their emails and the phone, seen way to many people on here have an issue and some of the sellers just brush them off after they get their $$$'s
Dax, Venice, Sick Bike Parts, Piston Bikes and JNMotorbikes... I have read good reports of all these venders and I have dealt with 3 of the 5 of them I listed here personally with no issues with parts or service.
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