I have a 68.5cc engine from BGF that I purchased last April. As far as I know, this particular engine does not have any mechanical issues as of yet. However, this is the 4th engine I have had since I built my first bike back in 2009, and this is only my second bike. Here is the breakdown: I gave my first bike to a friend that I was staying with in lieu of rent money so that I could build my second bike. The engine on the first bike was a SkyHawk GT2B 49cc. The first engine on the current bike was a SkyHawk GT5-A 66cc. My friend returned my first bike to me when the seat tube of the frame cracked. The rear mounting bolts had come loose and the engine moving back and forth more than it should is what cracked the seat tube. The 66cc snapped a rear mounting bolt, so the GT2 came off the first bike and went on the second bike. Shortly after that, the rings in the GT2 burnt up. It turned out that I had been using the wrong type of oil (marine 2 cycle oil). I then purchased a 49cc top end and a bottom end with centrifugal clutch from gasbike. I assembled the motor with the help of a buddy, and had nothing but problems. I blew a head gasket, stripped my exhaust studs, and after the 3rd or 4th ride, the one way bearing in the clutch failed, and since it wasnt a dual start, it was done for. I then purchased the engine that I currently have. It is a little stronger than any of the other engines, and ran better and smoother even out of the box. So far, I have considered it to be a great engine, and very reliable. My only concern now is this. I know that all these two strokes break down eventually. Some of them last a very long time, and others not so much. I have no way of knowing how many more miles I will get out of this bike, so I am considering either installing some sort of 4 stroke engine or just replacing my 2 stroke engine every year or so. I constantly debate with myself about how to deal with the issue of long term reliability.