My first china girl finally worked excellent after much advice from the forum, and lots of tinkering, although it did vibrate a lot. After 600 miles though it seized up solid at about 20mph bringing me to an abrupt stop. I blamed this on an incorrect break in, in which I had made a mistake with fuel mixtures. I had bought 2 more motors however, planning on 2 more builds, so I just swapped out for one of the new ones. I followed proper break in procedures. The new motor worked excellent from the get go. I rode to work daily about 20 miles each way almost nonstop on a rural road at aprox. 20-25 mph. After about 4& 1/2 weeks , over aprox. 2 days I noticed a significant loss of power to the point where it would not climb a small hill. I suspected a loss of compression. When I removed the head I discovered a not so small groove scored in the cylinder wall as well as small pits in the head. Even a small piece of metal embedded there. It seems that something in the bottom end has disintegrated. Is there any thing I can do ? Are there better quality parts to rebuild with ? Is there a better quality engine to start with ? Maybe 4 stroke would be a better choice ?