engine problem

You "might" be able to just get a new piston and rings and new head and base gaskets. Depends. If you think the motor is toast then take the head and cylinder off and have a look. if the piston looks burned or like the aluminum looks smeared over the rings then the piston is burnt up. Examine the cylinder walls, if their scratched up then a new cylinder is needed.

With a new crank case you might think on getting a new same sized motor and installing it then get into a real good rebuild with the best parts you can get.

http://motorbicycling.com/f13/66cc-engine-tear-down-setting-high-27007.html

This thread is excellent for what to do and what parts are needed. Many times, though, the aftermarket parts from the stock dealers are actually much better that stock. Like the boygofast on eBay exhaust gaskets are the highest quality exhaust gaskets I've seen. Metal cored with real exhaust gasket material. I used up a Dremel sanding drum grinding it to fit the port and that's a good thing. The stock gasket crumbles to bits if you remove the pipe after it has run a while.

With a motor on the bike and one to rebuild you have a safety margin. At least I felt better when I was raceing to have a motor in the car and another wrapped in plastic waiting to be swapped in when needed, sooo easy to do with a China motor as inexpensive as they are

If you plan on rebuilding it...I would contact Frank Bowman and get a custom Frank Bowman ring made for it. I convert weedwacker engines into RC aircraft engines and have used Franks rings in other smaller RC 2 strokes as well as my wacker conversions. His rings are the best and he can make rings for almost anything and he is really reasonable on price. Definately worth checking into if you plan to rebuild. Here is Franks email
[email protected]

What kind of engine are we talking about? What carb?

Just having a 40T shouldn't hurt anything. Actually that should have helped, because it lowers your revs.

peppers said:

on flat ground yes but having the smaller sprocket up hill makes it have to pull harder though right?

Click to expand...

That is correct.
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If your friend was riding at full WOT for a good distance while the engine was still breaking in, that is what caused the failure.
Riding at WOT while an engine is breaking in is actually a very good thing when done in short 5 to 10 second bursts, then running it much slower for a few minutes to let things cool back down. It allows the piston rings to break in properly.
The cool down part is extremely important though because if you don't allow that, you end up with what you have now.