2000 Yz125 Weird Noise

[JD Collins]

It ran great up until i had to do a top end, the cylinder looks ok so we put a standard piston in it and rings and it had loads of compression. Then is started making a weird clicking noise up by the head but was still riding fine. Finally i pulled head off and didnt find anything wrong and put it back on and then i had no compression. Got a new ring and the compression was back with the noise. Any suggestions of what to do. It also is really hard to hit the powerband up until its really warm.

 

[jaguar]

make sure the ring locator pins never cross an open port as the piston goes up and down. maybe the piston isn't standard. make sure the exhaust port has beveled edges so the ring won't snag. rings snagging the ports make a weird noise. also piston hitting the head makes a noise. look at the piston top to see if there is any bare metal that isn't coated with carbon. that would be where it is hitting. the piston rises .3-.5mm when its hot due to metal expansion.