im having complete hell with this motor. i tore the motor down to replace crank bearings. the motor was running before this but when it got warmed up and if for any reason it died, it wouldnt like to start back up. got directed to the seals. i grabbed the end of the crank and wiggled it up and down. good indication of bad bearings. after i got all that fixed i put the same piston and cylinder (bored 1 over) to take it up to a 73mm bore. motor fired off for about a second and died. never could get it to fire again. checked compression and only had 60 psi!!. cant see anything wrong with either the piston or the cylinder (almost new piston put it in last year and only rode it maybe 3 times). i have a stock cylinder and piston as a backup so i put it on. pulled the rope and it fires off for about half a second. just enough to get your hopes up. checked the compression and was only getting 90 psi. the cylinder was pulled from a running motor and put in my shop for storage the piston was ran for 1 season before putting the overbore setup on. so i guess what im getting to is, is there anything besides bad rings or cylinder that would make the compression fail? im at my wits end and i dont want to bore out bigger if i dont have to.

 

90 psi is way low something sounds fishy about the top end rebuild and you said you replaced the crank bearings and seals. hmmmm

 

Could be an error on your part or check the head gasket.

 

everything is torqued to the correct specs. both of the jugs and pistons were pulled from running motors. both only have 1 season of run time. brand new head gasket on the std jug. didnt replace the one on the bored one because it was running before hand. the only thing i can think of is take the 1 over piston/rings and the std jug to a machine shop and get them to machine the jug to the piston.

 

why not get the jug done and put a new piston and rings and you shouldnt have any problems