20 minutes of smoke seems excessive for stem seals , unless maybe they are totally gone , but then it seems it would never stop smoking -------- I got a 450 that was just rebuilt , it had a new upper a couple of years ago , the connecting rod which is pressed onto the crankshaft was worn , it allows the piston to wobble in the bore and eat away the piston and the wall of the bore into a out of round situation , so 2 years is actually a long time for a upper job to last with a bad rod , from talking around , it seems if you do a upper job with a bad rod you will get 20-40 hours and it needs to be done again ----take it a part and see whats going on inside