The International Cessna^® 170 Association

Continental O-200 here
replaced cylinders
ran engine about 4 hours
oil keeps fouling up plugs
detonations when leaning mixture
bore scope shows oil squeezing out between cylinder walls and pistons
piston rings are correct according to mechanic.

What could be the problem?

Rings aren't seated. You shouldn't be leaning. Run it at 75%+ power.

I am running in a top on my O300. At about 4 hours and 20 minutes I had a 60 or so degree drop in CHT. The CHT's were then back to where I was accustomed to seeing them. Oil consumption is still high but my mechanic said to keep running it at 75%+ and at full mixture. I'm at 75 hours on the top and oil consumption is starting to slowly go down. The good news? Lots of power!