I have my 17hp Briggs 31C707 all torn down after running it with a knock for a season. The knocking started recently after the head gasket was replaced last June and I have been running it since then. Its not used for mowing however it has seen heavy abuse from the snowblower attachment moving wet snow. It has never been low on oil and has been running on 5w-30 all winter as the manual suggests. Not sure where the knocking is coming from but it sounds like it could be the counterweight slapping around. What should I be looking for to locate the knock? I noticed the piston skirt and cylinder wall has a fair amount of light scoring; in addition the sump cover has scoring where I believe the counterweight rides. Is this scoring something to be concerned about?