We pulled the plugs and squirted a small quantity of motor oil into each cylinder. On a flathead, this oil reaches both the cylinders and valves. On engines with overhead valves, engine-machinist Bob Cherveny of B.C. Engineering recommends popping the valve covers and using carb cleaner to zap any sludge from the rockers and valve guides. Then he says to apply Marvel Mystery Oil to the moving parts (and down the plug holes, too) and let it soak in to loosen sludge, rust, and ring corrosion. Don't get in a hurry to fire the mill, he cautions.