Did you replace the little rubber washer on the drain plug? For that matter if that washer is missing it could make the threads over tighten due to metal to metal. What I have found to work for me best is to replace the washer, both upper and lower every time I change my lower unit lube, sometimes 2X a year depending on use. They only cost about .25 apiece and is cheap insurance to make sure no water gets in the LU.