I purchased my 1990 ProStar 190 just over a year ago. I discovered similarly corroded threads, as shown in the picture posted above by pbgbottle, in the exhaust manifold and engine block drain holes when I winterized it in the fall of 2010 so I switched to brass drain plugs for both locations. I did not have any problem getting the brass plugs out when I winterized the boat today, but I did find it difficult to thread the plugs back into the drain holes. Seems like the drain plugs wouldn’t hardly even turn a full revolution before they snugged up today, which is what I experienced when I first used the brass plugs last year as well.
Has anyone ever tried using a tap to chase the threads in those drain holes? Did it improve the situation?