Thanks, guys. In a desperation attempt at addressing the issue, Teflon tape was tried, which improved the situation, but coolant still wicks out around the threads.
And don?t think about cross threading... the plug screws in easily by hand for quite a few rotations before a wrench is needed to snug it up.
Two of us in our EAA chapter are at the same stage building iS?. One of us had no issue with just the 567 but the other still has the problem after trying the three solutions described. We?re really trying to find out if any others have had a similar experience.