The purpose of a good fit between the pan and the pan cover is to 1. prevent the priming powder from falling out, and 2. to prevent water getting into the priming powder. Keep in mind that light can be seen between two surfaces far too small for water to go thru. Capillary action of water going thru a space requires a space much larger than light going thru. Bottom line is just because you can see light doesn't mean you have a poorly fitted frizzen to pan.
We always try to fit our locks tightly enough for the two issues above, but not necessarily to the point where you can't see light.
One year at Friendship another lock company and I were discussing what we lock makers call the "Friendship Test". Guys at Friendship (and other places) will pick up a lock, put it on half cock, close the frizzen, hold it up to the sky, and judge the overall quality of the lock based only on seeing light or not. The tolerances of everything else can be junk quality, but in their opinion it is a good lock if they cannot see light. As a joke I suggested we make a little projection on the bottom of the pan cover that would defeat the "Friendship Test" no matter how good or bad the fit of the frizzen. I was joking, but the other company took it seriously and did just that. If you really want to test the fit, close the frizzen, fill the pan with water, and see if it leaks out. But, no matter how well the frizzen fits, when I go hunting I always put a little grease of some kind around the edge of the pan just to make sure my priming stays dry.
