Another day, another surprise. I found out that the presta valve on one of my
tubes screws out. In fact, it screws out and the tube loses pressure
appr. one out of ten times when I screw of the little plastic valve cap from
it. I never knew that those cores can be screwed out and it appears that on
all but one of my tubes it can't be screwed out, and are in fact not even
separate pieces but are part of the valve stem.

Let me try to specify. It's not the "core" what screws out but the thing
which holds the core and which itself is screwed into the valve stem.

So when this thingie can be screwed out, is this a "lower" or "higher"
quality indicator.

Also, how can I screw it in so it would never ever screw out again, I tried
to wrap a rubber piece around it and then use a wise grip plier but I am not
sure it worked.

Thanks, Paul