Originally Posted by BargainFittings
Don't try to force it to thread as you will eat up the male threads and jam it on after a couple turns.
If you have a vise you can grip the edge of the washer and use a file on it to open it up enough like you said. A drill with a grinding stone or burr bit would work too but it will eat up the stone fairly quick.
What size washer do you supply with your weldless sightglass kit?