When the butterfly (part 12) screws far enough in to part 11 to touch the back of the tube (part 10) it pulls part 11 forward. The larger diameter section of part 11 then binds on the inside of the main cross tube (part 3), holding the screen in position. Making the larger diameter section of part 11 longer adds greater friction area. Well, that's what I did and its worked well for many years. It would be advantageous in my mind to make part 11 out of stainless. When left after a good soaking parts 10 and 11 do rust togther a bit.
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