When I installed a backflow preventer on my hose bib, I did not tighten the set screw so it can easily be removed. That screw has no functional purpose other than to try to prevent removal of the backflow preventer. If it fails, you need to either replace the entire valve or start drilling! I know that many homeowners have no idea that such a device is required by code.
Not sure why code can dictate such things but not require hose bib manufacturers to make their products with built in backflow preventers. That's probably coming...
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