Scubamon

I received a set of these headphones and when screwed into the back of the equinox 800 I noticed a gap between the silver knurled knob and the base of the mount. It is tight and one thread is still showing. This could permit water to go down the threads. Should there be a washer in there or is this normal? Did not want to overtighten or submerse without knowing. Thanks for any help.

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