That's a bummer... A stock Harley muff should be blocked in the center with a solid round plate.. You can't see any light through it...
Sounds like your baffle has been knocked out...
I'd demand the money back...
That's going to be way too loud (obnoxious loud), and run like crap...
If you can't get a refund... go to the hardware/auto store and get a freeze plug that just fits and bash it down to the center of the pipe...using a broom handle or something...
Someone here may be able to chime in with the correct size plug...
Anybody?...
PS.. not sliding all the way up, is okay, as long as it seals good with the clamp...
You want a "carriage bolt" for the rail mount... (it has no screwdriver slot.. just a square under the head that will lock into the slot)...
Best luck... (modding is a hassle, but worth it in the end)...
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