I tighten without tying, Hold things together without glue. I circle you daily, Yet I’m not alive. Who am I?

Put your hands on me just right, Twist and turn and shut me tight. You can polish me all night. If you're having trouble thinking and your hearts a flutter, it's not me it's you, your mind is in the gutter.

I circle kings but wear no crown, A silent guard that won’t back down. Not wall nor gate, yet I defend, What am I that rings the end?

Add a k and I'm a clown, remove the y, and I am a noun. I have no breath, but I bleed red, my color changes in a threesome with two other elemental variables. What happens to nails, the adjective, that can cause tetanus if you step on one, afflicted with this?

As a verb, powerful undead might do this to your energy. As a noun, you likely have two, but maybe three or even four in your main bathroom. Make it an adjective with ing and it may be how this riddle affects you.