Bike part

[JohnTheBros]

I'm looking for the name of this. Anyone knows??

 

[DCLane]

Caliper brake recessed nut, also known as a brake bolt?

That's if it's for a brake caliper.

 

[wisdom]

If it's not for a brake calliper and it has a hex head it could be a seat post clamp bolt as used on older bikes it should have a mating part with a ridge on it to stop it turning.

 

[raleighnut]

It's actually called a 'Sleeve Nut' and is closely related to a spoke nipple.