Bolts
From what I understand the factory used studs, but I just used bolts. They were 6mm but I don't remember the tread pitch.
When I look at the pics, they look like the same size studs that is being used for the fuel rail to intake manifold. Which is 8 bucks a piece . So 100 bucks for coilpaks studs? Lol
PS
Not each I think it was $3 for all of them.

