I only show 3 bolt holes on this, but you could do all 6 if you want. I felt the more bolts the more difficult it would be to get the fit right. The center circle is what fits through the wheel for extra support and future attachments if you want them. It's not as big as the hole that's actually in the wheel because I wanted to have room to put some rubber or soemthing on there for padding. This piece is optional and may be replaced with a square piece on mine if availability or cost of the round piece is a problem.