That's a pretty thick plate, I'm gonna guess two things:) One is he counter bored the holes and two is that the bolts go up into the light as opposed to a threaded studd hanging down from the lights, so all he has to counterbore is a nut head. Or am I the nuthead? Either way it is SUPER clean looking. I've been debating the whole lightbar thing and also as an option for my signals, and I REALLY miss the machine shop at my old job right now... That looks great.
