I just noticed something diffrent about the bolts on my z6 head. The bolt in the picture is smaller than the rest. Is this correct? Or did the previous owner swap it?

 

That's not the head. It's the valve cover. Remove the bolt and take a picture of it. It doesn't look like any ordinary bolt.

 

looks like someone replaced it with a 8mm valve cover bolt from something else that had the same thread n pitch as the 10mm. or someone has stripped and heli coiled a smaller thread size.... that is a valve cover bolt... take a pic of it ... im pretty sure u go a junkyard u can get one the same as your other ones. as long as they didnt heli coil or anything...