z

Yep, heads are pretty common on a Z. This is primarily due to the different metals used. For instance, the engine block is iron. The head is aluminum. Due to the different expansion/contractraction characteristics, your head can warp.

Heads can also get warped because they weren't installed correctly. There is a certain pattern to tighten the head bolts. If this pattern isn't followed correctly, the head can be tweaked out of shape.

A head can also only be milled so many times before it's no good.