So I got a polished valve cover and today I swapped them out. On cover I took off of my car I noticed that on the bottom side on one hole, there was some rust. This concerned me, because none of the others had this problem. What could cause this and is this something I need to take it into the shop for? There was no rust that I could see on the head or the cylinder that goes down where the spark plug was at, so it looked like it was only on the bottom side of the cover.

Normal:





Rust:

 

technically thats not inside the valve cover. were that corrosiuon is (aluminum corodes, it cant rust) is were the valve cover meets the sparkplug tube. if you live in a wet climate mosture can get trapped there and cause the corrosion. i wouldnt worry about it to much, its not going to cause a leak.