Are those valve covers cast aluminium?
The reason I ask is that if they are cast aluminium with a high silicon alloy the chances of them anodizing well are not very good.
Aluminium comes in differing alloys depending upon the purpose. Some alloys are designed for high strength and or machinability, while others are designed for corrosion resistance. Generally speaking alloys used for castings have a high silicon content. The silicon content allows the aluminum to flow better. The downside of a high silicon alloy is that it tends to be very porous. This porosity holds onto the sulfuric acid used in the anodizing process, unfortunately during the sealing process the sulfuric acid can leach streaking the surface and marring the finish.
All that being said, I've seen beautifully polished pieces that after anodizing looked horrible, I've also seen beautifully polished parts that came out beautiful. Your mileage may vary, I wish you luck.
If you have them anodized and they don't come out well, strip the anodize off, repolish and then clear coat them with a clear lacquer. The lacquer will eventually yellow, but I've found that it holds up pretty well.
Here's a couple of articles that can help you make an informed decision.
http://www.anodizing.org/Publications/bulletin_4.html
http://www.anodizing.org/FAQ/faq_cont2.html
Regards,
Joe Dokes
