Cylinder Chrome

Hanging my head in shame with such a basic question but the two arrowed chromed "discs" on the cylinder housing are badly pitted on my Thunderbird 1600. Are they just decorative or do they serve a deep mechanical purpose? If I wanted to renovate them, would I need to take off the cylinder heads or do the "discs" unscrew/prise off? Engine mechanics are a dark art to me, unbolting something usually involves a lot of swearing & so I would probably mask and spray them (black) in place....

 

It's weird since the discs seem to be made of plastic not metal. It is just cosmetic but with those long winter months of just looking in awe at the bike, I'll get itchy fingers to sort it.

 

Just to finish this off, took heed of DD, I wandered in and spoke to the good folk at Mitchells of Inverness who identified the "discs" as being part of the gasket set (£30 plus) for the cylinders. They are "T" shaped internally, held in place by the cylinder head (glad I stopped trying to lever them off with a screwdriver...oops).