Rear Sprocket Nuts

[pekingduck]

As Ben says above, they are 12mm. And they have a stainless locking spring built in that means they can be re-used, unlike Nyloc, which deform and then don't grip anymore.

Since you are replacing them, you could powdercoat them also, as long as the threads and mating face are covered, as by threading them onto a bolt. The powdercoat should hold up as long as you use a 6-point socket, and tighten by hand. When I want to protect a bolt head's appearance, I have a piece of paper or rag between the socket and the head.