CMA Steam/water Gasket Holder
You will need a very thin 16mm open-end wrench to remove this (which you shouldn't really ever have to do), and then get "creative" with how to hold it still while unthreading the W_168. Clamping it in between two pieces of wood in a bench vice is about the only way. There really should be no reason to replace these, unless the stud for the gasket seat fixing nut snaps off or you need to replace the entire valve shaft. If that is the case, one may as well just get a complete valve when you consider the time and effort compared to the cost.