I think the top housing on most valves of cast construction is called "bonnet". It will be topologically equivalent to the "arcade" in this drawing, although it does not look like a bonnet anymore. It's like overgrown bonnet on steroids.

These names, including "arcade" are chiefly historical. Arcade originally meant something like this:

http://www.jc-valves.com/ignitionweb/data/boutique/106/thumb... valve-sp.gif

The most similar instance of "spring bonnet" I could find is shown in Fig. 1 here:

http://www.valve-world.net/srv/ShowPage.aspx?pageID=640

In more complex designs, it is called "spring module", or "spring chamber", as kashew suggests. The cast part is then called "spring module housing".

I think if you say "spring bonnet" you will be understood, however deformed this part has become in comparison to its original bonnet shape. It's not exactly an arcade either. Things evolve.