Here next week are my acolytes going to visit Cunmetal City, a lovely place of violence, an abundance of weapons (especially guns for some reason) and all of the lovely molten lava and metal that there now are when you have so manufactoriums on an active volcano and uses its lave as your fuel.

I do know that there is a thing names resistance heat, but looking through my book can I not find any rules myself regarding heat and what an GM should do when you (almost) are walking inside of a volcano.

So are there something that I have missed or are there no rules for this? And does anyone have any advices as to how I can add this element into the game via rolls and what not.

I would probably go with regular Toughness Tests to avoid gaining levels of Fatigue from the heat (i.e. heat stroke), with the Test modified by -[.5 x Armour Value] and frequency of Tests determined by level of activity (every hour of light activity, every half hour of moderate activity, and every minute of strenuous activity). Maybe include locally-made 'Chill Coats'- duster-like coats woven through with refrigeration coils to give a +40 to resist the heat (and they don't work over armour).

I like the sound of that, and I must admit that I already had something like the the "Chill Coats" planned, them meeting an NPC with rime on her armour to show that it is cooled down.

What about drinking water and such? Would it be too much to say that they now should drink a lot more than normally as well as taking their tests?

Logically yes, but you can discard logic in this kind of game. If you think you can make the game more interesting by challenging them with a lack of drinking water, you should. However, as there are countless billions living in gunmetal city, I'm assuming that the Mechanicus has installed some kind of water distribution system... But off course, this system can be broken (in some parts of the city).

Logically yes, but you can discard logic in this kind of game. If you think you can make the game more interesting by challenging them with a lack of drinking water, you should. However, as there are countless billions living in gunmetal city, I'm assuming that the Mechanicus has installed some kind of water distribution system... But off course, this system can be broken (in some parts of the city).

and surely used to cool down the manufactoriumthan to be wasted as drinking water...

The water issue could actually be a serious point about the city... does it go for drinking before or after manufacturing? What effect does this have on the population? Mysterious diseases and so on could be a result of this, although this may not be the focus of your adventure it could be an interesting side-note to be explored later.