[quote=@Wraithblade6]
[@Klomster] Awesome. Maybe you could tell me stuff a techmarine would know/do. I'm only partly knowledgeable. 

But yes, Your explosion was explicitly ginormous, so NOBODY should be ignoring it [s]unless they are not reading your amazing posts. [/s]
[/quote]

So what a frater astrotechnicus would know. More commonly known as and referred to as techmarines.

They are basically on the same knowledge level as an enginseer, meaning their knowledge is focused on battlefield repairs and equipment care.
So a generic techmarine, as in not one who's done that job a long time would mostly know how to keep the motor pool of the chapter running. Being able to repair all expected vehicles of the astartes.
Being imperial vehicles, they are however very similar in construction to other imperial vehicles. So a techmarine could manage a chimera without much trouble because of his great education and often intelligence. (Being space marines and all that.)

Other things like thunderhawks and drop pods are also expected to be kept in working order by techmarines and his serfs.
In the field however, serfs are far too weedy (often more badass than guardsmen though) to survive the combat conditions the marines are expected to face.
So in the field they mostly walk alone, rarely servitors join them.

Being physically more impressive than regular enginseers, they are more fit for battlefield duty, while a techpriest would struggle to reattach a track on a rhino, the techmarine would do it with ease.

Where techmarines lack however, is their half-half way of being both a space marine and in the cult mechanicus. Since they are still very loyal to their chapter, the mechanicum is apprehensive with giving out their deeper secrets.
Therefore compared to higher ranking techpriests, techmarines will have a far lesser knowledge of the secrets of the machine god. However, being cut off from most mechanicus in their chapter, and being forced to make do, they often figure out things by practicality.

Higher ranking techmarines, or in chapters with a crafting history. They begin to learn how to create their own armaments themselves.
Making impressive power armours (some even make artificer armours) and the weapons of the astartes.
Supplementing the deliveries most chapters get from the mechanicus, and making sure their chapter gets hold of specialized equipment they want.

So in short.
Is it a civilian day to day machinery, normal factories or a marine equipment. The techmarine will have perfect knowledge in it.
If it's some sort of larger more complex factory, making something like basilisks, the techmarine would need some time to figure out this particular thing.
And lastly, if it's something massively complex like a factory cathedral or a titan. They have little to no clue. Such secrets not (normally) given out to techmarines.

I'd say most of the things on the imperial parts of the hulk (including past chaos things, they are imperial in design) is something any techmarine can figure out without much trouble.
If you decide to go about and start mucking with the generators and other ship systems, i'd say tread carefully, and it will probably work out.
Complex things like modifying gellar fields is purely guessing.

Also, is it more complex than turning it on and off, that usually requires more experience.
So in the above examples, all the examples have been the techmarine repairing and/or modifying the mentioned things.
Techmarines are good, but a techpriest of similar rank will have more knowledge. Which many techmarines compensate for with practical tricks and knowhow.

For example, my character.
Before the fall, he was just a lowly mech-wright, a techmarines knowledge would normally far surpass his. Mech-wrights being focused on repairing structural damage and operating damage control equipment. Complex machinery being way out of their league.

Why my character is such an crazy techpriest, is that he's gotten knowledge from Tzeentch. So he has more knowledge than mortals are supposed to have.
So i guess he's equivalent to some arch-magos, or perhaps even an arch-magos veneratus.
Regardless of what, my character is not a normal techpriest.