Is it possible for ye tech-priests to make thing? Like if he's stuck in a warehouse, can he build a lasgun from the spare parts in the crates? Is there a written up rule for this in a published book?

I think there are some rules in IH.

In DH to make Arms and Armor you need the Trade (Armorer) skill. If that is true for a Techmarine, I don't know. Butgenerally speaking he should be able to build something if he's got the parts to do it.

Letrii said:

This is so. Pages 244 to 248.

Thanks guys, exactly what I need to make my own Good quality cortex implants.

Keep in mind that a task like that would undoubtedly require at least the 2d10 x Months timescale - making a new cortex implant, or even improving a common quality one, is not going to be something the Tech-Priest can complete when he's bored one night on watch duty.

Thank you down time in warp travel!