I tried adding a helmet to gelebor's outfit in the CK to cover his face but when i finally met him in game, his head is invisible, anyone know how to fix this and why it happen?

It once happened - in the vanilla game - with some of the masks. It also happens if you add a weird piece of clothing (like the draugur stuff) to a normal NPC.

So, my guess/assumption is that all NPCs cannot wear all outfits ... Have you tried with a different helmet?

(also; idk if the f4 thing has an effect here ... try it, just to be sure ... but I don't think so?)

Thanks man i'll probably try some other helmet and see if it has the same effect.