I i had to disconnect my downpipe from the manifold but now I cannot get it to seal completely. I tightened the 13mm nuts as much as I think I can,(didn't want to break them), but its still leaking.

Is it mandatory to change the whole bolt and nut to get it sealed properly? And the bolts are welded into the flange so would need to be drilled out from up top?

Thinking to take to a shop tomm so wanted to know if this was feasible to do myself or not.