I often worry the same....

So I take a wooden mallet and beat the ---- out of the manifold. Stuff DOES fall out.

 

TOILET SNAKE

 

this actually does work. i do it sometimes to get little beads that fell in and got stuck out.

 

Sand blast it out.

 

nice mani

 

take it to a car wash and blast it

 

You can get a flexable adapter for a die grinder and feed it through the runners, sort of like a tiolet snake. A 60 grit flat wheel and go to town.

 

This would be my idea!

The mani is now welded and corrected with tig welder, just to be safe + is getting the wastegate port welded and flanges are being resurfaced. Pics coming soon.
Then I will clean the inside and it should be done

 

Should have removed the excess as you went along welding the pieces together. Now, I think the snake and mallet ideas would be your best option. If it's still there after that, I doubt it will come loose.

 

i know it aint cheap but you could take it to a place that does extrude honing