What monster sized allen wrench do I need to get the plugs out of a merc factory exhaust manifolds? Where do I get them? Biggest I saw at the store was 3/8". These things are index finger sized.

1/2 in if square, I belive 7/16 hex if I remember right, but very hard to get out if been in a while you will need to use heat to get them out!!!!

The plastic ones that leak when the manifold rusts and the scale makes the threads no longer seal? I had to drill the centers of mine out, then used a screw driver to pry the rest of the plug out. Then ran a tap back thru to clean up the threads. The new plugs are brass.

They're allen head metal plugs. I'll try a 7/16" hex.