,

[peterman]

You might take a large chamfer bit to the hole and put a slight chamfer on the hole's edge. You could even do that with a ratchet adapter. Then it should be easier to use a tap after that.

[rex]

update- i used an expandable rubber plug. i had to do a little modification to the plug to make it work, but it seems to be holding. hopefully this will keep me on the road while i plan out how to do it the right way (i.e. if i'm pulling the exhaust manifold in order to drill and re-tap, i'm guessing im going to need a bunch of new exhaust nuts/bolts/gaskets/etc). thanks to all

[jeffnhiscars]

I'd watch how that plug responds to heat especially from the exhaust. You might want to find a way to shield it.