What he said, but especially the disc type core plugs are darn near
impossible to seat correctly if you don't have a straight shot at them.
 The cup type ones are better (used on most cars since the 60's) but
still not a piece of cake.  You can get expanding type ones that you
tighten with a nut, Dorman makes metal ones, they are a lifesaver for
tight spaces where you otherwise wouldn't be able to do them in the car.
 I've used the rubber ones before as well but they aren't as elegant.

At this point you might as well just put in a new plug and hope for the
best, but don't be surprised if there's collateral damage.  Keep an eye
on your coolant and oil for a while to make sure there's no
cross-pollination going on.

nate

.

As a "get you by" you can get a rubber freeze plug to replace it
temporarily.  You put the rubber plug in and tighten the bolt that
runs through the middle of it and it squeezes the plug to expand it
and make a good seal.

I would put that in first to see get you running again and see if
there is any more damage before taking more drastic measures to fix it
only to find that the block or head is cracked.

            Steve B.