Hi Don,
  I had that happen on my 79 bus. It was quite cold if I rememeber.
Luckily it was a Saturday morning and I wasn't going anywhere in
particular.  I didn't have the luxury to drill and tap so I replaced it
with a new stock plug. I didn't have any problems after that. Might be a
good idea to put some lock tite or high tack on the plug before
reinstalling. I also gave the center a good tap once it was in place to
make sure the outside edge was seated. I was a little nervous abnout the
plug because it was only a interference fit but it stayed in place. Good
luck.  Take care.
--

                          -Jim DiGennaro
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