I am trying to remove the threaded oil galley plug that is one of the three hex head plugs on the left side of the engine to install an oil pressure gauge.  The plugs have a hex or Allen head that is loose with a 1/4" Allen and 5/16" will not go in. It stripped the 1/4" one as I was trying to unscrew it.  It appears that the engine builder used brass plugs.  I want to get this out without drilling as I am worried about getting brass filings in the oil passages and eventually damaging the new engine. Any thoughts?