Do yourself a favor and use a 1/4" NPT tap to thread those holes, then use 1/4" NPT steel Allen plugs in them. Brass or aluminum will melt out eventually if any exhaust gas at all gets to them. Don't ask me how I know this!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!