Depending on the specific alloy, aluminum begins to melt at a temp. just under 900F to around 1200F. Brasses will melt starting at around 1600F, again, depending on alloy.
I would tap 1/4" pipe threads (7/16" is the right size) and put a (steel) socket-head pipe plug in.
And it seems high-temp. paints always burn off...I've been re-painting the pipes on my motorcycle every couple years since installing in the late 70's.
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