The header on my OS... is a relatively heavy guage (i belive stainless steel) tube Its thick enough walled that the header was easy to tap for threads. A bit of high temp Permatex (the stuff that hardens like a rock) and... the nipple is permanent.

Note... I already had a spare header pipe when I drilled and tapped one... so if it didn't work, I wasn't in trouble.

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Note also... this is NOT one of the thin wall header tubs associated with tuned pipes... DON'T try tapping into one of those.