remove the header before tapping the hole to 7/16 or helicoil
if you dont get it straight,and mess it up,the head will need to be replaced
to helicoil it you need the correct size drill bit with the helicoil kit
the kit has a tap and coils and the coil install tool
remove header,drill to size,tap hole,install helicoil with some rtv on the outside of the coil,put header back on and then use the stock size bolt with rtv on it..LET THE RTV CURE OVER NIGHT!...add water to rad and run engine and check for leaks
or like said,use a 7/16 tap and use a larger than stock bolt after drilling the header flange for the bigr bolt
