Originally Posted by PERRIN_Jarid
Factory exhaust manifold yes? If so there is much more material there to drill thru than an aftermarket stainless header.
There's a little raised up casting button on the stock driver's side portion of the manifold that most people use for tapping on the stock headers for an EGT gauge. There's plenty of material for tapping and threading in an EGT probe and I did this on my old WRX and it work great. I'm sure a welded bung would be ideal but, it's not necessary.