Since I'm going to be tapping into a big buck part and don't
really have any experience tapping cast iron, I figure I'll
just ask a few stupid questions.

I'm tapping a 1/4 NPT hole into the exhaust section of a
turbocharger. An EGT sensor will be put in there. The part
is cast iron, about 1/4 thick. The directions are to remove
the turbocharger, drill a 7/16 hole, then tap. The part
that screws into the hole has a shoulder which is
supposed to rest against the turbocharger.

Alternately they say I can drill and tap the hole
with the engine running. Once the drill bit goes
goes through, the exhaust pressure is supposed to
push any particles out. When drilling cast iron
they say it is supposed to turn to dust. I'd
be drilling from the top down.

I have to get a tap since I don't have one that size.
I assume HSS will be ok?

It's supposed to be a pain to get the turbocharger out.
But if it was out, I could finish the hole with a
7/16 HSS end mill. That I think, would give me a more
round hole than drilling by hand under the hood.
So I would think it would seal better that way.

Any thoughts?

Wayne D.