I am restoring a 1986 Dodge D150 with the 225 slant six engine. I purchased a NOS Chrysler 225 exhaust manifold. It has a pipe fitting hole on the right side of the heat riser that I believe is for an O2 sensor. My O2 sensor is further down the exhaust system. What kind of a plug do I need to plug this hole? Is it a 1/2 in NPT fitting or something else?

Move your O2 sensor to the manifold; you'll have better driveability and economy that way. And put in a new one -- not a Bosch; use an NTK, they're the best -- while you're at it.

You can leave the old O2 sensor in the lower location as an effective plug.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

IIRC O2 sensor thread is unique. There are companies out there that make plugs but you won't find one at a parts store... I would do as dan says and leave the old one in as a plug. He is also right about ntk vs bosch, you will find that same statement on many automotive forums from people in the know.

_________________
72 dart acquired 12/08 /6,holley2b, mopar perf intake, mopar perf header, cam, built motor,904 finally re-installed, 8.75....still needs sorting out.
99 dakota 318 5 speed reg cab picked up 9/09; Trans finally fixed! woohoo!