What nut?

[TRD_ZERO]

What size nut do i weld onto my exhaust for a oxygen sensor? For tuning purposes only.

[touge_ae101]

get the oxygen sensor and measure it? if you are using it for tuning you will need a wideband oxy sensor which sometimes come in diff sizes anyway.

[Malcolm]

The standard size is M18x1.5

[strx7]

The standard size is M18x1.5

yip, same as spark plug thread (cause they come in handy for blocking the hole when not being used)

[Malcolm]

and making flamethrowers. Wouldn't think you'd see many modern engines with those big 'ol sparkplugs in them though, I'd think most would be the smaller (m14?) plugs.