It's definitely NOT 1/4" NPT, it's definitely not 1/2-13...does anyone know the size of the threads in a Mercruiser big block and exhaust manifold? I have one that is kind of buggered and needs chased with a tap as I have new blue plugs to put in, all the others went in fine.

Thanks in advance!

Be more specific where these plugs are, and if adapter or brass/plastic oringed wing nut or ? thanks .

Survey says groogle

The thread of the blue plug is 1/2-13.

I didn’t read the title.
yes, merc blue oring plugs are 1/2-13.
So aren’t the ss replacements

https://www.hardin-marine.com/p-8108...74-5-pack.aspx

Hardin Marine SS ones are exceptional. Use a little anti-seize on them.

The Merc ones lately are so brittle. Maybe get a season out of them.

If you need either block adapter for blue plug or exh manifold elbow with blue pkug, they aren’t too expensive.

Interesting. I can't get my plugs to screw into a 1/2-13 nut but they go fine in a metric 14mm - 2.0 nut, like...perfect.

Sometimes with cheap pos manufacturing nowadays, you just gotta do what you gotta do. Unfortunately. Merc can't make sbc manifolds and risers line up, so I'm sure their blue plugs aren't all that exact.
I replace several a year with the boats I work on. I don't have any real problem with them until they just break while the boats under way. Old and brittle I guess. Whenever I remove them I clean off the thrads and squirt some wd40 or silicone or whatever is in he boat with me, therefore I don't have to tighten them much, which keeps them from snapping off.

I forgot to mention, last time I got the Hardin Marine SS plugs, I got them from Michigan Motorz because their shipping is fair, Hardin MArine / CP Performance usually not.
Here's the link per piece:
https://www.michiganmotorz.com/stain...ugs-mercruiser

Wow, thanks...I have tried to get these in the past only to be told they were sold out and on backorder...just ordered (5), so yeah...cool and thanks again.