3/8-16 which is 3/8 NC.

 

Did you put the extension and packing on yours. Camaro Central said with their exhaust it only needs a flat gasket.

 

Did you put the extension and packing on yours. Camaro Central said with their exhaust it only needs a flat gasket.
View attachment 337381

That insert is necessary to align the donut and piping for a seamless assembly line installation. My reproduction system had it as well. Use anti-seize on the threads and torque to spec. I have found you may have to recheck the nuts after a few cycles.

 

Exhaust manifold nuts are prevailing torque style locknuts. They are deformed or crimped to stay tight under many heat cycles. See the deformation in the picture beow.

You might want to swap in some brass nuts to make disassembly easier in the future. They work well too.

 

Exhaust manifold nuts are prevailing torque style locknuts. They are deformed or crimped to stay tight under many heat cycles. See the deformation in the picture beow.

You might want to swap in some brass nuts to make disassembly easier in the future. They work well too. View attachment 337426

I wound up using brass lock nuts