New run Kato NW-2.

[u18b]

Would trimming the post on the exhaust stack (inserted into the shell) to be flush with the inner shell surface solve the problem? You might also have to glue it to the shell so it doesn't pop out.

Yes.  That's what I did.
Covered here in this comprehensive repair doc in case anyone can use it.

https://u18b.com/wp-content/uploads/2024/04/NW2-Manual-Kato-pdf.pdf