So I'm leak testing my new engine. My leak testing rig is working great and is air tight. Almost 100% my engine is air tight ***BUT*** I can't quite get a good seal on the exhaust side. Not willing to settle for mostly air tight maybe air tight or even probably air tight. I need to see her hold pressure before I proceed.

I currently have a rubber bath plug that's flat on the bottom being pressed down by a flat piece of metal that goes on the exhaust studs and gets tightened down.

I tried grease, it helped but I could hear little grease bubbles. Tried an O Ring under the plug but that didn't quite work either.

Here's the question. Do most people fit some sort of bung or just use a piece of rubber? If bung, how do you get one that's oval shaped for the port? If rubber, how do you hold it down and what type of rubber i.e. thickness etc. ?

I found plenty of threads of people describing their rigs, but no one really went into how they block the exhaust port.

Thanks

Clancy