I purchased a non running, 78 280z from an estate.  The EGR had been removed and a block-off plate had been installed.  I put a solid fitting into the exhaust header to seal it off.  Under the area where the block-off plate sets, there is a threaded hole.  Originally it was fitted with the pipe that went to the header.  Do I need fill that threaded hole with a plug?

The block-off plate on the top is supposed to seal that passage from the one that connects to the intake manifold passage.  But sometimes they rust at the sealing surface or between the exhaust passage and the intake passage.  Shine a light down there and see if it's solid.  Check the top sealing surface.

Or just put a plug in it just in case.

Zed Head, The block-off plate is shinny and new looking.  The area where it sets on the intake is clean with no rust. I'm reluctant to remove it because I'm sure I will destroy the gasket.  Sure I can make a new one but it all looks very neat and tidy.  I have the reciept for the block-off plate.  It is from Zcardepot and was dated less the two years ago.  So if I understand what you have told me, just that plate and the plug installed in the exhaust header will do the trick?  Thank you for your help!

  

It should do the trick if the internal wall is intact.  If it was in my garage I might rig up a hose to that hole and blow on it to see if it's sealed.  A simple way to tell, I think.

Zed Head, Thanks. Good easy idea!