As I was in the process of pulling the E31 head on my ’71 240z I have been inspecting every part as it comes off.  The engine was rebuilt by the previous owner to stock specs but I have no paper work on it.  The reason for pulling the head is that I cannot get the intake vale #4 in to adjustment.  As I was inspecting the exhaust manifold there is a fair amount of rock hard clay like material on the inside of the manifold.  There is also traces of it in the exhaust pipe but not as built up.  Can someone tell me what this is from?  It appears it would come off rather easily but I have tried chiseling some of it off with a flat head screw driver but it is not budging.  Should I be worried?  Does it warrant buying a new header?

 

 

 

 

Edited May 3, 201511 yr by Hardway

Looks like ceramic muffler patch compound. The metal between the sides corrodes. I plugged an EGR port with that stuff.

Ash from combustion.

That looks similar to the crud around my EGR failure. It was a huge vacuum leak.

Ditto what Blue said.

From a book on performance tuning:

 

 

I think it initially comes from oil/carbon deposits that were never removed and eventually cooked over the years.  It can initially be deposited as oil from leaking valve seals or passing the rings.  I see similar when I clean the tubes in my oil fired furnace.

Thanks for the info guys.  Since it is on the exhaust side I don't think it is hurting anything.  I may take another stab at trying to get some of it off tonight.

I saw the same buildup in my manifold. Was curious as well.