One or two of mine did the same thing. I left some threads in my old system.

I used WD-40 and no heat. Mine were loose for half a turn, and then they would just lock up. I worked them back and forth until they were willing to come out. As you know they carbon and rust up in there, and you need to loosen all of that junk to get the threads to turn.

I had a heck of a time getting the half-thread one back into my Dynatechs, but it went...

 

I guess GM doesn't put anti-sieze on them at the factory. Those suckers aren't cheap.