I bought some brand new Edelbrock heads and the spark plug hole threads are great on the outer 4 but the inner 4 won't even start. What is the best way to fix this before I install them. I got them on Ebay so the manufacturer is out of the question. Strange that the outside ones are able to be tighten all the way down with my fingers but appear stripped on the other 4.

 

clean the threads with a tap

 

14mm spark plug tap....

 

I'd use a thread chaser rather than a tap.

 

Take this early opportunity to learn that you made a mistake in buying the Edelbrock heads. They are not any good. Best case scenario they could be called stock replacement.

Get rid of them before you make a bigger mistake by bolting them on.