Removing broken head studs from block

[alexiskai]

Originally Posted by Joe K

Put in to the "neck" of the thread is my usual discovery. This would be the two or three threads at the top of the threaded portion where the thread is not fully formed. The thread at that portion "jams" in the correct size hole, and causes an "interference fit" between the stud and block.

This is an interesting idea, can you clarify? The problem is that the top threads in the stud bore in the block are not properly formed? What exactly is jamming where?