Ron,
Yes the lack of anti sieze is the main reason for them breaking, followed closely by how long the suggested interval to change them.
Wayne,
The threads don't extend into the combustion chamber. Fowled or siezed threads is not the issue and is commonly suggested as the problem. The penetrating
oil is used to break down the carbon build up on the ground electrode shield not to free the threads. You must first just turn the spark plug 1/8 of a turn open very slowly this will allow the penetrating
oil to whick down the threads and end up breaking down the carbon below the threads on the electrode shield...If you look at the TSP it show this in detail...Mac