When the plugs "blow out", I'm assuming the threads on the spark plugs stay intact, and the threads in the aluminum are stripped out? You'll rarely find rolled threads on an internal tread, so that would indicate that the machined/tapped threads in the aluminum heads are where the problem lies, since they are the parts that are failing, not the plugs themselves... no? (bolts with rolled threads are screwed into nuts/holes with machined threads all day long with no failures).
