I have discovered that the life of the iridium plug seems to be dependent upon the porcelain.  I had several (three to be exact) when over 500 hours, that I would discover a mag drop that traced to a individual plug which had a cracked insulator. No other indications anywhere of detonation/mechanical/dropped plugs during its history.  Can it be that the porcelain becomes more brittle or soft or???  Any ideas??
I have replaced the iridium's several times now.  By the way, never do the new plugs show up a cracked insulator.

During their 500 hours the gap never changed and only rarely would I have to take bromide deposits out of the bottom of plug.  Thanks to running LOP.

FWIW

Charlie K.