I'm still perplexed by the "automotive" vs. "aircraft" plug debate.  I've heard that automotive plugs are bad BECAUSE they're cheap.  They're cheap because they're built in large volume.  They don't work because they're "designed for cars."  I wonder if the plug knows that.  They don't work because automotive engines run at a constant temperature - but air-cooled automotive engines use the very same plug designs as water-cooled automotive engines.  A real disadvantage of automotive plugs is that they have the wrong thread diameter, requiring an adaptor, which reduces the heat transfer to the head.  But then, as George implied - it's all about heat range.  Very true, but automotive plugs are available in all kinds of heat ranges including very cold ones.  And aircraft engines use leaded fuel - yes, a big, maybe THE difference, but that just lets the plug last longer, it doesn't change it's performance.  You can clean aircraft plugs - but you can also clean automotive plugs; it's just cheaper to buy new ones and a cleaned used plug is not as good as a new one.  The reason we clean aircraft plugs is that it is cheaper than buying new ones - not better.  So I could contradict an earlier remark by saying I'm taking care of my engine better by buying a new set of $1.67 automotive plugs every 100 hours than by cleaning my $25 aircraft plugs.  I'll guarantee you that the ceramic material is the same.  The sealing method (not really related to pre-ignition, etc) is probably better in the automotive plug.  The electrodes are of the same material.  Rare-earth electrodes are available in both types and they do help reduce firing voltages and increase life.  But if you are going to throw away the automotive plug early anyway, the extra life doesn't get realized.  And yes, as George said, there has probably been very little (or no) instrumentation of automotive plugs in aircraft engines.  There has probably also been very little instrumentation of aircraft plugs in aircraft engines, except for the recent most excellent work done by George and his crew.  On the other hand, visually "reading" plugs has been very successful over the last 100 years.  I look at the characteristic of my automotive and aircraft plugs run in the same cylinder and they look essentially the same - are my automotive plugs about to give up the ghost?  I still don't know, but so far I haven't heard any argument that convinces me they will.  There are two options that would be better, at least in my opinion, than running automotive plugs:  If a good adaptation could be made between the automotive coil(oops, mentioning an automotive coil might start a new thread!) and an aircraft plug I would go that route.  Also, if automotive plugs were easily available with an 18mm thread that would be a good way to go.  Incidentally, the plugs I have seen with a design that most closely resembles an aircraft plugs were used in pre-WW II farm tractor engines.  Are we flying with tractor plugs?  :-)