spark plug exploded?
 

Had a very unusual occurrence the other day. I actually blew up a spark plug. It didn't leave the car or anything. We just removed it and the white ceramic insulation completely separated from the metal. The center metal core and tip moved within the threaded shank. Any thoughts on how this could occur? They had about 29k miles and were due to be changed anyway, but I'm wondering if there is something else besides wear & tear that would explode a spark plug?
Wires, cap, rotor and plugs all had about 29k miles on them.
Never had this happen before.

Hot Rod!
 

It could have been over torqued with enough pressure to make the plug have an undetectable crack. Heat and time could have worn it enough that it finally grounded out the side and popped the plug. The correct torque with a torque wrench would be 18 to 22 Ft-lbs or 25 to 30 Nm.

I tend to agree. From the sounds of it, your plug got way too hot. Can you see rings near the base of the ceramic part of the other plugs left?

I always install spark plugs with a torque wrench, and during about a 4 month time frame on my 951 I went through 10-12 spark plugs because this same thing kept happening. After dumping ~$15 on spark plugs at one store I finally went to a different one and the issue disappeared. In my case I think they had been handling them too roughly before selling them. Not sure how much that applies to you, but there's my story on the same problem :)

Thank you all for the thoughts.
Hadn't thought of something as simple as torque or even how the plugs were handled. The plug other than being separated looked quite normal in it's burning.
I'm going to look a bit more closely at the the other 3 plugs and see what they tell us now that I know I may have a problem.

I went onto the NGK web site and they had a few other ideas, most of which will be quite time consuming at best and probably more expensive to resolve.
Here's the link if any one is interested.

This is the problem page.
http://www.ngksparkplugs.com/tech_support/spark_plugs/p5.asp?mode=nml

This is the beginning of the write up.
http://www.ngksparkplugs.com/tech_support/spark_plugs/index.asp?mode=nml

the same thing happens to my dad's benz. but at first they were the cheap auto zone plugs then we changed them to the Bosch, now instead of 4 going bad it's down to one. I think that cylinder has a burnt valve now :eek::(