I took out a set of plugs that may have had around 90K miles on them. There was absolutely no corrosion on the bases of any of them, yet there appeared to be a ring of what looks likes rust around the base of the insulator. I've never seen this before on any car.

Could it be Blow-by? What else might cause those stains?

BTW: After all those miles "and" after running lean for who-knows-how-long due to an intake leak, all eight plugs were still gapped to exactly .040". I really do love Iridiums.

 

It's not blow by it's a corona stain, see the explanation here https://www.google.com/url?sa=t&rct=...KdROYUj23Lfkxw

 

Well, wadiya know...

Always learning am I (Yoda speak).

Thanks Norri - That's perfect.

And with that, into the trash bin the good soldiers shall go. :-)