Spark plugs

[Tom Nestor]

I just replaced the plugs in the 20vt and noticed that the insulator (the
one at the ignition wire end) was rather discolored from what appears to be
heat where the insulator meets the steel portion of the plug. I have never
seen the ceramic insulator on any plug discolor this way. They are the
FD5POR, have about 40,000 miles on them, not chipped. Is this normal ?

Tom Nestor