Spark plug condition question

Iridium NGK plugs fitted 10k miles ago. Left hand cylinder plugs shown, primary on left, secondary on right. Primary condition is spot on. Thoughts re secondary? (Both gaps +/- 0.7mm which is bang on)

Strange that there are carbon deposits on the threads of the right hand plug. This would suggest that there was a flow of combustion gases past the threads. The only way that this would occur is if the plug was not fitted tightly enough as can be seen by the condition of the sealing washer on that plug which is not squashed.

The plug will not operate correctly if it is loose. There wil be a lose of compression on that cylinder. The plug will not operate at the correct temperature because it is not tight into the head. Hence the black carbon at the business end caused b the plug running too cool..

Secondary plugs are always black and fouled,, normal

As above..Standard NGK are just as good on the twin spark.

Condition of left hand secondary plug is a fair bit worse than right hand secondary. Torqued to 20nm as per specs.

Thought I would try the iridium last plug change. Frankly i haven't noticed any meaningful difference