For those who don't know, to avoid stripping or cross threading a spark plug when fitting (it's the same principle for any thread really) try to start screwing the spark plug in for the first few turns by hand. I know you're going to say you can't reach the spark plugs by hand on the Firestorm

but use your hand on the end of the tool kit plug spanner first. That way if you feel the spark plug go tight meaning you may have got it cross threaded you can undo it and start again. If you use a spanner straight off on the end of the tool kit plug spanner and it suddenly goes tight on the first few turns, you will have will have most likely damaged the threads

which will not be easy to repair.
How to spot non genuine NGK spark plugs from this link:-
http://www.ngk-sparkplugs.jp/english/techinfo/fake/
Chris.