I gave the answer to the center electrode growing....it sparks from ground to center....and takes material with it...the polarity issue...best plugs are NGK....worth the extra every time....have been using in stoc & performance/race engines for 30 years...

V6 MTX
'Don't p**s up my back then tell me it's raining!!!"
'Its only nuts & bolts!'
'If I build it,fix it,upgrade it or modify it...MAYBE they will come....!
Haines Motor Sports Inc,
Dealer for 'Quaife America' & 'Autotech Sport Tuning'
SOLE USA Dealer for the American Axle 'AUSSIE BAR'...
Get a Turbo for you Zetec from HMS Inc...by 'The Demon' ...www.DemonDynamics.co.uk
..don't talk about it DO IT !!!