Join Date: Dec 2004
Location: Altofts zx-12r B4 UK
NGK IRIDIUM CR9EIX 3521 are the ones I use on my zx-12r B4, and on my zx-9r's.
The gap on these with that number 3521 should have the gap preset to 0.7, so no adjustment needed.
These days there as cheap as OE ones, and in my opinion work better and last longer.
Just don't drop them as you can crack the inner core and they won't work so good.
For Tech Questions Bike Model letters and Country are more useful than saying the year, for UK owners anyway, as registration year is not always the year of manufacturer model.