NGK DPR8Z is what I use - what I prefer is Nippon Denso X24GPR-U - in a pinch I use AC Delco 4163 - for racing I use DPR8ZIX - no other spark plugs are listed as exact replacement for the 400EX and 400X - there are other plugs that will fit the threads, but not the design feature The DPR8Z, DPR8ZIX and X24GPR-U are the only ones you will find in my garage or tool box