I am replacing the plugs on my 99 7.4l mpi. They are NGK's, what is the correct gap? the only mod I have done is replaced the stock flame arrestor with a K&N.

I went with what was on the engine. 2000 454Mag MPI .035" Gap. Check your engine or manual to be sure

i also have a 99 7.4mpi it says on my engine that the gap is .045 and the plugs should be mr43lts. The manual doesn't have any info.

Most should be gapped at .35 max should be.45 you can always split the differance a .40

Thanx, fella's. I gapped them at .045. What would the diff. be if I gapped them at .035? The old ones I pulled out did not even look like they were gapped(most were .020). I let the marina do it last time. I f have started to do all my own maint.

The Mercury shop service manual indicates .035 to .040 is the stock gap for NGK or MR43LTS.

I just replaced my plugs with AC Delco Rapid Fire #1's. These are great plugs. Outstanding idle and startup. Gap for these plugs is .045

Rick

I just bought a set of the rapid fire #1's I'll let everyone know if there's a difference in performance. I'm running the original plugs now that are 2 years old. Haven't had any problems figure its just time to change them.