Just bought new plugs, and they are gapped from the factory at 42 thousandsths. Mercruiser suggests 32 thousandsths for my 420 hp engines. how important is regapping the plugs?

thanks.

Are you using Merc ignition & coils? If so, I would gap them at no more than .035. A larger gap, provided you have a "hot" coil produces a larger flame front for more complete combustion. I used .040-.045 on my Jacobs ignition w/MR43T plugs in my blower motors and worked extremely well.

Using merc thunderbolt ignition. Noticed that the 330 was at .035, and the 420 was at .032. Seamed a little odd to me that the lower hp engine would use a larger gap.

looks like a gappin we will go. . . .

I just picked up a set of NGK's and they said to gap them at.044. That seems a bit wide, what do you think.

THX
Sam

It's wider than what I've found, but that would prevent regapping 16 spark plugs. What kind of engines do you have?

Who is "they"

I just gapped my NKG's at .35. Same as what they were gapped at. And as was suggested.

I'm runing a SBC putting out around 360-370 hp. Griff, " they" are NGK .

Sam

I think he means who are "they" that told you to gap them at that. Dealer, Marina...

with most ignitions you'll never notice the gap change 35 works great .
On M S D and other high current ignitions they give enough spark energy to take some advantage of the larger gap.