I am running Gen 4 GM rect port heads, need to get some plugs for a boat equipped with Gen 6 or 5, sight unseen. Think they all have the same reach but not certain. Thanks

Ok I researched this and found that they all take .460 reach, standard or extended tip plugs

ngk R5673-7,8,9 or Ngk R5674-7,8,9

If it's a stock engine. the 7's will be to cold. In stock engine's, use an NGK R5673-6 or BR6FS.
Eddie

Thanks Eddie, 9.8-1 runs up to 5600 fairly similar to mine except for the Gen. His is 560hp. EGT's were high on his (due to crappy marine carb I think) so I was thinking 7's with out the extended tip would be a safe bet

the rect ports do, but the oval ports do NOT take the same reach plugs

Good to know, thanks.