I went thru a similar problem on my 292 when I first put it together. Had a lot of ignition problems & to make a long story short I found the head had tapered seats & required a plug 11/16 long from seat to the base. AC # R44LTSM fit this pefectly. I believe I have a Mexico production head, some auto parts website had this listed, I found it later. I live near the border & there is a lot of junkyard cars & parts back & forth here. I've since put about 25K on it with same plugs, no problem. It taught me to Always look at the seat & reach required. It's in my '55 Bel Air, BTW. Doug
