1. If it's a resistor plug ?
2. If it's hotter or colder plug than the one I run now ?
After a few prompts, I get this young sounding kid on the phone that's asking all sorts of stuff like, make, model, cc's, year . . . Hey, I just want to give you a plug number, and you can tell me what the next hotter plug in this reach style is . . . but I'm getting nowhere. He says I should talk to "I'll call him Jack" . . . so this guy Jack gets on the phone. Jack starts in with the same questions . . . . I'm trying to tell him the spark plug is for this special racing head . . . yada, yada . . . then he says, "Look Sonny Boy, if you'd just shut your pie-hole a minute and tell me exactly what you got, I can help you." So I say, ok - I got 1968 Harley XLR cylinder heads on a 1961 XLCH engine. He's like . . . "You need the Autolite 55, non resistor plugs which are correct reach for those heads. You can run the next hotter Autolite 4056, but they will take longer to heat up, and they take longer to cool down. I'd run the 4056's . . . I did in my XLR back in 1970. Anything else I can help you with ???? I about fell off my chair. It pays to be a good listener.