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Spark Plug recommendations?


Well-known member
Dec 30, 2002
Dallas County, Texas
'75 Modified Red Ragtop: "LEFTLN"
OK, I'm an idiot. I listened to the parts monkeys.

I think they went the wrong way on the heat of the RapidFires they gave me - anybody know the system on those?

Anybody have a general recommendation on plug selection and gap for a stock 350 SBC in a '75 auto with an aftermarket ignition (Mallory HyFire) and lots of decent bolt ons but no great guts, compression or cam?

These guys gave me RF #10's, which were gapped at 0.035". As my last plugs showed a little erosion, but not a lot and no black deposits, I said give me fairly COLD plugs. I altered the gap to 0.055" to make the most of the long spark duration, but a nagging midrange miss is increasing and I caught one reference to HIGHER number RF's being HOTTER plugs - but I couldn't follow it up.

I'm inspecting them tomorrow, but what are anybody's general choices?

Thanks for the input.
I'm not familar with the numbers on the later "peanut" plugs, but whatever the cross is to the "45" AC heat range should work fine for 99% of driving conditions; they don't make 46's any more, and 44's are a shade too cold for most normal driving and can have some light fouling problems. Don't waste your hard-earned money on "Rapid-Fires" or any of the other "whizbang" plugs; they won't work any better than ordinary brand-name plugs, regardless of the marketing hype, and I wouldn't go any more than .040" on the gap - I run .035" gap on all my '67-'69 small-blocks, no problems.
I agree...I user good old AC 45 in my 64. I do run a bit more of a gap because I have an HEI on my 327. You can't go wrong with what GM used originally.

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