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66 Vista cruiser, I'm putting a edelbrock performer and a HEI on my otherwise stock 330. As I remember we updated the spark plug gap to. 035 with HEI. Any body else do this.
.035 - .040 is all good afaik, don’t make it any bigger, well maybe .045 if you feel the need. After that you’re just kind of abusing the module, coil & carbon button.
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I second bccan’s comment about the gap .035 - .040 is just about right.
Been using HEI’s for decades and you can run a larger gap, but on the the street it eventually turns into a larger probability of misfiring.
In case you’re interested I was using R46SX plugs until I couldn’t find them anymore. Now I’m using NGK Iridium plugs. I liked both along the way about equally.
Somewhere on Motor Trend TV, David Freiburger has an episode of Engine Masters that compares various brands and plugs including the gimmicky ones. I came away with the impression that the differences in ability to ignite cylinder mixtures was very small.
The 0.060 and larger gap used on HEI systems was for emissions reasons and is NOT something you want to do on an older engine. People forget that the HEI system was developed to satisfy the EPA requirements for cars to meet emissions levels after 50,000 miles without a tune up. By that point plugs would be crudded up. The larger gap was necessary to prevent this crud from bridging the gap and shorting the plug. The higher voltage coil was needed to jump this larger gap. Normal engines want 0.035-0.040 max.
Make sure you put thermal paste compound between the body of the distributor and the HEI module. Back in the day you would get a ketchup style packet of thermal paste with a new distributor.
The old R46SX plugs are out there. Just have to sniff them out. I got over a dozen boxes of the green ring R46SX as it was standard equipment on the 83-84 VIN Y and 9 engines (and 85 VIN 9). Pregapped at 0.080". When they changed the specs, they also changed the recommended plugs to R46SZ, which came pre-gapped to 0.060". Always double check them to where you want. I just bend the SX's to 0.050" gap and call it a day.
The larger the gap, the more strain is put on the ignition module causing increased heat generation. Gap was monster-huge in the early 80s due to the ultra-lean mixtures they were dealing with. GM had a developing warranty problem with fixing module and coil failures, so late in the 1985 model year, they reduced the gap from 0.080" to 0.060" on Oldsmobile 307s as their fix. Seemed to help. IMO, no reason to go over 0.040" on a "normal" engine, though. They put new stickers on Chevy cars under the hood with the new gapping information of 0.035" when serviced, but for some reason, never did with Oldsmobiles to my knowledge. I know they did a reduction in gap, but I've never seen an Oldsmobile service bulletin regarding that. A guy I know that used to work on Oldses at a GM dealership back in the 80s said 307 module failures among others with the 80 gaps were not uncommon. But once they changed the gap settings, things got better.
Here's a Camaro that got a sticker.
And here's a Chevy bulletin below that alludes to the issue, but it specifically calls out "high-output" coils on the Chevies, and to change them out with "standard" output module and coil. Which, btw, the standard output coil (1985474) and module (1976908) was used on Olds 307s at the time. It doesn't appear to be Campaign 85-C-22, to which I cannot locate, but in effect it does the same thing as a last ditch fix in this service bulletin. I'm guessing this bulletin was first, then made it a campaign to take pre-emptive action on it. Just a guess. Note too, in the pic above, the car got a new emission tune-up label p/n 14086589. Trust me, even a standard output coil will make you stand up and take notice if you've ever been bitten by one.
Olds went to an FR3LS6 peanut plug in 86 for the 307 (Y and 9) swirl port heads and carried this on until the end of the 307s.
I have the same car with same setup, when I purchased the car, it had AC R44TS which are 5/8th tapered plug gapped @ 45 in it ? I had to scratch my head for a min, I immediately replaced them with AC R43S @ 38 gap and runs much better.
I've got t stockpile of green ring R46SZ's for both my 79 H/O (lists 35 gap), and my 260 equipped Calais & Supreme (60 gap). In the mid 80's I could not figure out why (in the blue Calais) I was burning through rotors and ignition modules more quickly than usual. I switched to using a Jacobs distributor cap/rotor set up which had the rotor lasting alot longer than OEM. But man, that, coupled with the Jacobs wires, wreaked havoc on radio noise.
Next spark plug change I will pull those SZ's down to 35 and report back on how the car does on it.
As said, the .080" was ridiculous and GM backed it down with a TSB. I remember our 81 Delta 88 not starting. I pulled a plug, they were worn, must have been .100"! I went back to the factory .080" but eventually went less, like .060. I think we used that on the Olds we fixed quite often that came into the shop. We saw almost no Olds ignition issues and they were massively abused cars, many didn't even get oil changes! Now the TBI chebbies always had blue corrosion in the small distributor caps, we used Blue Streak brass caps to fix misfites. Now I run a loose .045", zero issues with excessive wear. The only issues I had was a melted cap when I forgot the coil ground strap and multiple pick up coil failures. Also had one fail in Dad's chebby truck as well, so not gap related. I always run an Accel coil, currently running the red and white lead Brute Thunder model, their heavy duty brass cap and rotor and a Moroso module. Don't sweat .010" gap, it will be fine.
Last edited by olds 307 and 403; May 31, 2025 at 06:05 AM.
Somewhere on Motor Trend TV, David Freiburger has an episode of Engine Masters that compares various brands and plugs including the gimmicky ones. I came away with the impression that the differences in ability to ignite cylinder mixtures was very small.
