# spark plug gapping



## 67 GTO (Nov 26, 2011)

Hey guys! Spark plug gapping is something so small but also so crucial. What is the correct gapping for a 67 Pontiac 400 with a crane cam ,HEI distrubtor and sterling pistons. Thank you.


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## BearGFR (Aug 25, 2008)

It's less about "correct" than it is "optimal"... You want a gap that's wide enough to get a good fat flame started, but not so wide that being able to get a spark generated across the gap becomes iffy. Compression ratio and cam (dynamic cylinder pressure) along with ignition system (available "oomph" to create a spark), fuel type, combustion chamber characteristics, plug type, even head material (aluminum vs. cast iron) all come into play.

For an HEI system on a 400, I'd start with .045 on the gap and see how it acts.

Bear


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## Eric Animal (Oct 28, 2007)

Yep 045 to 060 I been told.:agree


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## geeteeohguy (Feb 2, 2008)

.045" is what I would run. But that's just me. Run what you wanna.


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## Original Goat (Apr 4, 2012)

Hi. I'm one of the new guys on the block just reading along here. For my 2 cents my drag racing guy runs NOS and is into it pretty big checks the gap after almost every run and plays with it based on the apperance of the burn on the plugs. As Bear stated it is what is "optimal". Back in the day for tune-up specs the books gave a .030" gap for the 335 hp 400 running AC RS44's and the 360 hp was .035" with AC RS45's. Course if you try to find that info now the books just recomend .035". Running HEI ignition would change that and you would need to experiment with what works best. Takes time. Hey thanks for putting up with my dribble.


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## 67 GTO (Nov 26, 2011)

thanks for all the feedback guys! great info once again!


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## pontiacsniper (Jan 20, 2012)

Not to hijack but my motor is similar. What part number spark plug do you recommend and what brand?


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## Original Goat (Apr 4, 2012)

pontiacsniper - being a GM guy I have to say AC/DELCO for the brand. When I was running my car and maintaining it I found that AC's lasted about twice as long as the other brands of the 60/70 era. I'm going to be getting into the maintainance again on the car as I get it restored so I will probably find out the technology has got better on other brands. So if someone wants to let us in on the techo stuff it would be nice.

Bill


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## 67 GTO (Nov 26, 2011)

Original Goat is correct i used AC/Delco i got all 8 for like 16 bucks. As soon as im by th car again ill see what number they are.


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## pontiac (Mar 6, 2011)

spark gap is based on what plug you use and what distributor you have.


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## stevenray122 (Feb 15, 2014)

do points come pre gaped. for a 69 400


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## BearGFR (Aug 25, 2008)

Never assume that. Always check them yourself. It's preferable to use a dwell meter instead of measuring the gap, but good luck finding one if you don't already have one or know someone who does.

Bear


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## younggto (Nov 22, 2013)

Dwell exists on points type systems, not HEIs


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## geeteeohguy (Feb 2, 2008)

No, they don't come pre- gapped. Sometimes they are so far out that the car won't even start. But you can set the dwell while cranking it over. Never just bolt them in and run them. Always check/set the dwell. And then set the base timing.


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## 666bbl (Apr 13, 2014)

The late great Bill "Grumpy" Jenkins said to run the widest gap that your ignition system will fire consistantly. I've run as high as .040 on point systems with no ill effects and lovely performance and driveability. Big gaps will take a toll on stock wires so get the best wire you can get that suits the engine dress. On HEIs I've been as high as .055 just to see if it would work but it was pretty cold natured. Some also swear by (or at!) plug indexing, which is to point the opening of the gap in the same place on each cylinder. I used to do it "just because" but never did "A-B-A" testing. I love ignition system challenges...


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