351c Iridium Plug Gap Size - Vintage Mustang Forums
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post #1 of 8 (permalink) Old 07-10-2019, 03:53 PM Thread Starter
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Confused 351c Iridium Plug Gap Size

Hi all.
I've just bought some Iridium plugs for my '71 Mach 1 (351c, Fat leads, Pertronix, uprated coil) and the gaps are preset at .043, 10 thou more than recommended gap for the normal copper plugs. Would it be ok to fit plugs as is? It's just I remember reading somewhere that you can widen the gap a bit over stock with Iridium plugs? Is 10 thou too much? Stick to recommended oem gap?
Thanks!

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post #2 of 8 (permalink) Old 07-10-2019, 05:01 PM
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With a hotter than stock coil, that gap should be fine.
But you can gap them tighter if it makes you feel better.
I think .038 would be perfect.

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post #3 of 8 (permalink) Old 07-10-2019, 05:34 PM
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Throw them in the trash and go get some copper plugs.
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post #4 of 8 (permalink) Old 07-10-2019, 09:55 PM
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Originally Posted by sportsroof69 View Post
Throw them in the trash and go get some copper plugs.

Platinum and Iridium are a sales gimmick used to double or triple the price of a spark plug. I have heard that copper is actually a better conductor of electricity than either of the precious metals. So maybe a copper plug only lasts half as long as a precious metal plug. How many miles do you drive your car every year?
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post #5 of 8 (permalink) Old 07-10-2019, 10:42 PM
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No need for iridium plugs unless you car requires them . The iridium plugs take more current to fire
than a copper core plug .
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post #6 of 8 (permalink) Old 07-10-2019, 11:44 PM
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They are actually meant for higher energy ignitions. Not only do they work better with them, they actually require higher energy spark to stay clean. With a lower energy ignition system they tend to foul more easily than copper or traditional plugs. I found this out myself on several vehicles. If you are running something like an MSD ignition with a high output coil they would be OK. But really they are intended to simply last longer, not perform better. It's not unusual to see them go well past 100K miles in late model vehicles. In our other drivers that's fine, but in my older Fords, they don't quite fit my needs. Copper plugs do for me, even WITH an MSD box.
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post #7 of 8 (permalink) Old 07-11-2019, 08:58 AM Thread Starter
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Cheers guys - think I'll stick with the copper ones

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post #8 of 8 (permalink) Old 07-11-2019, 12:37 PM
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Cheers guys - think I'll stick with the copper ones
Wise choice

The exotic metal plugs are fine for the highly controlled fuel delivery of an EFI car with a high power ignition. On carburetor equipped cars such as ours, they do not do well and will readily foul in a short period of time. So stay away from platinum or iridium plugs.

Autolite 25 is a good start for a basic resistor plug. Cheap and historically correct for a Cleveland.

As for gap, a wider then stock gap is preferable for electronic ignitions (i.e. Pertronix). Your Pertronix equipped Cleveland should like 10-15 thou over the stock gap.

And here's a link to everything you ever wanted to know about Autolite spark plugs but were afraid to ask:
https://www.autolite.com/resources/tech-specs/

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