Spark plug change interval

[ew1usnr]

My 260 was rebuilt four years ago. It is using Autolite 45 spark plugs that now have 17,500 miles on them.
It had been my impression that the spark plugs should be changed every 25,000 miles.

5953

A friend at work gave me a Chilton's repair manual for Ford Pinto/Bobcat, 1971 -1980. That manual says:

"The average life for a spark plug is approximately 12,000 miles. This is, however, dependent on the mechanical condition of the engine, the type of fuel that is uded, and the driving conditions under which the car is used. For some people, spark plugs will last 5,000 miles and for other, 15,000 or 20,000 miles.

So, Chilton's says 20,000 miles maximum. How often do you guys change your spark plugs? Do you go by mileage intervals or just wait until the car begins to run unsteady?

Thanks, Dennis.

[SmithKid]

I figure it'll be around 12 1/2 more years before I get the miles on mine to warrant changing the plugs. However, I do have Pertronix point replacement unit and a hotter coil, so I feel with the coil, my plugs should last approximately my expected life span. Maybe I'll leave a note in the car for whoever ends up with it.

Seriously, I feel that with an occasional 'look see' for confirmation, the plugs autta last at least the 20K you're talking about.

[ew1usnr]

I feel that with an occasional 'look see' for confirmation, the plugs autta last at least the 20K you're talking about.

Hello, Gene.

I agree. The plugs I have could easily last another 2,500 miles and make it to 20,000 miles. But, ..... my car has started to idle a little unevenly while in neutral. It seemed like it was probably caused by the spark plugs and I was a little disappointed that they were beginning act up at "only" 17,500 miles instead of lasting until 25,000 like I thought they were supposed to. They have actually done considerably better, though, than the 12,000 mile average referenced by the Chilton's manual.

That being said, a new set of spark plugs is cheap. I bought a set of eight Autolite 45 spark plugs from PepBoys on the way home and will try to install them this weekend. They were only $1.64 each with a 25%-off promotion. 8*$1.64 + $0.92 tax = $14.04.

5955

I will change the spark plugs every 12,000 miles from now on. It is a very inexpensive performance enhancement. I have already seen definite performance gains simply by installing a new air filter and by advancing the timing. It will be fun to seen how my little Challenger 260 runs after it gets a new set of plugs in addition the other improvements. :)

[Luva65wagon]

Newer cars they have a very long lifespan on plugs.

Plugs "fail" for at least the following reasons:

1) Internal. They open (as-in like a switch) and no longer have a connection between the center electrode and the terminal nut/stud end.
2) They foul, which can be cleaned. You can adjust heat ranges to minimize this potential.
3) They corrode/burn away. This is the visible reason to replace them. Rounded edges on everything don't give a good, reliable, spark. As soon as the edges start getting too rounded over, it's probably time to get a fresh set.

The hotter the spark the longer they last, so any improvement you've done in this regard increases your plug's lifespan. That's also a reason new cars have a longer service life on spark plugs. Back in the day mechanics would just sand blast plugs and back in they'd go. As long as they were clean and made spark, that was good enough. Today, you rarely see this being done, but there is no reason to toss a good plug just because it has reached mile XX,XXX.

[ew1usnr]

... there is no reason to toss a good plug just because it has reached mile XX,XXX.

Dang. I changed the Autolite 45 spark plugs in my 260 yesterday and was very pleasantly surprised by what I found.
The old spark plugs had been set to the standard 0.035" gap. They had exactly 17,345 miles on them and looked perfect. They were clean, not fouled with deposits, and not burnt, or sooty, or oily.

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5959

I installed a new set of Autolite 45's, but tried something that is a bit "controversial". The standard gap for the Autolite 45 is 0.035".
The previous owner had replaced the points ignition with a PerTronix Ignitor electronic ignition module and a 40,000 volt Flamethrower coil.
Pertronix says: “The Ignitor has no set specification in which the spark plugs should be gapped. Every engine responds differently to spark plug setting. In most cases increasing the factory recommended gap by 0.005" improves the engine performance.” See: Troubleshooting Tips (Ignitor)

Adding 0.005" to 0.035" = 0.040". I found a graph on the internet which indicates that a 0.40" (1.02 mm) gap increased the required volts to 17,000 compared to 14,000 volts for the 0.035" (0.89 mm) gap. This is a 21% voltage increase.

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A 21% increase did not seem excessive, so I set the gap on my new Autolite 45 spark plugs to 0.40".
I took the car for an 8-mile test drive in traffic. It started instantly, ran smoothly, and accelerated with no hesitation. It runs perfectly.
I do not anticipate any problems, but at 1,000 miles I will examine the inside of the distributor cap and will remove and look at a spark plug for any evidence of arcing or burning.

[Jeff W]

Dennis, as long as you are spending this much attention to the plugs and ignition system it may be worthwhile to try indexing your plugs. This article says a 1-2% performance loss can occur from poorly oriented plugs.

Looks like a fun Saturday afternoon project for little or no money.

https://fordmuscle.com/archives/2000/07/indexplugs/

[ew1usnr]

Dennis, as long as you are spending this much attention to the plugs and ignition system it may be worthwhile to try indexing your plugs. This article says a 1-2% performance loss can occur from poorly oriented plugs.

Thanks, Jeff.

Before changing my plugs I had watched a video about indexing plugs using washers to change the final spark plug tip orientation. That did not seem like a good idea because it would change the depth of the plug into the combustion chamber.

The article that you provided described how to index the plugs without using washers. It explains that all the plugs are different and to simply use trial and error to see which plug fits best into each hole.

Darn. I wish that I had marked the direction of the plug tips with a sharpie.

Logically, it seems that it would make a difference if plug tip faced towards the center of the combustion chamber rather than facing backwards towards a cylinder wall.

Indexing the plugs is supported by this explanation where a guy tried changing the position of the spark plug while working on a one-cylinder engine. He said that the orientation of the spark plug tip made an obvious audible difference and changed the RPM from 2000 to 1700:

"A few years back, I played with a single cylinder ATV and noticed that a spark plug fired best at a certain rotational position. The results were measured in idle RPM. At one idle setting, the RPM ranged from 2000 down to 1700. The change was audibly different - very obvious.

When I hit the optimum spot, I pulled the head off the cylinder and looked where it was. The arm holding the gap was pointed AWAY from the center of the combustion chamber.

It's not about increased horsepower anymore. It's about SAVING GAS. Burning your fuel more efficiently will increase your gas milage, with fewer trips to the pumps. Mind you it's not a dramatic change.... But a lot of little still makes a lot."

See: http://www.doityourself.com/forum/passenger-cars-mini-vans-suv-service-repairs-no-trucks/349781-spark-plug-indexing-washers.html#b

[ew1usnr]

Dennis, as long as you are spending this much attention to the plugs and ignition system it may be worthwhile to try indexing your plugs. This article says a 1-2% performance loss can occur from poorly oriented plugs.

Hello, Jeff.

You planted a bug in my brain and I had to take another look at my plugs to see what was going on.

Ford Muscle recommends aiming the open end of the plug at the exhaust valve.
See: https://fordmuscle.com/archives/2000/07/indexplugs/

5965

I removed the plugs and marked them to indicate the open end:

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A friend at work had given me a pair old 260 cylinder heads. I screwed a plug into one of the old cylinder heads to get an idea of how the plug would be situated and set it to the same position as the one installed in the car:

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That showed that the open end of the plug was aimed at the black exhaust valve, exactly as was recommended by Ford Muscle as being the ideal spark plug position. I shuffled the plugs around so that the open sides of all the spark plugs on the left side were pretty close to this same position.

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The plugs on the right side of the car, however, all tended to be aimed at the intake valves. That is evidently alright also. Another web site recommended positioning the open side of the plugs towards the intake valves:

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See: http://www.enginelabs.com/news/getting-the-most-out-of-your-ignition-spark-plug-indexing-101/

So, the open ends of my spark plugs are aimed at either the intake or exhaust valves, which is good. None of them are positioned so that the spark is shielded from the combustion mix, which is bad.

The Wonder Falcon has indexed spark plugs!

[Jeff W]

Hello, Jeff.

You planted a bug in my brain and I had to take another look at my plugs to see what was going on

My apologies to your wife for pulling you away from your household chores and back into the garage.[yay]

[ew1usnr]

My apologies to your wife for pulling you away from your household chores and back into the garage.[yay]

Well I was going to pull weeds and clean the driveway.
Instead I carried a 45-pound cylinder head and a spark plug over to my neighbors garage. He has owned a Plymouth Laser, a Thunderbird and Mustang since I have known him. We drank beer while I explained spark plug positioning. It was all good.