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  1. #1

    Spark plug change interval

    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.

    Autolite 45.jpg

    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.
    Last edited by ew1usnr; October 17th, 2016 at 03:07 AM.
    Dennis Pierson
    Tampa, FL
    "The Wonder Falcon"

    '63 Futura Hardtop (260, Ford-O-Matic, bench seat)

  2. #2
    Join Date
    Feb 2007
    Location
    Fredrickson
    Posts
    977
    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.


    Gene Smith
    Fredrickson, WA
    '65 Ranchero Deluxe
    302, EFI, 4-Spd
    Granada Discs

  3. #3

    Spark plugs

    Quote Originally Posted by SmithKid View Post
    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.

    Autolite Copper Core Spark Plug 45.JPG

    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.
    Last edited by ew1usnr; October 18th, 2016 at 02:03 AM.
    Dennis Pierson
    Tampa, FL
    "The Wonder Falcon"

    '63 Futura Hardtop (260, Ford-O-Matic, bench seat)

  4. #4
    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.
    Roger Moore

    63 "Flarechero"
    powered by: 347ci stroker | Tremec T5 | 8" 3:45 TracLoc rear



  5. #5

    Super Sparks

    Quote Originally Posted by Luva65wagon View Post
    ... 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.

    IMG_0559.jpg

    IMG_0560.jpg

    IMG_0561.jpg

    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.

    spark plug gap to voltage impact.jpg

    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.
    Dennis Pierson
    Tampa, FL
    "The Wonder Falcon"

    '63 Futura Hardtop (260, Ford-O-Matic, bench seat)

  6. #6
    Join Date
    Oct 2006
    Location
    Seattle
    Posts
    1,430
    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/

    Jeff Watson
    Seattle, WA
    '63 Tudor Wagon (170 - 3 spd.)

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