My replacement

I decided to check my spark plugs in my 2018 F150, 3.3L, non turbo, non eco, non flex, 2wd, at 194,500 miles yesterday. I bought the used truck one year ago. The plugs I pulled out were Motocraft CYFS-12F-5. They looked horrible.

Autozone couldn’t find my vehicle engine exactly and tried to give me many choices.



I left and went to Advance and they offered me the Motocraft SP-589. I stepped outside and ran that through Gemini AI. Here is AI’s answer:



“Yes, the Motorcraft SP-589 is the functional replacement for the CYFS-12F-5.

In the Motorcraft part numbering system, CYFS-12F-5 (and its successor CYFS-12Y-T6) are the "engineering" or "service" part numbers, while SP-589 is the "short" or "stocked" part number used for retail and ordering.

Key details regarding these plugs:

• Supersession: The SP-589 has effectively superseded several older part numbers, including SP-520, SP-551, and the original CYFS-12F-5.

• Compatibility: They are commonly used in Ford 3.3L, 3.5L (non-turbo), 3.7L, and 5.0L engines.

• Specifications: They are platinum-tipped plugs designed to meet the exact heat range and gap requirements originally specified by the CYFS-12F-5.

If you are looking at a box or a listing, you will often see both numbers printed on the packaging to confirm they are the same part



The standard gap for the Motorcraft SP-589 is 0.051 inches (1.3mm).

While these are typically pre-gapped from the factory, it is a good practice to verify them before installation. For a 2018 F-150 with the 5.0L engine, the specification remains 0.051 inches.

Quick Specs:

• Gap: 0.051"

• Material: Platinum

• Hex Size: 5/8"

• Thread Reach: 0.984"

If you are using a gap gauge, be careful not to apply pressure to the center electrode, as the platinum tip can be fragile.”



I wrote back later to AI: Based on your excellent research, I bought the plug you recommended for my replacement of my CYFS-12F-5.



My new question today is this. Could the old, dirty, worn out plugs I removed yesterday from my 2018 F150, 194,500 miles, with all giant burned gaps of .075”, be the original plugs from the time of manufacture in 2018? I bought the F150 in March 2025. It was owned by an electric contractor company before my purchase.



Gemini AI’s answer:

“It is highly probable that the plugs you removed were the original factory-installed set from 2018. Given that you purchased the truck in March 2025 and it was previously a fleet vehicle for an electrical contractor, several factors point toward those being the original parts.

Evidence for Original Equipment

• The Part Number Match: The CYFS-12F-5 number you found on the old plugs is an "engineering" or "service" number. These specific alpha-numeric codes are typically what the factory installs on the assembly line, whereas a technician performing a later tune-up would likely have used the SP-589 retail part number.

• The Extreme Wear: A gap increase from .051" to .075" represents a significant amount of metal erosion. This level of degradation—where the platinum tips are essentially gone—usually takes 70,000 to 100,000 miles to occur.

• The Service History: Fleet vehicles owned by contractors are often maintained on a strict "if it isn't broken, don't fix it" schedule or follow the manufacturer's maximum service interval. Since the truck "ran fine" despite the wear, it is very likely the previous owner never saw a reason to change them.

Why the Gap Grew So Large

• Spark Erosion: Every time a spark jumps that gap, a microscopic amount of metal is blasted away. Over several years and tens of thousands of miles, those microscopic losses add up to the .024" of extra space you measured.

• Platinum Survival: The only reason the truck was still running smoothly at .075" is likely due to the high-voltage capability of your 2018 F-150’s ignition coils, which were working overtime to bridge that massive gap.

The Benefit of the Change

By installing the SP-589 plugs and returning to the correct .051" gap, you have significantly reduced the electrical stress on your ignition coils and improved the efficiency of the combustion cycle. This change is the most logical first step in resolving your P0420 and P0430 codes, as the old plugs were likely allowing trace amounts of unburned fuel to reach your catalytic converters.”

 

My guess, they are not original, likely a replacement that was close and good enough. I would simply us SP589 and carry on. Rock Auto is a great source, be careful buying online, lots of knock offs.

 

Way too many variables in spark plugs for a life prediction. Top that with Motorcraft constantly updating old plug numbers. OP I take is using a NA engine vs an EB, makes a big difference in cylinder pressure and the reason NA plugs use wide gaps vs EB and tight gaps, A general consideration in plug life is the electrode material used. From short to longest life there is copper then up increasingly, platinum, iridium, ruthenium. Iridium and ruthenium have longer life (near the same) versions in their style single and double tip with rare metal. Ford is fine with the life of a platinum plug in NA engines but switches to double iridium in EB engines. NGK makes a lot of Motorcraft spark plugs and offers multiple styles of tip materials and construction. I use NGK iridium single electrode Iridium IX 6510 plugs, shorter life cold plug with easy ignite-ability. The are the same plug as Motorcraft Cold Ecoboost Racing plugs SP542-28, these would be considered short life plugs at 20000, these can get sooty. Current double iridium factory plugs run close to 100000 miles, ruthenium the same, but often use extended tip electrodes for better ignition. Quite the change since my youth changing plugs and tuning dads Pontiac in the 1960"s, those old copper plugs went 10000 miles at best and in high compression performance engines maybe 5000!
If you are looking for long life spark plugs look into NGK Ruthenium HX LTR6AHX 91276, double ruthenium extended tip plugs. I recommend these as Motorcraft does not offer ruthenium double tip plugs yet. KM

 

I think they were the original plugs. I got 180,000+ miles on the original set of plugs in my 2011 XLT 5.0...and it was still running strong when the truck was totaled.

 

Ran my 2019 2.7 EB to 100k but then started having a miss. Changed plugs and coils to be safe. 135k now with no issues.

and yes, the old days. Not only did we change plugs frequently, but we cleaned and re-gapped them in between. Wire brush, pocket knife or a “high tech” sand blaster did the job.

 

Make sure you buy plugs from a reputable source. There are a lot of counterfeits out there.