Spark plugs for Mercury 150 4S...

[Johnnyred]

Mercury 150L 4S, Model: 1150F13ED, SN: 3B020418

I own a 2021 Mercury 150 fourstroke outboard that has NGK ZFR5F spark plugs in it stock. I am doing maintenance and was wondering if NGK IZFR5G plugs (Iridium long life enhanced electrode) were a drop-in replacement, compatible with my engine that would give longer and better service?

If yes, do I gap them the same 0.032" as the ZFR5F plugs?

[EuropeanAM]

NO SIR. Use only the recommended spark plug in your engine.

[Johnnyred]

I contacted NGK and they said, "IZFR5G is the exact same specifications as the ZFR5F down to the pre-gap size of .032”. They are ready for you to install."

I thanked them and replied, "So the stock 5F has a different, bigger electrode than the 5G and I assume is a standard nickel material instead of the Iridium in the 5G. These difference are OK?"

NGK replied, "Absolutely! All precious metal, Iridium, Platinum and Ruthenium all have fine wire center electrode which is designed to allow larger flame expansion."

I had also sent them a screenshot of their website's part-finder page only showing the ZFR5F for the Merc 150 4S. They said the part-finder only showed the OE plug.

[Johnnyred]

Don, I certainly rely on your knowledge and advice when it comes to my Mercury motor. Thanks you for replying to my question and for the advice you have shared with me in the past. Not trying to stir the pot... just gain knowledge.

I contacted NGK again after confirming on their website that the heat range for the IZFR5G and ZRF5F are the same, "5" on the NGK rating system for both. I wanted to make sure I was not missing something about the 2 plugs. Their reply:

"There are no differences between the two plugs besides the firing tip metal. They are identical in heat range, projection, resistance value, etc. I would never confirm that they would work if I see any differences."

One of the major Mercury Marine 3rd party suppliers' websites recommended the iridium version as interchangeable with the standard nickel... <EDIT- READ ANNOUNCEMENTS>... I cannot remember which.

[Pulsare2020]

Just a guess, but one possible reason that Don said to use the "recommended plug" could have been for warranty issues. If some engine issues were to arise, or damage for whatever reason, let's assume due to detonation for arguments sake. Even if it had nothing to do with the iridium plugs, Don (or any Mercury dealer) would have a weaker warranty case when he called Mercury and they found out it had a non recommended plug in it. There might be many other reasons as well.

[EuropeanAM]

There are a number of reasons, but perhaps the most important to me is that, for some reason, engineers determined that the less expensive Platinum Plugs provided excellent service and longevity in your engine over many, many hours of pre-production testing. They have now been in service in this engine model for more than a Decade.

In the case of this application- had there been any justifiable reason (or benefit) to use a different spark plug, it would have been listed.

Optimax models (specifically) benefited from the smaller firing pin and more durable Iridium material, especially at idle. Those concerns are not at play in this engine.

ANY Spark Plug (regardless of what the manufacturer might "claim") should be gapped prior to installation. From my experience, it's extremely rare for the gap to be correct (as in less than 10% of the plugs we get match the recommended gap straight from the box).

Be careful who you take advice from. Just because a company sells Mercury or Quicksilver parts does NOT mean that they know diddily-squat about the engine they're going in.

Look up parts in the official Mercury Parts Catalog- which can be found on Mercury's Website, and is also incorporated into ours at: www.mercurypartsonline.net (ours will actually handle super cessions as parts are added to the shopping cart).

Above all, support our BBC Sponsors where possible.

[Johnnyred]

Done deal. No iridium plugs for my 150!

Don, one more ? on this topic: I assume there is a "trick" to get a tool and some leverage on the plug in cylinder #4 w/o having to take off the lower cowling?

Thanks as always.

[EuropeanAM]

Yes- by using the shortest extension that can make the reach and assembling the ratchet to the extension "while on the job". Remove the ratchet as soon as you can spin the plug by hand.

[Johnnyred]

Yes- by using the shortest extension that can make the reach and assembling the ratchet to the extension "while on the job". Remove the ratchet as soon as you can spin the plug by hand.

[JS!]

This extension from Harbor Freight works perfectly on the #4 plug with no removal of the chaps.