Finding the correct spark plugs.

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J

jonnie Gofaster

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Hi all, I'm a first-time poster and I have a "Head Scratcher". I'm a relative novice, not a true gearhead. I'm building a 1969 318 to swap into my 1974 Plymouth Valiant slant 6. I ordered spark plugs - RN14YC, but the diameter of the threads (14mm) is too small. The plug falls right into the threaded hole. I went to a parts store, and they confirmed the plug reference number and brought out the same plug I already had. The guy cross-referenced it, and came out with a 18mm threaded plug, but the reach was different. What the heck is going on?!!
 
Thise should be the right plugs for a 69 318 unless you have an engine out of a truck. (like a motor home) which may have the big plug heads.
 
He says 69 318, poly's stopped in 66, unless maybe Canadian because they continued with poly's for a while.
 
Sounds like the engine has motor home or industrial heads on it, which can have 18mm plugs.

Those heads usually have a water crossover where the exhaust heat cross over is.

Might be worth pulling a valve cover and getting a casting number off the head so you can ID what you have.
 
We could use some photos. Are the plug holes machined for a tapered seat plug, or flat gasket type?
 
One of the plugs I pulled a few months back. It has a crush gasket. I cross-referenced it and found a couple dozen options.
1724789031520.jpeg
 
You should carefully inspect the area where the spark plug seats against the head to see if it’s flat(to use a gasket), or tapered(tapered seat plug) no gasket.

The reason I mention that is because it appears that Champion plug you have there isn’t supposed to use a gasket.


Edit-
The digging around I did showed a 67-74 318HD used tapered seat 18mm plugs.
To be used in a non-HD application, I’d suggest some NGK WR5’s
39EE01F3-AEFA-4116-A843-F35D8A10C846.png
 
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I found the correct spark plugs for my project in my parts pile...they are likely only 20 years old .

I'd use the new ones I have but those are even older.
 
Age doesn't matter with spark plugs. It's heat range and mileage that makes a difference.
 
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