Based on what I read here, I went over to Napa to order some ZFR5N's. My friend who works there was very reluctant to order them for me because they aren't the "correct" plugs and he says the extended electrode will give me problems. He was sure that I had the wrong number or something, so I told him to wait and I'll check again. So here I am asking what is probably a stupid question: is the ZFR5N recommended for use with a 1970 225, or not? has anyone actually had a problem with this spark plug?

The ZFRN is "stock" for most AMC/Jeep applications and the late Dodge engines. These seem to work great in stock/close to stock applications where mixture problems and low compression stratify the air fuel mix in the combustion chamber...they were designed to work most efficiently with the turbulence caused by the closed combustion chambers fo the late heads, so on the highway/at high speed these work better...A series of exchanges over on one of the late Dodge Truck forums also had speculated that the extended tip would also place the flame kernel at a point that would be similar to advancing your timing a couple of degrees (so in essence the flame would start at a spot that the standard plug's flame front wouldn't reach for a few fractions of a second)... Where the plug does not work is the higher compression engines (so depending on your build if you are running super as the lowest octane to avoid ping, these might not be your plug).

If you have a stock car with 1 barrel, give them a try... it's part of "super tuning" your engine...it's not different than having a new engine build and having to buy a set of plugs that are hotter and colder than your regular set to see what the engine really likes. Many people on here have used them, I have used them and they worked fine in my stock application, they did not work well when my compression ratio went over 9.5:1...

If your friend is worried that the extended tip might impact the piston...let him know that the typical engine from your era has a -.17-.185 deck height and that the valve at max lift is still going to be .2 from the deck at least...having "mocked up" other lengths of spark plugs...you will need to worry if someone screws in a set of plugs from one of the late corvettes....

Stay away from the autolites, install and test drive the car...if you get a little hesitation you may need to adjust the carb a bit and then test again...If worried...find a "test" road...warm it up drive it for a while, kill the ignition, put in neutral, pull to side of road... pull a couple of plugs #1,#3, #5...and check the color...if tan/green you should be OK...if white or ashen you might need to richen the carb a bit....

This all assumes that your car is in good working order (no burning oil, etc...).

-D.Idiot

Thanks for all the info. The car is a 1970 duster that I just picked up last week, 52,000 original miles, owned by an old lady until a couple years ago, it was driven and tuned up from time to time a little rather than just sitting for a decade or more at a time (I have the service records for the past 40 years). However, when I got it the carb was poorly tuned- mixture screw all the way in and idle turned way up high to compensate. I did get the mixture and idle dialed into the ballpark at least and the engine now runs very smooth. Seems like a very good healthy engine.

I still have a lot of investigating and tuneup to do, but spark plugs are cheap and simple so I figured I would go ahead and toss some new ones in there. Right now it has champions in it. I still have to check the valve clearance, timing, points etc. and I will probably end up getting new plug wires and a distributor cap too, though I might wait on it because so far the engine does seem to be running really well. I will have to see what gas mileage I'm getting and if I'm happy with it I might decide "if it ain't broke don't fix it".

Welcome on the board. ZFR5N is yes, and you'll need to remove the metal ring washers. Thank your friend for helping make sure you get good parts and tell him to give you what you asked for. Tune-up parts and technique suggestions in this thread. The engine will need periodic valve adjustment. Get the books described in this thread. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

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