So I'm changing out the plugs on my, later 60's 225, when I notice a HUGE difference in plugs. The plug on the right, obviously, is what came out. The new one, NGK ZFR5N, and the older Denso T14PR-U. Forget the significant length difference, what about the tapered seat?
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1963 Plymouth Valiant 225, convertible

Close up of hole.

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1963 Plymouth Valiant 225, convertible

The tapered seat plugs were for the later tubeless heads. If your 60's engine has its original "drool tube" head (the removable plug tubes that use the rubber seal) and you were using the tapered seat plug, then you were using the wrong plug for it.

I bought this engine used. I don't know what year it is. Can you tell by the whole if this head will take the newer plug? I'd hate to find out after special ordering these plugs, and cutting the crush washers off, that I can't use them.

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1963 Plymouth Valiant 225, convertible

Sorry I didn't see your close up pic earlier, it showed an error. Now it shows clearly. Yes that is a later model (1975+) head. The ZFR5N is not appropriate, that's for the older heads.

The stock replacement taper seat NGK plug is a UR4. If you wanted fancy you can get the iridum UR4IX.

Be glad you bought the cheaper zfr5n and not the zfr5fix-11 ($7-9 each).

Where are the head casting numbers? Under the valve cover? I used a very in scientific way of measuring the threaded length of the spark plug holes. It appears to be the length of the newer plugs. Is it possible that the old plugs were wrong? Or am I just wishing, really hard?

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1963 Plymouth Valiant 225, convertible

Yes that hole in the head is for a 1975+ BL plug .... do not install the ZFR-5N plug it does not have the tapered seat, and the thread length is longer than the depth in the head...the only way you could make them work is if you machined the head to accept the larger body and spot faced the plug receiver and used the crush washer...a lot of $$$ that could easily buy and recondition a pre -1975 head.

Note:

Casting number is under the valve cover.

Start with the stock UR-5 since that is the middle ground for that plug...Ur-4 is hotter (better if you have oil fouling), and there is a UR-6 if you need colder....

Your wishing really hard. I'm telling you without a doubt the pic you showed is the later head that takes the taper seat plug.



This is one of Doc's pics - this is the early head. There is a stamped aluminum tube that connects the outer head surface to the plug seat that isn't shown. The ZFR5N is for this type head. Clearly different then yours.

Edit: oops, looks like DI and I posted at same time.

Damn. I definitely see the difference. My old 170 has the tubes. Well I have brand new set of plugs now. I guess the car that this came out of, got it from a newer car? I thought it was a late 60's Duster? It obviously had an engine transplant too.

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1963 Plymouth Valiant 225, convertible

But you got pics to help!!!!

Jinx!!!

There it is, plain as day. Thanks for your help.

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1963 Plymouth Valiant 225, convertible

Curious: what did we learn from the head casting number?
Also, if the longer threaded plugs were used in the newer peanut head, would the piston hit the plug tip?

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Nope. Not a late 60's duster. Duster's existed from 70-76. That style head was used from 75 onwards. It's also possible to swap heads, so you may have an older block with newer heads.

No, on stock 225's the piston is so far down the hole it's almost comical...a peanut head guy could install a set of TR-4/5's which have a bit more thread (they are for the late 400 Chevy I think with the tapered seat), the problem that occurs is the end of the threads are exposed to the combustion process, so if they get carboned up or damaged from detonation you won't be able to wrench them out without issues with the threading on the head, or you might have to remove the head to clean them and get them out properly. They will be to long if the engine has been cut for a high 9-10's SCR.

You can even install a set of those 1" long threaded plugs for the modern Corvette, but those will clash with the pistons on the stockers.

With proper timing (distributor recurve), there is no need to run anything but the stock plugs in the late engines...