Help - Spark Plugs 1972 440 RJ12YC

[Karl5203]

Pictures show old plug that is in car now RJ12YC and the new off the shelf RJ12YC at 3 parts stores.
The tread Diameters are wrong on the new plug. Old plug 1.5 centimeter new plug 1 centimeter

What the heck is going on? Looked up 72 AC Delco plug and its too small as well.

Thanks for any feedback.

 

[69Bee]

A couple of things:

1) Get rid of the "Plastic" calipers, they are junk and totally inaccurate.
2) Measure round items perpendicular to the jaws and use the wider flat area for better accuracy.
3) We live in Murca', and we use inches here (even tough plug threads are metric, 14mm-1.25)

That being said, the new plug visually looks proper, and the old plug looks like 18mm like a ford. Seems, I think I have seen an industrial head with larger plugs, but would expect the part numbers to be different.

 

[4speed68rt]

What heads do you have? You are going to have to do some creative cross matching. Like 69bee said, maybe with a Ford 300 six or something like that.

 

[493 Mike]

Contact Champion and ask them.
Mike

 

[kiwigtx]

spark plug cross reference

R = Resistor

 

[lewtot184]

A "J" plug will have 14mm thread 3/8" reach. The big problem I have with champions anymore is their high resistance . An autolite 85 seems to be a little better made. The autolite resistance can vary some but doesn't go the extreme that champions do.

 

[PRHeads]

Have you pulled any other plugs out for a peek?

Looks like the old plug might have a thread repair insert stuck on it?

Edit- after blowing the pic way up, you can see the serrations/teeth of the thread insert that are supposed to help lock the insert in place, in the head.

If you can get the insert off the old plug without damaging it, you could thread it onto the new plug, and install it as a unit back into the head.
Not perfect, but will be the same as it was.

The other option is to get a new insert and try it again.

 

[RemCharger]

Good call PRH.
Yes if that's the case, get a new kit, it comes with a swedging tool, so to speak, that flares the the locking teeth out into the head.
Brake clean it all up, and green loctite it.
I've done aluminum heads, but never a cast iron one .

 

[1967coronet]

Have you pulled any other plugs out for a peek?

Looks like the old plug might have a thread repair insert stuck on it?

Edit- after blowing the pic way up, you can see the serrations/teeth of the thread insert that are supposed to help lock the insert in place, in the head.

If you can get the insert off the old plug without damaging it, you could thread it onto the new plug, and install it as a unit back into the head.
Not perfect, but will be the same as it was.

The other option is to get a new insert and try it again.

View attachment 1536142

Great eye and catch !

 

[BSB67]

No offense to the OP, but just in case you’re not following:

The new plug is correct
The old plug is also correct
The Head has been repaired with an insert because the spark plug hole has been stripped
The old plug has an insert for the head stuck to it.

The head now needs to have an insert reinstalled.

You have three options: get a new insert. Reinstall the old insert, or what PRH suggested, Remove the insert from the old plug and put it on the new plug and install the plug.

 

[RJRENTON]

spark plug cross reference

R = Resistor

Info re Champion spark plugs
BOB RENTON

Champion Spark Plugs Decoded
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