correct size description for old spark plugs

[mrpushbutton]

say gang, I'm about to list these old spark plugs online and want to know the correct size designation. The larger two on the left measure .872", which converts to 22.148 mm (22mm?) and the smaller two on the right measure out to .7" which converts to 17.78 mm (18mm?)

what is the correct designation for these? the Champion on the right is a C-7

[oldford]

The two larger plugs are 7/8" thread size and the two smaller ones are 18 mm. I can use the 7/8 AC. I just bought one like it at the Quakertown swap for $5. It too was NOS.

Frank

[MCHinson]

Is the second one from the left perhaps labeled Champion 3X?

[mrpushbutton]

The second from the left is not labeled anything--no evidence of any printed part number, manufacturer, nothing.

[MCHinson]

It looks like a Champion 3X to me. (They were used in Model A Fords.) I have never seen one that was not labeled though.

[Willy]

Just a bit of info - I have a 1927 Whippet, and I can buy spark plugs - in stock, from Napa. They also fit tractors, which is why Napa carries them, but they have cross -referencing. They are pricey, tho - around $6 - 7, but it's nice, because if there is something wrong with them, you can take them back. Can't do that at a swap meet. I can also get spark plugs for my 1938 McCormic Deering there.