1931 Plymouth spark plugs

[Bill Davis]

According to the NAPA Auto Parts website, Autolite #216 is for the
aluminum head and Autolite #295 is for the cast iron head.

[32cabrio]

AC 45 is what came on my '32 PB. I don't know if they are correct for '31 but I think the engines are pretty much the same. Others may have more accurate information.

[TodFitch]

My notes indicate the original were AC G12 plugs with 18mm threads. http://www.ply33.com/Models/PA/specs.html

Not sure what the modern cross reference for that would be. But some older cross references are at http://www.ply33.com/Parts/group8#607121 You could maybe start with those at your local auto supply to see if they match up to anything.

If your auto supply can't match based on the above, then: On my '33 (which uses a 14mm plug) a gasket is used (versus a tapered seat). Assuming that is true, I'd probably start with something like https://www.summitracing.com/search/brand/ngk-spark-plugs/part-type/spark-plugs/universal/yes/spark-plug-thread-size/18mm/spark-plug-seat-style/gasket?N=400395%2B4294926774%2B4294951518%2B4294926719%2B4294926765&SortBy=Default&SortOrder=Default

You'll need to measure how long the threaded distance is on the head to determine the reach.

[Lincoln52]

Can someone tell me the correct spark plug for a 31 Plymouth PA?
Thanks!