Packard service literature in the early 1930's recommends AC spark plugs--- type K9, K10 or K12 for my 1932, depending on the heat range.
I've yet to find a list of what the modern designation for these plugs would be.
These are 14 mm plugs but what's important is the heat range --- K9 is cooler, K12 is hotter --- and especially the reach into the cylinder.
I'd like some NOS plugs. Nothing much on eBay. . . .