How Do You Date Spark Plugs

[Tom Wesenberg]

At the last swap meet I bought 4 AC spark plugs mainly because they were only a buck each and NOS in the box. They are number 83 SPECIAL, 18 MM thread and 7/8 Hex. I was wondering what time period they were made and what they fit? I know they fit the Model A, but don't know if they are correct for it. The last patent on the AC box is 2,272,338.

What's really unusual is they are sealed in cosmoline, so I'm not sure if I should open one up.

The small Autolite plug is PR-4 10 MM resistor. I know Chevrolet used a small thread for a short period in the 40's, but don't know if this plug would be correct for them.

Plugs change shape on the porcelain every so often, and I'd like to find a chart or pictures to show when these and other plugs might have been made. Does anyone know of a site with such information, or a good book on spark plugs? Thanks.