This is from a 1941 shop manual
" The spark plug used in this years engine is of a 10mm size instead of the 14mm size used previously.The smaller size spark plug warms up faster but operates cooler under load. The heat range of the 10mm plug is about that of a number "47" during the warming up period but on heavy service such as in trucks and at high speeds it about equals a number 42 plug in the 14mm size.
In the 1941 chevy truck data book it says it used ac plugs.
I am learning as we are going so correct me if I am wrong. Good thread.
Rob
