Why the 10mm spark plugs?

Posted by Dave Brownell On 2017/1/9 12:20:45

Compared to the great Packard mysteries (e.g. Why did Packard go out of business?), this is almost non-consequential in importance. Why did Packard go from a standard sized sparkplug in 1937 to a much smaller 10mm plug and have that last until mid 1950 production, when it went back to a 14mm? I have looked through the various Counselor and other service documents, including the Bob Aller CDs, but cannot find the justification for these changes. How prevalent were the smaller plugs throughout American car makers of the eras? Did the smaller plug fitment apply to all the Packard engines of the period, or just the 282 and 288 c.i. ?