The N-18 (N18), which has a 3/4" reach, was originally specified for ALL 56th Series models.
The H-10 (H10), which has a 7/16" reach, was originally specified for ALL 55th Series models.
However, there is an exception that involves ONLY 1955 Clipper Deluxe and Super models - a running change in cylinder head design for the 320-cid V8, which was covered in
SC Vol. 29, No. 7 (duly noted in this site's EXCLUSIVE
Packard Service Index). Spark plug supplier catalogs seem to be ignorant of details regarding this change.
Meanwhile, Kanter's staff advised me, about 25 years ago, that H14Y was the replacement for the H10. However, I found H10s readily available at the local parts store, back then - allegedly used in agricultural applications. I can't speak to present availability of any spark plug, today.
Also, DO NOT use a LONG reach plug in an application that specifies a short reach plug.
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