Guys please take a look at Mercury Master Spec Chart?
KD-4 shows Champion J7-J spark plug, J8-J for singles.
Anyway here is a pic of four different spark plugs side to side.
AC 45M, Champion J8-J “Per Box” NOS (Phos Plated), Champion J8 (Zinc Plated), Champion J8-C (Zinc Plated)
The AC 45 M is actually shorter than old style J8-J
Looks like the post war (Zinc Plated) Champion J8-J or J8 will work, but must have removable tip (Red & White Packaging) Not Blue & White (Fixed) solid “Non- removable tip”!
I also find you have to use a smaller diameter brass nut for proper clearance!
