Its because of the firing order, but I think there may have been some with even numbers that have been tried. That 28 cylinder R-4360, has 7 cylinders in each row, so it is uneven number of cylinders. Unlike something like the R-2800 that is only 2 rows, the 28 cylinder job has 4 rows.I was told that radial engines can't have an even number of cylinders because of the way the rods hook up to the crankshaft? Somewhere on YouTube there is a video of a rebuild on a Wasp engine. I'll also have a look when I go down to the aircraft museum at Boeing field next time.