when you here nodular, think nickel.
they have a higher nickel content than typical housings. Nickel enhances the castings strength/density. This term is also used when describing certain early model and aftermarket engine blocks (from variety of mfgs.).
Another nicety of most "N" housings (common only to Ford 9" drop out centers) is that they were coated on the inside with a slipery orange colored coating to enhace oiling of R/P (ring and pinion).
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