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You are looking for a nodular iron case, with its telltale “N” cast into the front or inside wall. These third members are the strongest of the production cases and the most desired. Aftermarket vendors are actually re-casting these cases in very strong nodular iron, so more often than not it is easiest just to purchase a new case.

But if you’re looking for an original, they can be spotted quite readily even without an axle identification tag. The N cases had two vertical ribs, three horizontal ribs, and a machined-in fill plug. There are four versions of the nodular iron third members that were produced from the factory. These first became available on the 1964 Galaxies with the 427-ci engine and were found on many higher horsepower cars and trucks.

Of the four versions of the N case, three have an actual “N” cast into it right above the pinion cartridge.

The fourth version did not have the “N” in the casting on the outside but still retained the rib structure. In order to verify a nodular unit, you need to look on the inside of the casting. These have a C4AW-4025-B casting part number next to the adjuster nut.

Of course, if your budget allows, you can purchase an aftermarket iron case that is stronger than any factory case. If you are looking for an N case or think that you have found one of these desirable axles, make certain to examine it closely. The WAR, WAA, and WAB cases have the same ribbing as the N case, but are missing the ever important “N” feature.

Any of the W-series cases do have a fill plug machined in them. Both standard and W-series cases are made out of gray cast iron. Only the N-series cases are made out of the stronger, more desirable, nodular iron. While the iron is molten, magnesium is added to increase the shear strength of the alloy. The magnesium addition causes the grain structure to change from flakes to nodules and thus the name “nodular” iron. The N cases typically came with the larger Daytona-style pinion support bearing, and 31-spline axle shafts.

This chart references the connection between the month codes and the actual month. The letters I and O are skipped to avoid confusion with the numbers one and zero.