These three versions are all dimensionally and functionally interchangeable. Although the steel crank is desirable for roughly 600- to 800-hp small-block engines, the nodular-iron crankshafts are very durable in the range of build that the gas 350 block will tolerate. The 350 Olds main bearing webs in the block will likely fail before your 350 nodular crank. The nodular-iron cranks support about 600 peak horsepower. The gray-iron cranks are well suited for stock and moderate performance builds up to about the 400-hp level.
Factory Big-Block Crank Identification and Application
The 1964–1967 “short-stroke” 400-ci engines were produced with forged-steel cranks and are the same as their 425-ci brothers. Later-model (1968–1969) “long-stroke” 400-ci engines could have either cast nodular-iron or forged-steel crankshafts. Most of the big 455 Olds engines produced from 1968 to 1972 were equipped with a nodular-iron crankshaft, but a forged-steel unit can be found. There is wide speculation on exactly where these forged crankshafts were used, but ultimately, there seems to be no specific application where you can be sure to find one, and they’re very difficult to find.
The thin parting lines on the ends of the throws indicate 350 nodular-iron crankshafts.
Notice the undercuts in the ends of the rod and main journals on this 350 N crankshaft. At a quick glance, even with no micrometer, you can tell if the shafts have been ground undersize.
The 330 Olds crankshafts were all steel with the 3.385 stroke. They always have the wide parting lines at the ends of the rod journals and have a smooth finish compared to cast-iron crankshafts.
This part number, 388776, identifies a 330 Olds steel crankshaft.
The older 400 short-stroke and all 425 Olds cranks were steel and appeared very smooth (shown). You can verify that it is a 400/425 versus a steel 455 crank by the lack of a lightening hole in the side of the rod journal.
As with small-block cranks, steel big-block cranks have a smooth look, a wide parting line, and rounded crank-throw noses. The cast 455 cranks have narrow parting lines and squared crank-throw noses. The 455 steel crankshaft is extremely heavy. The weight possibly negates any material advantage there may be over a nodular-iron unit in applications less than 600 hp.
The 400/425 needs the counterweights cut down about .300 inch (shown) to fit in a DX block. You only want to cut as much as you need to fit in the block to avoid having to add expensive Mallory metal.
Depending on the bobweight of the rotating assembly, you need to install Mallory slugs in the counterweights. Expect to pay about $25 to $30 per slug plus the cost of installation. A balance job can be pretty pricey.
The 1973–1974 455 crankshafts could be either nodular iron or gray iron, and they were made exclusively of gray iron material until the end of the original 455 production run in 1977. As with the small-block crankshafts, the nodular-iron 455 cranks are identified by a large “N,” “NA,” or a small “CN” cast into the number-1 counterweight. You may notice that “CN” crankshafts have four lightening holes in the rod journals rather than two holes in the front and rear rod throw as on “N” cranks. Most of these are also machined for pilot bushing installation. The more of these I see, the more I think that they are the nicest of the Oldsmobile 455 cast crankshafts.
This “CN” crankshaft is easily identifiable by the four lightening holes in the rod journals. They are the lightest, best-cast Oldsmobile crankshafts of all.
The nodular cast crankshafts for 455s have a rather large “N” cast into the counterweight. In addition, they can have a small “CN” cast into the counterweights on later models. If you cannot find either, you have a low-performance, late-model crankshaft.
Unlike the small-block family, 425 and 455 cranks do not directly interchange. The 425-ci version has a 3.975-inch stroke with 2.50-inch-diameter rod journals, 3-inch-diameter main-bearing journals, and a flywheel/flexplate bolt pattern that matches the old 330 crankshafts. The 455 has a 4.250-inch stroke with 2.50-inch rod-journal diameter and 3-inch mains, and is drilled for the later flywheel bolt pattern that matches all the later small-block flanges.
The 455 forged-steel crankshaft is the heaviest crankshaft; these weigh about 90 pounds.
The 455 forged-steel crankshaft is very rare and often described as rather ugly.
This 425 crankshaft was knife-edged on the CNC machine. Forget doing this operation by hand unless you have nothing but time.
The wide parting line on the rod journal and lack of a lightening hole easily identifies a 400/425 crankshaft.
The easy way to identify a 455 forged-steel crankshaft is by the lightening hole in the rod journal.
Both the steel and nodular-iron crankshafts typically support more horsepower than the 455 blocks handle in ungirdled applications.
The biggest single performance improvement that can be made to the factory Oldsmobile crankshaft is to have the crank properly and professionally ground. This is the only machining investment necessary on a factory cast-iron crankshaft.
It is not necessary to cross-drill factory Oldsmobile crankshafts for increased
