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 oil flow; the main bearings are fully grooved and oil the connecting-rod journals 100 percent of the time. Upon grinding journals to final dimensions, it is not necessary to nitride treat nodular-iron crankshaft journals. Nodular cranks are very durable, and hold up just fine through the 600-hp level, as do the forged-steel cranks. Beyond these power levels, it’s a coin toss as to which will fail first, the block or the crank. I am not saying nodular crankshafts absolutely fail at horsepower levels beyond 600; I have seen them live at higher outputs. I just have never seen a crankshaft break at the 600-hp level, and consider it a safe target for those choosing to use a factory nodular crankshaft.
As of this writing, the only aftermarket production Oldsmobile crank-shaft produced is the big-block crank made by Eagle. It is characterized by Eagle as being a “cast-steel” crank. I have used some of these cast-steel crankshafts in some Pontiac engine builds making about 800 hp (with nitrous oxide) and they’ve never failed. I have found that the grinding job on these units is not necessarily flat, round, or sized properly, and required rework to be perfect. When purchasing these units, you should inspect them closely, and expect to grind them to suit your particular application.
Bryant Racing and Velasco Crankshafts make top-of-the-line billet crankshafts for Oldsmobiles. This 4.600-stroke model is made with premium material, center counterweights, hollow main journals, and many other features. The stiffness of the crankshaft reduces cap walk and main-bearing issues especially on the number-2 and -4 main bearings.
Crower has also manufactured a few Oldsmobile crankshafts. This model has big-block Chevrolet rod journals and a 4.250 stroke.
On most of my 750-hp or more powerful builds, I use Billet Bryant crankshafts that have a 4.600-inch stroke. You can make one with more stroke and get it to fit, but the 4.600 version fits well in the block without too much headache.
Custom-crafted Oldsmobile crankshafts are available from Bryant Racing, Velasco, and Moldex, all of which are experienced in manufacturing premium-quality competition crankshafts for all big- and small-block applications. These crankshafts are expensive; manufacturing lead times range from 8 to 16 weeks, depending on the manufacturer’s plant loading. For those who need them, they are worth the wait, and they represent the absolute peak of quality.
The price of these premium aftermarket crankshafts goes up as you add features. Expect to pay between $2,500 and $3,500 for a billet Oldsmobile crankshaft as of this writing. These more expensive billet crankshafts incorporate lightened, scalloped, and knife-edged counterweights. They feature hollow main journals, a center counterweight, drilled and lightened crankpins, a scalloped rear flange, and a surface-hardening treatment (such as nitriding).
The biggest advantages of using these billet units are less rotating weight and less crank flex. These two points are reasons why your high-performance Oldsmobile engine may live longer once equipped with one.
If your goal is to rebuild a stock street cruiser that will never see more than 3,000 rpm and will use a stock stall converter, there is no reason to read beyond this paragraph. Factory clearances and standard crank-grinding procedures are fine for your application.
However, maximum-performance engines require additional bearing clearance. You cannot check clearances at the time of assembly. By then it’s too late. Proper clearances must be designed into the build and be part of the plan from the start.
You can’t just take your crankshaft to a crank grinder and tell him to grind your crank 10/10 (.010 inch removed from the main-bearing journals and .010 inch removed from the rod journals). You will get your crankshaft back with standard crank journal diameters and should result in factory clearances with a standard bearing. Performance Olds engines require additional clearance. Your typical high-performance Oldsmobile engine cannot live with standard factory clearances.
As we know, all gasoline production Oldsmobile blocks are weak. Factory crankshafts, both iron and steel, flex when used in performance situations. As more torque is produced, both are stressed and the crank starts to bend and flex like a piece of linguini while the block is also flexing and moving. Therefore, allow enough bearing clearance so the flexing doesn’t result in the engine clearancing itself because of the crank journal contacting the bearing. The higher the RPM, and the greater the weight of the reciprocating components, the more the connecting rods stretch and the sides of the bore close in. This requires more bearing clearance