Cast Iron Crankshafts

Cast iron crankshafts are most commonly found in ordinary passenger car engines and are favored by automobile manufacturers because they are inexpensive to produce. A cast iron crankshaft is made from molten iron or steel and simply poured into a mold. The raw casting is then rough machined so that it can be ground to its finish dimensions and then balanced. These types of crankshafts are relatively inexpensive and can be purchased new for $200 or even less. Since cast iron crankshafts contain flakes of graphite flakes, these crankshafts often have a grey visual appearance.

Nodular Iron Crankshafts

Nodular iron crankshafts are manufactured in the same way as cast iron crankshafts, but the composition of the iron is slightly different. The inclusion of graphite nodules, instead of flakes, adds strength to this component. Cerium and magnesium are also added to further strengthen the crankshaft. Nodular iron crankshafts typically have a carbon content of 3.3% to 3.4%. Many aftermarket cast crankshafts are composed of nodular iron materials for added strength. Although nodular iron crankshafts cost slightly more than a cast iron crankshaft, the added expense is well worth the increased tensile and fatigue strength of the crankshaft.

4340 Forged Steel Crankshafts

Unlike cast steel, forged crankshafts are produced by using hydraulic presses to compress molten steel into its final shape using dies instead of molds. Unlike a casting process, which leaves a sandy grain in the crankshaft, forged steel crankshafts, have a uniform grain structure that adds a significant amount of strength to the final component. Most forged crankshafts are made from 4340 steel, which is a low alloy steel that containins nickel, chromium and molybdenum. 4340 steel can also be heat treated and does maintain an acceptable fatigue strength for moderate to high performance applications.

Billet Steel Crankshafts (4330 Steel)

Billet steel crankshafts are by far the strongest crankshaft available and are used for extreme high performance applications such as competitive racing. A billet steel crankshaft starts out as a large round ingot of forged steel and is machined to produce a parallel grain that is consistent. The grain structure of a billet steel crankshaft makes it more durable than any other crankshaft option currently available. However, this strength comes at a cost that can exceed $2,500.00 per crankshaft. It is important to note that the steel used to produce billet crankshafts also commonly differs from forged crankshafts. 4330 steel, for example, is a nickel, chromium and molybdenum alloy steel. The carbon content in this grade of steel is on average .30%; making it quite durable for engines that are designed exclusively for race applications.

Please note that specific steel designations, such as 4330 and 4340, refer to SAE International’s specifications regarding the grades of steels, their composition and metallurgic qualities. While there are various types of materials used to produce crankshafts, of varying fatigue and tensile strengths, they are typically hardened to produce a final crankshaft that is even stronger.

For performance applications, crankshaft flex is not uncommon at high RPMs. Forged and billet steels are better equipped to flex and return to their normal state. Cast and nodular iron cranks, on the other hand, are extremely brittle and more susceptible to cracks.

Identifying cast and nodular iron crankshafts is quite easy when you look at the casting lines. Thin casting lines, as in the image to the left, indicate that the crankshaft has been produced using a casting process. Steel forged crankshafts, on the other hand, tend to have wide forging lines. Billet steel crankshafts are typically smooth and will not have any visible casting or forging lines.

Now that you have learned more about the different types of crankshaft material compositions, and how to visually distinguish them from each other, please take this opportunity to rate the information on this page below.

In the diagrams below you will find coded arrows that point to specific areas on the crankshaft. The arrows are identified by industry standard terminology used to describe the area being viewed. A brief but concise description of what this area is responsible for is available below the crankshaft diagrams.

Balancing Holes

When the crankshaft rotates, at high RPMs, vibration can occur. Balancing the crankshaft, which requires that weight is either removed or added to the crankshaft, is often accomplished with the pictured balancing holes.

Connecting Rod Journals (Pins)

The connecting rod journals, often referred to as pins, are where the connecting rods are attached to with bearings. To maintain adequate timing, the connecting rod journals maintain a specific degree apart from each other, which does vary for specific engines and ignition firing orders.

Counterweights

Counterweights, as their name implies, adds weight to the crankshaft so that it reduces vibration at any RPM or position. Counterweights help to offset the weight from connecting rods and pistons.

Crankshaft Bolt Hole (Balancer Bolt Hole)

The crankshaft bolt, or as is commonly referred to as a balancer bolt, is used to secure the harmonic balancer (damper) to the crankshaft. The balancer bolt hole is located on the snout of the crankshaft and is typically torqued to 150 ft lbs or greater on many crankshafts.

Flywheel/Flexplate Bolt Holes

A flywheel or flexplate contains a ring gear which a vehicle’s starter turns when the ignition is turned into the starting position. Manual transmissions use the flywheel surface to engage the clutch so that the vehicle will move. Regardless of what type of transmission your vehicle has, these bolt holes are used to secure either the flexplate or flywheel to the crankshaft.

Key

The crankshaft key, which fits into the keyway as a press fit, is an important part of the crankshaft as it helps to align the harmonic balancer in the proper position. In addition to the properly locating the harmonic balancer, the key is also an important engine timing reference point.

Keyway

The keyway is machined into the crankshaft snout and accepts a special key to help align the crankshaft timing gear sprocket and harmonic balancer in a precise location.

Main Journals

Each main journal of a crankshaft runs in line with each other. This is the portion of the crankshaft that bolts into the engine block by main caps, with bearings installed, and must run true.

Oil Passages

Oil passages on the connecting rod and main journals help to feed oil directly to the bearings. The thin film of oil that forms between the bearings and the journals is what protects the crankshaft from damage.

Pilot Bearing or Bushing Hole

Manual transmissions utilize an input shaft that aids in the alignment of the clutch assembly to the flywheel. The input shaft alignment is made possible with the use of a pilot bearing or bushing, which is normally a pressed fit into the hole on the crankshaft’s rear flange. Because manual transmissions use a torque converter to connect the flexplate to the transmission, the use of a pilot bearing or bushing is not necessary, although some crankshafts have these installed from the factory even though they are not used.

Radius or Rolled Fillet

On each journal, where the bearing surface meets the counterweight, there is a radius or rolled fillet. Although small and typically measured with a radius gauge, this area adds a great deal of strength to the crankshaft. By minimizing 90 degree angles on each journal, the force of the combustion process is evenly distributed throughout the crankshaft.

Rear Flange

The rear flange of the crankshaft provides a strong surface area to accept the flexplate or flywheel bolts. The rear flange is often machined to help in balancing crankshafts as well.

Seal Surface

The seal surface on a crankshaft is responsible for helping to keep oil within the engine. The rear main seal is installed in the last main cap and gently rides over the seal surface to eliminate any voids where oil may exit the crankcase.

Seal Surface Oil Grooves

Especially on those engines which use rear main seals made of rope, the directional grooves help to redirect oil away from the seal as the crankshaft rotates. Many crankshafts do not have these grooves as they use a more efficient rubber seal.

Snout (Nose)

The crankshaft snout, or nose as many people refer to it as, provides a location for the crankshaft timing gear sprocket and harmonic balancer to attach to. The crankshaft snout contains a keyway and key so that the sprocket and balancer may be accurately positioned to ensure proper timing and balance.

Thrust

Every crankshaft installed must have a minute amount of clearance between the journals and bearings for oil. A crankshaft must also have a limited amount of backward and forward motion, which is commonly referred to as endplay. Most crankshafts are installed with .005-.010” of endplay, and the thrust surface of the crankshaft is what prohibits excessive endplay.

It is our hope that the diagram and terms noted above will better help you understand your crankshaft and the various parts that are often referred to as being in need of repair. If you found this information helpful, please consider rating this page below.

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