The main strength advantage to an aftermarket crankshaft is the increased journal radius..... stock cranks actually have a undercut here
as this radius is increased the bearing shells must be reduced in width to allow for
the oil to exit, bearing manufacturers offer race bearings designed to clear for most popular applications.
Tensile Strength- see chart above
Forgings generally produce a flowed grain structure, which is stronger than a casting. Even so, the forging process stretches, pulls and deforms the grain structure, and subsequent machining cuts through the grain structure. The strength of the forging also depends on the metallurgy of the alloy used, and the heat treatment that is applied to it after it has been shaped. Forgings require a die to shape the metal. Dies and forging presses are expensive (which adds to the cost of the crankshaft), so the availability of forgings for various applications depends on their popularity and how much people are willing to pay for a forged crank.
by comparison, are CNC machined from a solid chunk of forged steel. The grain structure is not stretched or deformed, and machining leaves fewer residual stresses in the metal. Consequently, some crank manufacturers say billet cranks are the strongest cranks available. Most Top Fuel drag racers run billet cranks, as do many circle track racers. Another advantage with billet cranks is that CNC machining allows a crank to be custom made with virtually any stroke, journal diameter, configuration or countershaft placement that will fit the engine. A billet crank can be one-of-a-kind or mass produced.
Most racing cranks are heat treated and case hardened to provide additional strength and durability. The journal surfaces may be hard chromed, nitrided or induction hardened. Some crank manufacturers use a "plasma nitriding" process that vacuum-deposits ionized nitrogen on the surface of the crank inside a high temperature oven. Others use a process called Tufftriding that soaks the crank in a hot "ferric nitrocarburizing" salt bath, or heats the crank to 950° F in an oven filled with nitrogen. Nitrogen penetrates the surface of the metal and changes the microstructure of the steel. This roughly doubles the hardness of the surface from about 30 to 35 Rockwell C to 60 Rockwell C, and increases fatigue life up to 25 percent or more.
Surface coatings like nitriding and nitrocarburizing can increase the surface hardening to any number necessary. And there are coatings that will repel oil from the non-machined surfaces to help improve the crank's ability to move through the air with the least amount of parasitic drag. Another choice that is seeing more use is cryogenic processing where ultra-low temperatures are used to alter the state of the metal. The real advantage is that this is not a surface-only process so further machining will not eliminate the beneficial effects.
Another area of the crank that receives a lot of attention is the un-machined surface of forged cranks. To take another step closer to perfection with a forging the un-machined surface can be polished to remove the stress risers from its surface. Then those same surfaces can be shotpeened to form a tough skin over the surface that increases its fatigue resistance. Note - When these steps are included the crankshaft appearance is unmistakably different from the stock appearance
Identifying crankshaft material / manufacture :
Cast ( stock )
Forged Wide Line
stock and lower cost Aftermarket
Premium forged-steel race cranks
* KEITH BLACK
Premium forged-steel race cranks
(Note mallory metal to balance)
( * These names will be stamped on the aftermarket crankshafts )
CARRILLO( K1), BRYANT, VELASCO, KEITH BLACK, CROWER, LA BILLET, EAGLE, AND SCAT
A) Knife Edged counterweights
B) Gun Drilled (lightened) center of crankshaft is drilled from rear to front snout - frost plug is installed at rear so oil doesn't exit motor
C) Full counterweights on center arms - less flexing
D) Lightened throws
KB crank is also micro polished, has a peened surface, nitrated, hardened and double keyways that are separate from cam gear keyway
Left - Forged stock 440 crankshaft Right Keith Black Billet crankshaft
Rod Journal Sizes
Main Journal Sizes
Wrist Pin Sizes
2.750" / 3"
3.5 "- 5"
1.095" / 1.156
Dodge Hemi *
Dodge RB-440 *
Dodge B-400 *
Chevy BB rods on aftermarket crank
3.380 " / 4.75"
7.120 " +
Chevy BB rods on reground stock
1.015"( ? )
2.060"( ? )
3.380 " / 4.15"
( ? )
* = stock
Note after market rods may have additional side clearance and chevy rods add .040 to side clearance
A little information from a reliable manufacture :
Failures: All bearing failures are caused by one thing. A lack of oil between the bearing and shaft. Of course, why that occurred isn't always easy to determine. We continue to be amazed by the number of ongoing bearing problems that are solved simply by adding more oil to the crankcase. This is particularly true on drysump systems.
Lightening and knife edging: These do nothing to improve the reliability of the shaft, and very little to increase performance. Most racers would be better served to spend their money on other areas.
Bearing Information :
Mahle/Clevite has a very nice guide to help you diagnose bearing wear patterns and failures. To view their guide, click HERE