The Crankshaft Journal Radius

Crankshaft Journal Radius Close-Up View
As most automotive technicians and machinists understand, the components inside an internal combustion engine are subjected to a lot of force. Crankshafts derive much of their strength from the material they are made of. For example, most passenger car crankshafts are made of cast iron while performance crankshafts are made of 4340 forged steel and even billet steel. Though the composition is responsible for much of the crankshaft's strength, each main and rod journal also has a radius that enhances its ability to withstand significant amounts of force from the combustion process.

The Role of the Radii

While the purpose of radii is to add strength, many people do not understand how a small radius can really improve the strength of a crankshaft. Quite simply, force is distributed throughout an entire radius. If no radii (plural of radius) were present, then that force would be targeted onto a very small point where 90° angles originate. Clearly there are advantages of spreading force throughout radii to eliminate weak points within crankshaft journals.

Pictured above you will see a close-up photograph of a main journal radius. By clicking on the image you may view a larger image with an arrow pointing directly to the journal's radius. Nearly every automotive crankshaft we have seen through our automotive machine shop has some type of radius. Many passenger car crankshafts have a 3/32” radius while performance crankshafts tend to have a larger 1/8” radius. Other crankshafts may have a rolled radius, which is rolled into the ends of each connecting rod and/or main journal.

When grinding a crankshaft, great care must be taken to preserve the existing radius. Often the grinding wheel must be dressed according to the existing radius, which is confirmed by using a specialized gauge. If the radii is not maintained, a freshly ground crankshaft will lose some of its strength, which is something we as automotive machinists are consciously aware of at all times throughout the machining process.

Before machining any crankshaft, it is wise to inspect it with a magnaflux machine. Such a machine allows us to apply a magnetic charge to the crankshaft, which will highlight any cracks under a black light bulb, after we apply a metallic spray solution to the journals. As you may suspect, the radii are the primary areas where surface level cracks can be found. If cracks are found in any radius, the crankshaft must be replaced as no weld can satisfactorily repair a crack of unknown depth.

Now that you understand what a crankshaft radius is, and its role in the strength of a crankshaft, we encourage you to visit our terminology page to view other areas of importance on a common crankshaft.
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Journal Radii