Crankshaft Rod and Main Journals

Illustration of Crankshaft Journals

One of the most critical areas of a crankshaft is its rod and main journals, which are often referred to as pins. Journals are the surface areas where main and rod bearings are used, which requires that precision tolerances be maintained. Ensuring proper bearing clearances is accomplished by making certain the crankshaft's journal diameters are within specification, that there are no cracks in the radii and that the oil feed holes are free from obstructions.

Generally speaking, a properly maintained engine will protect rod and main journals from wear. Since an oil barrier occurs between the engine bearings and crankshaft journals while the engine is running, the journals should never come into direct contact with the bearing. In fact, an engine that has regular oil changes and regularly scheduled service can often last for hundreds of thousands of miles. On the other hand, improper care of an engine can cause catastrophic engine failure in a brief period of time.

Orientation of Journals

The number of journals does vary by the type of engine and number of cylinders that it has. However, all crankshafts have main journals that are parallel to each other (inline). Connecting rod journals are offset to accommodate the engine's stroke. All journals are machined for side clearance as well, which limits endplay for connecting rods, main caps and also the thrust.

Understanding Journal Wear and Repair Options

Changing an engine's oil regularly is essential in protecting all of the internal engine parts, especially the rod and main journals. For example, sludgy oil can restrict the flow of oil through the rod and main journal's oil feed holes. Such restrictions can weaken the protective oil barrier and cause damage because of improper lubrication. A failed or defective oil pump can also cause similar damage to the journals rather quickly.

As automotive machinists, we often see crankshaft journal damage resulting from a wiped lobe on a flat tappet camshaft. Although oil filters help protect engines, they do not trap all particles and the debris from a worn camshaft lobe can quickly find its way between the bearings and journals. This debris often causes gradual damage, such as producing grooves in the journals and premature bearing wear. As more grooves are created and/or get deeper, the oil pressure begins to drop and this protective barrier loses its ability to lubricate the engine. Once the bearings are so severely damaged that their outer layers have been worn away, the crankshaft journals may come into direct contact with what is left of the bearing(s). When this situation occurs, and the bearing heats up, it's quite common for the bearing to catch the journal and spin. This is where the term "spun bearing" comes from.

When engine bearings spin and journals wear, the heat produced from this can also bend crankshafts or cause them to fatigue and crack. Although a bent crankshaft can be straightened so that the journals may be machined, cracked crankshafts should always be discarded.

In most cases, a slightly damaged crankshaft can be repaired simply by grinding the journals. When journal wear is so severe that oversized bearings can't be used, then the damaged journal can be welded and the entire crankshaft ground, polished and put back into service.

Now that you know more about the importance of a crankshaft's rod and main journals, and why they get damaged, we encourage you to visit our "what is a crankshaft page" to view the rest of our educational series pages about crankshafts.

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