Crankshaft Oil Feed Holes

Diagram of Crankshaft with Oil Feed Holes Highlighted

As most people know, engines must be properly lubricated to function normally and last for many years. Part of this lubrication process involves directing motor oil between an engine's bearings and the crankshaft. Oil feed holes, which are drilled directly in the crankshaft's journals (pictured left), allow oil to flow to all the journals so that they remain properly lubricated as the engine is running.

How Oil Feed Holes are Designed

Oil Supply Hole in Main Housing BoreTo direct oil to the crankshaft feed holes, the engine block's main housing bores must have holes drilled in them as you can see in the diagram to the right. All engine blocks have this feature, although some may have multiple holes to feed other areas of the engine. The oil holes drilled into the main housing bore are typically fed by a large main oil galley, which receives oil directly from the engine's oil pump.

Calico Coated Main Bearings Close-UpFor motor oil to leave the main housing bores, the crankshaft bearings must also be provisioned with their own holes. In the picture to the left you will see some Calico coated main bearing shells photographed with holes in the center. Not only do these main bearings have holes to receive oil from the main housing bore, but they are also grooved to allow the oil to fully extend to the upper and lower halves of the bearings. Because there is a small clearance between the engine's bearings and crankshaft journals, some oil is allowed to escape and properly lubricate all of the crankshaft's journals. The bearing to journal clearance is also largely responsible for the engine's oil pressure.

To supply the connecting rod bearings with oil, there are holes drilled in the rod journals that extend into the main journals. Once again the clearance between the rod journals and bearings allow enough oil to escape to maintain an acceptable level of lubrication at all RPM ranges.

In some performance applications, automotive machinists prefer to cross-drill the main journal oil holes to allow oil to travel to both sides of the main journals. Such a process involves specialized tooling, such as a special crankshaft fixture and Bridgeport vertical milling machine, and special drill bits. Since a cross-drilled crankshaft will be directing less motor oil to the connecting rods, without any further modifications, the engine builder will often increase the oil pressure by using taller oil pump gears and a stiffer pressure relief spring. Altering the oil pump characteristics to accommodate a cross drilled crankshaft will lubricate the engine properly with the crankshaft's enhanced oil feed hole configuration.

Repairing Oil Feed Holes

Oil Feed holes on their own do not wear and normally do not need any special machining process when a crankshaft is ground. However, grinding a crankshaft's journals will leave a small burr over the holes that can obstruct the flow of oil or possibly become dislodged and damage the engine's bearings. To remove this burr, the holes are often chamfered just prior to the final polishing process. If a crankshaft journal has received a repair weld, which uses a carbon insert to protect the feed hole from filling up with material, then the top section of the hole will have to be either drilled or opened up with a small die grinder once the journal has been roughed in. Once the welded journal reaches its finish outside diameter, the oil feed holes can then be chamfered and the entire crankshaft polished.

To learn more about the other important areas of a crankshaft, please visit our crankshaft information page for more details and links to other in-depth pages that contain a wealth of information.

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Oil Feed Holes