Main bearings are the critical components inside an engine that hold the crankshaft in place and allow it to spin freely. Every piston engine, from a lawnmower to a diesel truck, relies on a set of main bearings mounted in the engine block to support the crankshaft while it converts up-and-down piston motion into rotational force. Without them, the crankshaft would grind directly against the engine block and destroy itself within seconds.
How Main Bearings Work
A main bearing is a sleeve-style (plain) bearing, meaning it has no balls or rollers. Instead, the crankshaft sits inside a smooth, curved shell that lines the bearing saddle in the engine block. The shaft slides directly against this shell’s inner surface, separated by a microscopically thin film of oil.
That oil film is the key to everything. As the crankshaft spins, it drags engine oil into the narrowing gap between itself and the bearing surface. This creates what engineers call hydrodynamic pressure: the oil gets squeezed into a wedge shape and generates enough force to actually lift the crankshaft off the bearing surface. The load the oil film can support depends on the oil’s thickness (viscosity), the speed of the shaft, the surface area of the bearing, and the size of the gap between the two surfaces. At normal operating speed, the crankshaft essentially floats on oil and never touches the bearing metal at all.
Why They’re Plain Bearings, Not Roller Bearings
Roller bearings use balls or cylinders that roll between two rings. They’re excellent for many applications, but most automotive engines use plain bearings for the crankshaft because plain bearings are more compact, quieter, better at absorbing vibration, and capable of handling the rapid load reversals that happen thousands of times per minute inside an engine. They’re also cheaper to manufacture and replace. Some motorcycle engines and small industrial engines do use roller-type main bearings, but they’re the exception in cars and trucks.
Materials and Construction
Main bearings are engineered in layers, each with a specific job. The outer shell is typically steel for structural strength. Bonded to that steel is a softer lining material designed to be gentler on the crankshaft than the crankshaft is on itself.
The most traditional lining material is Babbitt metal, a family of alloys based on tin or lead mixed with copper and antimony. Babbitt works so well because it’s softer than the steel crankshaft, which means any wear happens to the bearing rather than the expensive crank. It also conforms well to slight misalignments and has a naturally low friction coefficient. Within the soft tin matrix, tiny hard particles form during manufacturing. These particles increase the alloy’s hardness and further reduce friction, helping keep operating temperatures down.
Modern engines often use aluminum-tin alloys or copper-lead alloys instead of traditional Babbitt, offering better fatigue resistance at higher loads. Many current bearings also receive a polymer coating on top of the metal layers. These coatings improve corrosion resistance and reduce friction during the moments when oil film protection is weakest, such as engine startup or the repeated start-stop cycles in hybrid vehicles and cars with automatic engine stop-start systems.
Oil Clearance: A Precision Gap
The space between the crankshaft journal and the bearing surface, called oil clearance, is measured in thousandths of an inch. A good starting point for most engines is 0.00075 to 0.001 inches of clearance per inch of shaft diameter. So a crankshaft journal that measures 2 inches across would need roughly 0.0015 to 0.002 inches of total clearance. For context, a human hair is about 0.003 inches thick, so the entire oil-filled gap in a main bearing is thinner than a single strand of hair.
Too little clearance restricts oil flow and causes overheating. Too much clearance, generally anything above about 0.003 inches in a passenger car engine, lets oil pressure drop and allows the crankshaft to move around excessively, creating noise and vibration. Factory specifications tend to be on the tighter side because modern engines use thin, energy-saving oils and run at high temperatures, both of which demand precise control of that gap.
Signs of Failing Main Bearings
Worn or damaged main bearings produce symptoms you can hear and see on your dashboard. The most recognizable sign is a deep, rhythmic knocking sound from the bottom of the engine that gets louder with engine speed. This knock happens because the worn bearing allows the crankshaft to slam against the bearing surface instead of floating on oil. A light tap might indicate early wear, while a loud, heavy knock usually means the bearing is seriously damaged.
A sudden drop in oil pressure is another red flag. As bearing clearances increase from wear, oil escapes the bearing faster than the pump can supply it, and the oil pressure gauge drops or the warning light comes on. If you notice low oil pressure combined with a new engine noise, worn main bearings are a likely culprit.
What Causes Main Bearings to Fail
Oil starvation is the most common cause. Running the engine low on oil, using the wrong viscosity, or having a clogged oil filter can all starve the bearings of the film they need. Without adequate oil, metal contacts metal, and the bearing surface overheats and smears away in a process called wiping.
Fatigue is the other major failure mode. Over tens of thousands of miles, the repeated loading and unloading cycles crack the bearing surface from within, eventually causing pieces to flake off. Contaminated oil accelerates this process because abrasive particles score the bearing surface, creating weak points where cracks start. Cavitation erosion can also occur: rapid pressure changes in the oil film cause tiny vapor bubbles to form and then violently collapse against the bearing surface, pitting the metal over time. Harder bearing materials resist cavitation better, which is one reason modern alloys have gradually replaced softer traditional Babbitt in high-performance applications.
Replacing Main Bearings
Replacing main bearings is one of the more involved engine repairs. In most cases, it requires either removing the engine from the vehicle or at least dropping the oil pan and removing the crankshaft main caps from below. Each main cap is unbolted, the old bearing shell is removed, a new one is set in place, and the cap is reinstalled and torqued to a precise specification. On large diesel engines, those torque values can reach 300 foot-pounds, requiring significant tooling.
Some engines allow individual bearing shells to be “rolled in” without fully removing the crankshaft. A mechanic loosens one main cap at a time, slides the old upper shell out by rotating it around the journal, and rolls the new one into place. This approach saves time but still demands careful measurement of oil clearances afterward, typically using a thin plastic gauge strip crushed between the bearing and crankshaft to verify the gap is within specification.
If the crankshaft journals are scored or out-of-round from running on bad bearings, the crank needs to be machined (ground down slightly) and paired with undersized bearings that are thicker to compensate. At that point, the job becomes a full engine overhaul rather than a simple bearing swap.