In engine rebuilds and performance upgrades, even the smallest parts have a huge say in overall outcomes. Though pistons, conrods, and cranks are the first parts to get a do-over, minor but related parts that separate these components – the engine bearings, allow the whole engine assembly to spin freely to redlines. Often the devil is in the details, and without bearings, you’ll be going nowhere, fast.  

Signs that your main or connecting rod bearings are worn or about to call it quits are as long as a Sunday sermon. There’ll be knocking noises as conrods thump against cranks, grinding sounds from timing belts and tensioners, ineffective lubrication and a loss of oil pressure, abnormally rising engine temps, grinding of metal parts against each other, and finally complete engine seizure. Better pray that you caught the issue on time. Issues in car bearings arise from ineffective lubrication or contaminated oil, bearing metal fatigue, sourcing bearings in incorrect materials, and errors during assembly.  

Main Bearings 

Main crankshaft bearings are tasked with supporting the rotating motion of the crank as it is pulled and pushed by conrods moving up and down thousands of times each minute. Main bearings line the crankshaft journals, and are held in place by the crankcase or engine block and bearing caps. They are of the split-sleeve design, consisting of upper and lower semi-circular bolted halves. Grooves bored partially or along the whole length of the upper bearing and feed holes in the lower section allow for optimal lubrication of both the bearings and the crankshaft. Some main bearings may have flanges that lock them in place and prevent axial play against the crankshaft.  

The number of main bearings varies. Engines have at least two main bearings (one at either end), but higher cylinder count and engine configurations (inline engines vs V6s and V8s) raise that number. Engines with higher loads also have more main bearings, as seen in diesels.  

Big End/ Conrod Bearings 

These slide into the big end of the connecting rods. Their purpose is to guide, support, and connect the conrods to the crank journal while being subjected to the downward thrust of the pistons. Conrod bearings are split-sleeve designs, with an upper and lower part. The connecting rod cap encloses the bearings and is secured with bolts. To maintain proper function (absorbing radial and axial forces in engine movement), the inner sections of conrod bearings have oil grooves and this allows them to slide smoothly against the crank. 

Choosing the Right Bearings 

Bearing Materials 

Bearings are designed to last the lifetime of the engine. Problems, however, come about in modified engines putting out more power, and this puts additional stress on the bearings. Choosing the right materials means bearings that are suited for the application.  

Buyers have two choices here – bi-metal and tri-metal beatings. Bi-metal bearings consist of aluminium tin on a steel backing and this is what you’d find the stock in most production cars today. They’re simpler and cheaper to make but still strong enough to last hundreds of thousands of kilometres with proper maintenance (read regular oil changes). Some may also have higher traces of silicon for improved flexibility at higher engine speeds. 

Tri-metal bearings consist of three metal layers. There’s the steel backing, a copper-lead middle layer, and a top layer of babbitt, or soft tin, lead and nickel alloy. The purpose of the softer top layer is to eventually wear out under extremely high loads from twisting cranks at higher revs, before being replaced. This is what you’ll find in more costly racing car bearings, meant to last the duration of one or a few races without seizing the conrods and cranks together.  

Bearings may also have thin coatings to protect against wear in high friction scenarios with increased heat and low lubrication levels.  

Bearing Requirements 

Bearings are battered at high speeds, pressures, and temperatures with the risk of increased contact with the crank and poor lubrication leading to severe engine failure. Manufacturers, therefore, define the properties that ensure parts’ longevity and improved performance.  

Bearings need to have high fatigue strengths to endure the different loads acting on them as the engine spins. Increased engine inertia can lead to flaking, or more seriously, cracking. The bearing also needs to have adequate compatibility or seizure resistance that prevents increased friction and high heat from metal on metal and low lubrication. With the tendency for lower oil clearances (see below), they also must endure higher rates of wear. This can be from direct contact with the journals, or contaminants in the oil. Related here is corrosion resistance from oil impurities.  

Bearing embeddability refers to the bearing’s removal of oil contaminants before they reach the crank. Conformability is the ability to maintain shape under twisting and misalignment in assembly.  

Clearance and Lubrication 

How bearings sit in relation to connecting parts and are separated by a thin film of oil is called bearing clearance. Practices have differed, and most recommendations are for tighter clearances with thinner films of oil spread over a larger area. This reduces oil pressure and allows for smaller oil pumps (hence less weight) and lower viscosity of synthetic oil.  

Retailers have simplified buying. Main and conrod bearings are available as regular replacement parts for stock production cars or as performance upgrades in higher revving and higher output vehicles. What’s harder is choosing a brand, as prices won’t differ all that much.