There are a lot of moving components in an internal combustion engine and they need to operate smoothly and efficiently in order for the engine to be stable and powerful. This is especially the case with modern-day engines which are known to be especially quiet and smooth.
This is thanks to a certain type of bearings known as conrod or main bearings. These engine bearings are essentially semicircles made of metal that go on the bearing journal of the crankshaft to provide support for each connecting rod. This way the journal is protected from the impact of the conrod and less noise is created too.
What Matters in Rod Bearings
Material
The material main engine bearings are made of can change how well and how long the engine can run for without needing replacements. You can either bi or tri-metal bearings made of a steel backing and a substrate that supports different overlays. For instance, you can get an engine connecting rod bearing with an overlay made of lead, tin, copper and lead indium. Copper, tin and lead bearings are the best in most scenarios while those with a lead indium overlay are usually made for use in high-end performance vehicles.
Construction
Depending on how automotive engine bearings are made they can be either cast or sintered. Cast bearings are made with a steel strip that has its edges folded. The overlay materials are melted and then poured onto the steel strip so that can then be formed by quenching it in oil. This makes for bearings that can withstand structural pressure up to 12,000 PSI or 827 bar.
Sintered automotive engine bearings are also made with a strip of steel but it's not exposed to high temperatures when it comes to making the overlay stick to it. Instead, high pressure is used to make the bond between the two happen with the help of a powdered substrate. Thanks to a series of rollers the two materials are put together to form what is a bearing that can take anywhere from 8,700 PSI (600 bar) to 10,000 PSI (827 bar).
Clearance
The contact area of the end of the rod with the bearing is what bearing clearance affects most. A bearing with higher clearance means the point of contact is quite narrow but there is also high film pressure. This was once thought to be the solution to engines prone to bearing issues but that just wasn't the case. Bearings with a low clearance have a much wider contact arc and therefore a low film pressure. This means they are able to match the shaft curvature much more closely and reduce the load caused by the road which results in a more durable engine rod bearing.
Cavitation
There are two types of cavitation bearings that can be experienced and those are gaseous and vapour cavitation. Gaseous cavitation occurs when oil pressure falls below the atmospheric pressure in the engine which makes gasses escape out of the oil resulting in voids known as cavitations. These are harmless as no damage occurs thanks to the recirculation of oil which redissolves them.
Vapour cavitation is the result of the load that's being applied to the bearing starts to fluctuate at high RPM. This results in oil pressure dropping rapidly causing fast evaporation and the formation of vapour cavities as the pressure rises. These bubbles can cause high impact pressure on the bearing which can cause the overlay material to start eroding over time.
Signs You Need New Rod Bearings
Oil Pressure
The most apparent and common symptom of rod-bearing failure is low oil pressure or loss of oil pressure. Not enough of it means the oil in the engine is not recirculating fast enough to lubricate components including the bearings. Without enough lubrication the more wear the bearing will have on it and the higher the clearance you'll get on them which means you'll need new ones much sooner.
Noise
Engine noises are plenty because they can vary greatly in pitch and loudness but one thing is for sure and that's a knock is almost always a rod or its bearing at fault. Rod knock usually occurs because of excess clearance created by the bearing which has been worn much more than what low oil pressure causes. If not replaced on time, you can end up needing new conrods too.
Belts
It's also possible that worn-out engine bearings cause issues with belts as the former have two faces. You can also end up having transmission issues since rod bearings also affect how the transmission works and if any of the faces starts to wear out it can affect it greatly. You can even end up damaging the transmission if not replaced on time.
Silver Shavings
Probably the most apparent symptom of worn-out rod bearings is silver shavings in the engine oil. When doing an oil change make sure to run some of it through your fingers. Get a feel for it and see if there is an unusual metal shine to it. This can be checked when you pull out the dipstick too.
