When it comes to the engine in your car, most guys think that a bearing is a bearing, is a bearing. However, contrary to what you might think, bearings do not come in a one-size-fits-all configuration.
While engine bearings appear to be a simple engine component, the conditions that these two halves of a circle of metal have to withstand is unbelievable. It might seem like an easy task to simply reduce friction and support moving parts, but the engine bearing truly is a complex part of the engine, which is why manufacturers continue to refine bearing design and technology.
There are a lot of factors that come into play when selecting engine bearings. So, in order to help you guys make the proper choice when selecting engine bearings, we will take some time to cover topics like, bearing construction, coatings, clearances, and even mixing and matching sizes.
To find out more about bearing selection, we contacted Ron Sledge of King Bearings and asked him to enlighten us about bearing construction materials, and the limitations of each design.
“There are two types of engine bearing construction, bi-metal and tri-metal,” Sledge said. “Bi-metal is usually constructed using an aluminum alloy with a steel backer, and tri-metal, is usually made up of layers of a lead, tin, and copper combination – also on a steel backer.”
We wondered how a metal bearing material could be used, and not destroy engine parts? Sledge explained, “Bearings have to be both hard and soft at the same time. The multi-layer approach allows bearings to be strong, yet soft enough to be wear and seizure resistant, because the different layers have specific duties.”
The materials used in engine construction are not the only variable that must be taken into consideration. Peak cylinder loads, engine RPM range, and operating temperatures, all fit into the equation.
When lubrication is less than adequate, dry film, polymer-based coatings help the bearing and crankshaft survive without damage. – Ron Sledge, King Bearings
Let’s take, for instance, an engine that will be used for drag racing. This condition involves operating in an environment that has fewer airborne contaminates, than a car raced on a dirt track. The engine raced on the dirt track must use bearings with more embedability, so that any dirt reaching into the engine oil will embed into the bearing material and not cause damage to the crankshaft.
According to Sledge, “For normal street applications, a stock replacement bearing like King’s AM and SI series will work fine.”
That means wall thickness consistency from one bearing shell to another is very close, which also means that fewer bearing shells are needed to arrive at a desired oil clearance.
When building a racing engine delivering higher horsepower and torque, a different bearing is needed to handle the increased loading. The bearings must be made of stronger materials to handle the extra pounding without experiencing fatigue. King offers two racing-oriented bearings, the HP and the XP series.
The Main Properties Engine Bearings Must Posses:
Load Capacity (fatigue strength) is the maximum value of cycling stress that the bearing can withstand without developing fatigue cracks after an infinite number of cycles.
Wear Resistance is the ability of the bearing material to maintain its dimensional stability (oil clearance) under the conditions of mixed lubrication regime and in the presence of foreign particles circulating with the lubricant.
Compatibility (seizure resistance) is the ability of the bearing material to resist physical joining with the journal material when they come to a direct contact.
Conformability is the ability of the bearing material to accommodate geometry imperfections of the journal, housing or bearing itself.
Embedability is the ability of the bearing material to absorb small foreign particles circulating in the lubricating oil.
Corrosion Resistance is the ability of the bearing materials to resist chemical attack of the lubricants.
Cavitation Resistance is the ability of the bearing material to withstand impact stresses caused by collapsing cavitation bubbles, which form as a result of sharp and localized drops of pressure in the flowing lubricant.
Although the technology behind bearing design has not changed very much during the last several decades, the manufacturing processes and the actual materials used do continue to improve engine bearings and their performance. A development that has affected bearing constructions in the last decade concerns coatings used on bearings. Engine builders almost unequivocally swear by the positive results they get when using coated bearings.
Sledge agrees, “When lubrication is less than adequate, dry film, polymer-based coatings help the bearing and crankshaft survive without damage.” Improvements like coatings are an ongoing part of bearing design, and most of the time, are driven by the automotive manufacturers. In fact, many of the coatings that bearing manufacturers are looking at right now are to solve specific problems that have not even arisen yet, as engine design changes and improves. Bearing coatings can also help manufacturers address the use of very low-viscosity oils. Since the elimination of friction is directly related to building horsepower, the use of thinner and thinner oils is being employed in engines, and the thinner oil film-thickness between the bearing and operating surface means coatings are a “must have.”
When it comes to keeping your rotating mass (crankshaft and connecting rods) and the block itself happy, the amount of open space between the main and rod journals on your crankshaft and the bearing shells is crucial. This space is filled with oil when the engine is running, and is known as bearing clearance. This oil fills this intentional gap, and provides a cushion between the actual journal and the bearing. When everything in the engine is working as it should, the oil keeps the steel of the crankshaft journals and the material of the bearings separated so they never touch each other.
But, if the bearing clearances are incorrect, you will experience engine troubles like keeping your oil temperatures in check, or low oil pressure, and engine failure. Sledge agrees, “One of the main things to be concerned about when installing engine bearings is building in the right amount of oil clearance for the application. Bearings have to operate on very thin film oil to survive.
Without a full cushion of oil, even at peak loading, metal-to-metal contact will occur and result in bearing failure. Oil clearance has to correspond with oil viscosity.” This means, when installing an engine bearing that is utilizing a tight clearance, the engine must operate while using a low viscosity oil in order to build the proper oil film. Engines built with larger bearing clearances have to use a heavier viscosity oil for the same reason.
Sledge continued his explanation, “Tighter clearances usually generate higher oil temperatures, because the oil molecules are causing more friction. But, the engine runs smoother, due to less vibration and peak loading. On the other hand, larger clearances allow for more oil flow through the bearings for better oil cooling. However, peak loading on the bearings is higher. Having the right amount of clearance is a delicate balance, depending on the application. If there is a lot of crankshaft deflection and debris in the engine, then larger clearances are better. If the engine stays clean internally, and has a good cooling system, then tighter clearances will work.”
Adjusting bearing clearance is easy, as most bearing manufacturers make bearings in standard, oversized, and undersized dimensions. For example, you can actually make clearance adjustments in .0005-inch increments. To do this, you would use half the shell of a “one-over” or “one-under” bearing with half the shell from a standard bearing. This practice can get expensive though, as you will need to purchase two sets of bearings to mix and match.
Keep in mind, when mixing bearing sizes, you always want to keep the bearing-half sizes in the same alignment. If you’re using one half of a “one-over” and one half of a standard bearing, it is imperative that you keep all of the dimensionally-same halves positioned the same within the journal opening. “Different shell thicknesses can be mixed in the same housing to achieve a desired oil clearance,” says Sledge. “When two shells of the same thickness generate too little or too much overall clearance, then mixing only one half of another size to increase or reduce the clearance is acceptable. But, always install the thicker shell in the loaded half. In the case of connecting rods, the upper side, and for the mains, the lower (cap side). Finally, never mix two halves that are .002-inch or more difference in thickness.”
It’s also important to recognize that connecting rod and main bearing housing bores need to be properly conditioned and sized to match the bearing outer diameter. Bearing shells are manufactured to be slightly longer than half of a circle, to give the two shell halves what is called a crush fit in the housing when torqued. Main bearing housings also need to be in perfect alignment to prevent abnormal wear to the main bearings.According to Sledge, “Both connecting rod and main bearing housing bores need to have a surface finish of approximately 60 to 90 micro-inches. Proper housing bore size and cap fastener torque will ensure that the bearing is properly crushed for maximum retention.” So, failure to properly size the bearing opening, and improper torque of cap bolts will result in premature bearing failure. Basically, too much crush will buckle the bearing causing oil starvation, and too little crush will allow the bearing to chatter and overheat.
Competition Or Street
For some reason, when most people are rebuilding an engine, they feel that they have to use a competition-style bearing. Unless your engine will be tasked with running in situations not like it was originally designed, a standard, OE-style bearing is perfectly acceptable. Sledge agrees, “Competition or racing-series bearings are needed when the load on the engine bearings is greater than what was designed for in a stock application. Fatigue strength of the bearing material has to be increased in high-performance engines to prevent failure.” That is why an engine’s planned horsepower should also be taken into account, and matched to the correct bearing material.
Choosing the correct viscosity oil your engine needs is typically based on desired oil pressure, oil clearances, and engine operating temperatures. While most multi-viscosity racing oils are quite capable of providing adequate protection and contain friction-fighting additives, important factors like whether to use synthetic or mineral-based oil, oil viscosity, and the basic design of the oiling system must all be considered when choosing a bearing. According to Sledge, “Oil selection has more to do with oil clearances and applications than actual bearing construction and materials. Oil selection for a high-performance engine should include a good multi-viscosity racing oil with friction fighters. Oil viscosity should match oil clearance.” For more on oils, checkout this article comparing racing oils to street oils.
After you have chosen your bearings, when installing them, there are certain aspects of which you will want to pay special attention. At all times, everything must remain impeccably clean. When actually placing the bearings in the engine, they should be positioned in the bearing saddles while dry, and then lubricated before crankshaft installation. Finally, all bolt threads must be cleaned with a thread chaser, and lightly lubricated during assembly, and the engine must be lubricated (primed) before it is ever started.
Hopefully, this tutorial about choosing the right bearing for your engine has helped, and now you can rebuild your mill with the confidence of knowing that the bearings you chose are perfect for your application. Remember, the bearings in your engine are a very fragile yet durable piece of the entire assembly, and using the right bearings will definitely help your engine live a long and trouble-free life.