Back to Basics: Bearings
Bearings are one of the most important parts inside an engine, so it’s important to understand their role in the overall operation of the engine, their design features, and how to install them properly. Engine bearings are a relatively inexpensive component compared to the cost of labor and many of the other parts that go into rebuilding an engine, but if one fails or causes a problem that results in a warranty claim, it can cost you plenty! So with that in mind, let’s review some of the basics about bearings.
Bearings actually have a variety of roles inside an engine, including:
- Supporting the crankshaft and camshaft;
- Limiting the fore and aft movement of the crankshaft (this job belongs to the thrust bearing);
- Reducing friction;
- Lubricating the rotating shafts and connecting rods;
- Providing splash lubrication for the pistons, rings and cylinder walls (which also helps cool the pistons);
- Conducting heat away from the rotating parts; and
- Affecting how much oil pressure the engine develops at idle and higher rpms.
Sleeve type bearings are used in most engines to support the crankshaft and camshaft. Most crankshaft main bearings are a two-piece (upper and lower) split shell design, while most cam bearings for pushrod engines are a one-piece full round design.
In overhead cam (OHC) engines, the cam bearings may be either type, or there may be no bearing inserts at all, i.e., the machined surface of the cam bores serve as the bearing surface for the cam(s).
Sleeve style wet bearings are used instead of ball bearings or roller bearings in most automotive applications because they are cheaper, lighter and are capable of supporting high loads. Even so, ball or roller bearings are sometimes used to support balance shafts in some engines, as well as the crankshaft in some motorcycle, marine and racing engines.
Though ball bearings and roller bearings are called "anti-friction" bearings because they spin easily and produce little drag, a rotating shaft supported on a film of oil inside a pressurized sleeve style bearing also spins with minimal resistance. The only drag on the shaft is that created by the shearing characteristics of the oil film. Also, the oil film helps spread the bearing load over a broader surface unlike a ball bearing or roller bearing, which concentrates the load at a single point or line.
At a microscopic level, oil molecules are like tiny ball bearings and glide easily past one another. That’s why oil feels slippery and makes such a good lubricant. And, the thinner the viscosity of the oil, the more easily it shears and the less friction and drag it creates. But, the oil must have a certain amount of viscosity so it can maintain film strength and not gush out of the bearing too quickly.
Temperature is also a factor to consider, too, because the hotter the oil gets the thinner it gets. If low viscosity oil is subjected to too much heat, it may not be able to maintain adequate film strength at high temperatures and loads causing a breakdown in the oil film and possible bearing failure.
In recent years, most vehicle manufacturers have switched to 5W-30 multi-viscosity oils for new cars and light trucks. The lighter oils make cold weather starting easier, produce less friction and drag to improve fuel economy slightly, and circulate through the engine more quickly after a cold start to bring up oil pressure as quickly as possible.
This is especially important with overhead cam (OHC) engines because the cam bearings are a long way from the oil pump. It can take several seconds for sufficient oil pressure to reach the bearings, and even longer during especially cold weather. If too thick an oil is used in such applications, e.g., 20W-50 or a straight 30W, the cam bearings may experience some wiping or seize.
Bearing construction and selection
The type of bearings used in an engine will depend on the vehicle manufacturer’s preferences and the application. So when the bearings are replaced, you can either follow the OEM lead and use the same type of bearings, or you can install whatever type of bearing you or your customers prefer.
Most late model passenger car and light truck engines have aluminum bearings as original equipment. However, "trimetal" bearings are still a popular replacement choice in the aftermarket.
A typical trimetal engine bearing has a three-layer construction. The steel backing plate is covered with a layer of copper/lead overlaid with a thin (.0005˝ to .0008˝) coating of babbitt. The bearing may also have a thin flash plating of tin for cosmetic purposes. The three-layer construction provides a good combination of strength, surface action and embedability.
Copper/lead can carry 12,000 pounds per square inch (psi) versus about 7,000 to 8,000 psi for an aluminum bearing and can better resist wiping and scoring under heavy loading according to manufacturers of the trimetal type of bearings.
Aluminum, on the other hand, has a higher temperature rating than copper/lead. The melting point of a typical aluminum bearing alloy is over 1,100° F, which is almost three times as high as babbitt. This provides added protection against localized overheating due to detonation, overloading, misalignment and similar conditions.
Most standard trimetal bearings work fine up to about 500 horsepower, but for higher output engines and racing applications, a “high performance” bearing is usually recommended. Such bearings typically have more eccentricity to handle rod distortion that occurs at higher rpms. One brand of performance bearings also uses a thinner babbitt overlay (only .0005˝) to reduce fretting and fatigue under high loads.
As an engine accumulates miles, the crankshaft and camshaft bearings eventually become worn and need to be replaced. That’s why engines have bearing inserts — so the worn surfaces can be restored by simply replacing the bearings. If there were no bearing inserts, every worn surface would have to be remachined or replaced. Imagine how expensive that would be!
Most bearing wear occurs immediately after a cold start because there’s little or no oil film between the bearing and shaft. As the shaft begins to rotate, it turns against the bearing and attempts to climb the bearing wall until there’s enough oil pressure to form a wedge of oil underneath the shaft. This separates the shaft from the bearing and allows it to float on a thin film of oil much like a bald tire hydroplaning across a puddle of water. This eliminates metal-to-metal contact and wear, and cuts friction to almost nothing.
As long as the shaft is supported by the thin film of oil, bearing and shaft wear is virtually nil. But under high loads and high rpms, the oil film can be squeezed down to almost nothing. Under such conditions, metal-to-metal contact can occur causing wiping and wear. To counteract this danger, zinc dithiophosphate is used as an additive in motor oil to provide "boundary lubrication" when the oil film is too thin to provide normal protection. However, even this may not be enough under severe operating conditions (such as racing or hard driving) to protect the bearings.
Bearing wear can also occur if contaminants find their way into the bearing via the oil supply. The oil filter will usually trap most particles large enough to cause problems, but if the filter is not maintained properly and becomes plugged, unfiltered oil can bypass the filter and carry contaminants directly to the bearings. The main bearings will usually suffer the most because they’re the closest to the oil pump and the first to receive oil.
Most bearings provide a certain amount of "embedability" which is the ability to absorb small particles of debris into the surface of the bearing. Embedability depends on the hardness of the bearing surface, and the depth of the surface layer. The softer the bearing alloy and the deeper the top layer, the more easily the bearing can handle contaminants that are too large to flush out.
If a particle is too large to be flushed out of the space between the bearing and shaft, and becomes embedded in the surface of the bearing, it can score and damage the shaft journal if it protrudes above the surface of the bearing. It will act like a cutting tool and scrape away at the shaft journal until one or both wear down. Embedded particles can also breakup the oil film causing a localized hot spot on the bearing that can lead to bearing failure.
Oil breakdown is another condition that can cause bearing wear or failure. Severe overheating can cause the oil to burn (oxidation) resulting in a breakdown of the lubricant. Maintenance neglect (not changing the oil and filter often enough) can also allow a build-up of sludge and contaminants in the crankcase which will accelerate bearing wear and increase the risk of bearing seizure and failure.
Dilution of the oil with raw fuel (from piston blowby or an overly rich fuel condition) can also break down the lubricant and cause the bearings to fail. The underlying problem may be a misadjusted carburetor float or choke, leaky injectors or a plugged or defective PCV system or valve.
Oil contamination by antifreeze is another problem that can wipe out a set of bearings. Antifreeze can enter the crankcase through a defective head gasket, a poor seal between the head and block (usually the result of undetected head or deck warpage, improper head resurfacing, loss of head bolt torque or head gasket leaks) or through cracks in the head or block water jackets.
Anything that causes a drop or loss of oil pressure can also prove fatal for the bearings. This includes oil leaks that lead to a low oil level in the crankcase, a worn or broken oil pump, an obstructed oil pickup screen or tube, or an oil galley plug that blows out under pressure.
Wear isn’t the only reason why the bearings may need to be replaced. Because of the heavy loads the bearings carry, metal fatigue eventually occurs after enough miles of service. In most cases, the bearings will wear out before fatigue sets in. But detonation, severe service and racing can all pound the life out of bearings rather quickly. In Top Fuel drag racing, the average life expectancy of a set of rod and main bearings may be only a couple of runs!
Tiny cracks that form in the surface of the bearings can cause the surface layer to flake away. Corrosion and pitting from exposure to acids in the oil will pit the bearings causing clearances to increase.
When clearances become too great, it’s difficult to maintain the oil film between the bearing and shaft. This causes a loss of oil pressure and increases the risk of metal-to-metal contact when the engine is under load. It also increases the volume of oil being thrown off the rod bearings, which can overwhelm the piston rings causing an increase in oil consumption. Noise will also increase as clearances increase and oil pressure drops.
Heat is another factor that accelerates bearing wear and may lead to bearing failure. Temperatures in excess of 620° F can melt away the lead in copper/lead bearings and those with babbitt overlays. Because copper doesn’t melt until 1,980° F, burned copper/lead bearings will typically have a copper appearance instead of the normal dull gray appearance.
Reading the bearings
The old bearings should always be examined to see if there are any underlying problems that might have contributed to their wear or failure — or might cause a repeat failure. A bent crankshaft can really chew up the center main bearings in the block. Likewise a bent camshaft (or warped head in the case of an OHC engine) will cause the greatest wear in the center cam bearings. The straightness of both shafts should always be checked using v-blocks and a dial indicator. If the crank or cam are bent, they will have to be straightened or replaced.
Main bore alignment can be checked by inserting a bar about .001˝ smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar doesn’t turn easily, the block needs to be align bored. Alignment can also be checked with a straight edge and feeler gauge. A deviation of more than .0015˝ in any bore calls for align boring. Line boring must also be done if a main cap is replaced.
The concentricity of the main bores is also important, and should be within .0015˝ If not, reboring will be necessary to install bearings with oversized outside diameters.
Connecting rods with elongated big end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted or bent. Rods with elongated crank journal bores or twist must be reconditioned or replaced. Straightening the rod may temporarily solve the problem, but many engine rebuilders say it’s safer to replace bent and twisted rods because they often return to a distorted condition when returned to service.
On some newer engines such as Ford’s 4.6L V8 with powder metal rods and "cracked" caps, rods with elongated bores cannot be reconditioned by grinding the caps because the caps do not have a machined mating surface. So the big end bores must be cut to accept bearings with oversized outside diameters if the bores are stretched or out-of-round.
Causes of bearing failure
The leading cause of most bearing failures is dirt – 43% according to one bearing manufacturer. This includes not only unfiltered oil reaching the bearings and a lack or proper maintenance, but also abrasive residue left over from machining operations that was not completely removed when the engine was last rebuilt.
Insufficient lubrication is the next leading cause of bearing failure (nearly 17% of cases), followed by misassembly (12%), misalignment (12%), overloading (7%) and corrosion (4%).
If the bearings are badly discolored with a dark, burned appearance, they may have been wiped out by a dry start or oil starvation.
When the bearings are replaced, in most cases the crankshaft will also have to be reconditioned. On most passenger car and light truck cranks, the easiest and least expensive fix is to grind the crank journals to a common undersize (.010˝ or .020˝), but on heavy-duty truck cranks the journals may be built back up by welding so the crank can be ground to a standard size.
To check the roundness of the crank journals, measure each journal’s diameter at either bottom or top dead center and again at 90° either way. Rod journals typically experience the most wear at top dead center. Comparing diameters at the two different positions should reveal any out-of-roundness if it exists. Though the traditional rule of thumb says up to .001˝ of journal variation is acceptable, many of today’s engines can’t tolerate more than .0002˝ to .0005˝ of out-of-roundness.
To check for taper wear on the journals (one end worn more than the other), barrel wear (ends worn more than the center) or hourglass wear (center worn more than the middle), measure the journal diameter at the center and both ends. Again, the generally accepted limit for taper wear has usually been up to .001˝, but nowadays it ranges from .0003˝ to .0005˝ for journals two inches or larger in diameter.
The journal diameter itself should be within .001˝ of its original dimensions, or within .001˝ of standard regrind dimensions for proper oil clearances with a replacement bearing.
When the crankshaft is machined, careful attention must be paid to the journal fillets. The proper radius is needed to maintain strength in this critical area. If the radius is too small, it can create a weak point that may eventually lead to crank failure. And if the radius is too large, it can interfere with the bearings.
Crankshaft journals should always be micropolished to provide the smoothest surface possible for the bearings. The journal finish should be in the single digit microinch range. Oil holes should also be chamfered to maximize oil flow to the bearings.
After all machine work on the crank has been finished, all the oil holes need to be thoroughly rinsed and cleaned with a bristle brush to make sure no metal chips are left inside. Just blowing out the holes with compressed air isn’t good enough.
Proper fit is absolutely essential for the bearings to do their job. If the crankshaft journals have been turned to undersize, the proper undersize bearings must also be used to maintain oil clearances. If standard bearings are accidentally used with an undersize crank, the engine will have low oil pressure and throw oil all over the inside of the engine. The excessive splash will probably overwhelm the rings and cause the engine to use oil. Worse yet is accidentally installing an undersize bearing on a standard sized journal. It’s a prescription for instant seizure.
To prevent such assembly mistakes, shaft journals should always be measured and bearing clearances checked to make sure everything fits together properly and clearances are within specifications. Most rod and main bearings on passenger car and light truck applications should have about .001˝ to .003˝ of installed clearance. Many race builders will "select fit" bearings by mixing and matching bearing shells until they get the optimum clearances throughout the engine.
Using a test stand to lubricate and spin a freshly built engine is a good way to check oil pressure, drag and compression.
Other things to watch out for include getting the proper amount of bearing crush in the rod and main bearings. Too much crush can distort the bearings and cause a overly tight fit. Insufficient crush can result in a loose bearing that conducts heat poorly and may spin because it is not properly supported. If the rod caps and main caps have been ground to compensate for wear, the bores must be cut accurately to provide the correct amount of bearing crush.
Always check the alignment of oil holes with the bearing. This is especially important with cam bearings that are installed "blind" into the block. Once the bearing is in place, a small allen wrench or similar tool can be used to make sure the oil hole in the bearing lines up with the oil hole in the block.
The importance of cleanliness also cannot be overemphasized. If a piece of dirt gets behind a bearing, it can distort the bearing and create a small hump that rubs against the shaft. The result will be a localized hot spot and possibly premature bearing failure.
Bearing faces should always be lubricated with oil or assembly lube to protect the surface, and most experts recommend prelubing or pressurizing the engine’s oil system prior to the first start. Dry starts probably ruin more newly rebuilt engines than anything else.