Bearing Up: What You Need To Know Before You Buy That Next Set Of Engine Bearings

Not all engine bearings are the same, and price is not the best way to decide which bearings are best for your application. A quick jog through this story will answer many unanswered questions.

Not all engine bearings are the same, and price is not the best way to decide which bearings are best for your application. A quick jog through this story will answer many unanswered questions.

Story and photos by Jeff Smith

 

It was a great day on the dyno. Our 502ci big-block Chevy was singing. It had just picked up a solid 50 hp from a cam and valvespring swap and power was about to break into the 600 hp range. We had previously run this engine with a supercharger, and it had made an awesome 830 hp. The next step was to tune this new normally aspirated package before bolting the blower back on the motor. That’s when it all went sour.

A backup run on the dyno lost 40 hp, and we knew something was wrong. A quick check revealed shiny metal bits in the oil. We’d lost at least one bearing – maybe more. We cut the oil filter open and our suspicions were confirmed. It was time to take the engine apart.

The teardown revealed that Number Two main had failed. This was the second big-block that we had worked on in less than a year that had killed Number Two main bearing, and we began to suspect an assembly issue. But a discussion with bearing expert Bill McKnight of Mahle Clevite revealed some information that was so interesting and important for part-time engine builders that we felt we had to pass this along. While we knew there was a difference in price between production style “P” bearings and their high performance cousins, we didn’t know the whole story. And if you build engines, this is important information beyond just the fact that better bearings are more expensive and worth the extra investment. We also learned new information about when production bearings are worthwhile – and when they aren’t.

The only right way to assemble an engine with new bearings is to not only choose the correct bearings but also spend the time to carefully measure everything with precision tools. There are several different types of engine bearings, and choosing the right one can make all the difference.

The only right way to assemble an engine with new bearings is to not only choose the correct bearings but also spend the time to carefully measure everything with precision tools. There are several different types of engine bearings, and choosing the right one can make all the difference.

In the case of our big-block, this was a brand new ZZ502 crate engine from Chevrolet Performance. Rated at 500 hp, we had bolted on a supercharger and pushed the power past 830 hp. In performing our Rat autopsy, we learned that these crate engines are assembled using production style aluminum bi-metal bearings. The shell is steel, but the bearing material itself is an aluminum alloy. According to McKnight, nearly all production line engines are now fitted with aluminum alloy bearings not just because they are inexpensive, but also because European laws ban the sale of production car engines with lead alloy bearings. This is understandable as it eliminates the potential for scrapped engine parts putting lead into landfills. When engines are built along production engine assembly lines, these are the bearings that are often employed.

McKnight said there is nothing wrong with using bi-metal bearings in the ZZ502 application as long as the power remains around 600hp or less. However, when we pushed our engine past 830 hp, crankshaft deflection (even with the ZZ502’s forged steel crank) can cause contact with the main bearings. Scat crankshaft owner Tom Lieb told us that when big-block Chevy bearing failures occur, he most often sees them in mains 2 and 4.

We also spoke with renowned engine builder Jon Kaase about this phenomenon, and he believes this may be due to where the thrust bearing is located on a big-block Chevy. Because the thrust is located in the rearmost position, leverage from increased power levels tends to load the Number 2 main bearing. To reinforce his point, Kaase points to the big-block Ford that rarely has main bearing problems. Kaase says this is because the Ford places the thrust bearing in the center main, which shortens the leverage arm and therefore reduces the bending moment that is applied to the crankshaft at high horsepower levels. He says that he often sees production 429/460 Fords with 2-bolt mains make over 900 hp with no problems.

Regardless of why big-block Chevys tend to do this, we wanted to know more about the difference between bi-metal aluminum bearings and their high performance tri-metal cousins. We initially thought that aluminum bearings must be softer than tri-metal bearings, but the truth is exactly the opposite. According to McKnight, bi-metal bearings have virtually the same load carrying capacity as tri-metal versions, but the aluminum alloy is much harder. This is intentional because bi-metal bearings are engineered to last decades in a production engine.

This durability is a great asset, but it comes at a price. For high-output applications, McKnight says engineers design the bearing softer to “wipe” when extreme loads push through the oil film between the bearing and the crankshaft journal. When this wiping occurs, the softer tri-metal bearing will absorb the load and the bearing material will deflect or wear off. Harder bi-metal bearings are less forgiving. Instead, a portion of the bi-metal bearing face can peel away and begin a microscopic micro-welding process between the bearing and the crank journal.

Micro-welding can be described as high heat and load removing small bits of aluminum from the bearing surface. Once this process begins, the crank quickly peels away portions of the bearing. This is what we saw with the Number Two main. The metal immediately progresses into adjacent rod bearings and is immediately destructive.

It’s important to mention here that this is not a condemnation of the ZZ502 crate engine or its aluminum bi-metal bearings. The bi-metal bearings in that engine will last a long time at near-stock power levels. Chevy has tested this engine for durability and found the bearings are fine at that power level.

We pushed that engine over 50 percent past its intended power levels and abused it beyond its intended capabilities. Had we understood the risk with bi-metal bearings, we would have replaced them with tri-metal versions and our sources all agree that the engine would have survived with no problem with perhaps only a mild wiping of the Number Two main.

Rod and main bearings can be condensed into two basic categories – production style bi-metal aluminum versions (left) and the more performance oriented tri-metal bearings constructed of a lead-copper alloy with a very thin lead-tin overlay (right). These different bearings may appear to perform the same job, but their performance characteristics are completely different.

Rod and main bearings can be condensed into two basic categories – production style bi-metal aluminum versions (left) and the more performance oriented tri-metal bearings constructed of a lead-copper alloy with a very thin lead-tin overlay (right). These different bearings may appear to perform the same job, but their performance characteristics are completely different.

There are many companies making both stock replacement and high performance bearings with the major players being Mahle-Clevite, Daido, Federal-Mogul (Speed-Pro), and King, among others. In talking with King Bearing’s Ron Sledge, we discovered that King makes three different styles of bi-metal bearing. The standard replacement bi-metal bearing uses an AM suffix. The SI suffix bi-metal employs 4 percent silicon and is suitable for daily driven, high mileage engines and mild performance applications.

Sledge said silicon works as a polishing agent for nodular cast crankshafts. If you were to look at the bearing journal surface of a cast crankshaft under a microscope, it is full of peaks and valleys. Nodular cast cranks contain tin ferrite pockets that are ruptured during grinding and polishing, causing the formation of jagged edges. The silicon in the SI and HP materials acts as a polishing agent to round off the jagged edges. The King HP suffix bearing is the performance version bi-metal and could be used in a small-block Chevy, for example, making 550 hp or less. According to Sledge, using the HP bi-metal bearing will be an advantage in conformability. Above this power level, you would want to use a tri-metal bearing.

According to Scat Crankshaft owner Tom Lieb, he really likes the King SI bi-metal bearing for non-racing, mild performance cast cranks applications because with these bearings he has had far fewer problems than other bi-metal bearings. As soon as he converted to the King SI bearing, his bearing-related crank issues dropped nearly to zero.

Tri-Metal Bearings
So what is a tri-metal bearing? This style of bearing is not really new. It has proven its design as an ultra-high performance bearing for decades. Starting with a steel back for stability, the base is a copper-lead alloy as the second layer with a very soft, electroplated lead-tin alloy as the top layer. This is a generic description with each company modifying this basic layout to a specific formula. The two upper layers consist of very soft materials that will deform and accommodate deflection in the rotating journals that naturally occur in highly-stressed engines.

This is important because during engine operation what keeps bearings alive is a thin film of oil that creates a hydrodynamic wedge of lubrication that both lubricates and cools the bearing. While softer bearings are more likely to absorb crank deflection, this comes at the price of a shorter overall lifespan compared to a bi-metal bearing. But in the case of high output performance engines, their lifespan between rebuild is far shorter than a traditional production engine that could easily be asked to support 200,000 miles.

If you are dealing with bearings from an unknown source, many factory aluminum bearings are stamped with an “A” or “AU” prefix that quickly informs you this is an aluminum or bi-metal bearing.

If you are dealing with bearings from an unknown source, many factory aluminum bearings are stamped with an “A” or “AU” prefix that quickly informs you this is an aluminum or bi-metal bearing.

As you may already know, tri-metal bearings come in many different and specific forms. Crankshaft companies have known for years that creating a large fillet radius between the crank journal and the vertical portion of the counterweight increases the crank’s strength. This narrows the bearing surface area slightly, requiring a narrower bearing. Most bearing companies refer to this style with an N somewhere in the part number.

The most common application for N bearings is for connecting rods. These bearings come with a chamfer on one side that dictates an upper and lower marked insert. This is critical because if these positions are reversed (an upper bearing shell installed in a rod cap, for example) the chamfer will be located on the wrong side and the 90-degree edge could contact the fillet area of the crank journal. While this is rarely fatal to the bearing, it’s an indication that the engine builder was not paying attention. Other important variations on the traditional tri-metal performance bearing include under- and over-size options that allow the builder to customize bearing clearances. Even half-shells can be used to make very slight bearing clearance adjustments.

Coated bearings are another option available to the engine builder. For example, Speed-Pro offers a DuroShield coating that is only 0.0003-inch thick but offers an added layer of protection. This polymer coating has the ability to absorb oil to improve lubricity and potentially reduce friction. There is far more information on bearings, coatings, and different chemistries relating to bearings, but our main focus here is to identify the difference between tri-metal and bi-metal bearings. You can decide whose product works best for your application. Talking with professional engine builders will often point you in a specific direction.

Clearances
The classic recommendation for rod and main bearing clearance really hasn’t changed much in the last 40 years. The standard rule is 0.001-inch per one inch of journal diameter. This is a standard that still works, but with advances in oil quality and viscosity, these recommendations are becoming more specific. The most important factor to remember when establishing bearing clearances is to match your intended oil viscosity to a bearing clearance. If the engine is already assembled, then you are forced to choose the viscosity that best fits the existing clearance. King Bearing’s Ron Sledge has created a chart that lists clearance recommendations based on engine oil viscosity. This is a generic chart and also refers to main bearing journal diameters less than three inches in diameter. Generally, journal diameters larger than three inches require more clearance. Examples would be Olds or Pontiac engines with the larger main bearings.

These are Speed-Pro performance tri-metal high performance rod and main bearings. Some people mistakenly think there is something wrong with these bearings because of their odd coloration. Federal-Mogul does not flash coat its tri-metal performance bearings, which is why the bearing looks like this. This is completely normal appearance for a tri-metal bearing.

These are Speed-Pro performance tri-metal high performance rod and main bearings. Some people mistakenly think there is something wrong with these bearings because of their odd coloration. Federal-Mogul does not flash coat its tri-metal performance bearings, which is why the bearing looks like this. This is completely normal appearance for a tri-metal bearing.

Oil viscosity plays an important role in bearing clearance because tighter clearances will demand a thinner oil in order to provide the necessary protection. Another way to think about bearing clearance is to consider the three critical aspects of clearance: bearing load capacity, oil flow, and oil temperature. Load capacity generally peaks with tighter clearances, but minimal clearance reduces oil flow (generally expressed in gallons per minute, or gpm). Tighter bearing clearances also increase localized oil temperature since the oil flow has been reduced. So clearances become a balancing act between all three components. This generally falls, as is indicated in King’s bearing clearance chart, around 0.0020 to 0.0025-inch as an acceptable clearance for street engines.

With the trend of high quality, thinner viscosity race oils, there is a solid case to be made for tighter clearances that can take advantage of slight power gains from reduced windage in the crankcase from less oil but also improved power from reduced pumping losses due to the lighter viscosity.

An oil viscosity test performed by Steve Brule at Westech Performance a few years ago found that high quality 0w-20 race oil produced an average of 3 hp more compared to a straight 30w street oil and an average pressure drop of nearly 8 psi compared to a 20w50 race oil. Ironically, the average power between the 0w20 and 20w50 oils was a mere 0.5 hp despite the difference in pressure, so gains are small when contemplating power increases through oil viscosity.

Oil pressure is another aspect of engine performance that is changing. The old standby of 10 psi per 1,000 rpm of engine speed is now considered to be outdated. According to Sledge, NASCAR engines are now running 9,000 –plus rpm with oil pressure in the 40 psi range.

Performance bearings come in multiple sizes. For performance applications, often a 1X bearing can be used to add 0.001-inch additional clearance. A -1 bearing will reduce the clearance by 0.001-inch. When mixing shell halves for custom clearances, always place the thicker shell half in the loaded side of the housing. In the case of rod bearings, this would be in the connecting rod side. Over- and under-size bearings halves should only be used in conjunction with a standard bearing half. Never combine a +0.001 with a -0.001 half.

Performance bearings come in multiple sizes. For performance applications, often a 1X bearing can be used to add 0.001-inch additional clearance. A -1 bearing will reduce the clearance by 0.001-inch. When mixing shell halves for custom clearances, always place the thicker shell half in the loaded side of the housing. In the case of rod bearings, this would be in the connecting rod side. Over- and under-size bearings halves should only be used in conjunction with a standard bearing half. Never combine a +0.001 with a -0.001 half.

If you think about it, pressure is just the indication of resistance to flow. Pressure is important, but it’s clear that 50 psi for most street performance applications would be more than sufficient. This also means that oil pressure at idle does not have to be 40-plus psi. At idle, the engine only has to make enough power to spin itself and any attending accessory drives. With this minimal load, oil pressure at 10 to 20 psi would be more than sufficient. This is why nearly all OE manufacturers are switching to 5w-20 or 5w30 oil for production engines.

Thinner oil requires less power and therefore results in better fuel economy and power. We once asked Kaase what the typical oil pressure was on an 800ci Pro Mod style mountain motor. Kaase says he gears his dry sump oil pressures for peak rpm and they fall wherever they may at idle. Often this means the engines may only make 5 psi of pressure at idle, which he feels is more than sufficient.

Conclusions
We’ve only barely touched on several aspects of engine bearing applications and usage, but perhaps we’ve piqued your interest and you can take the next few steps in determining what’s best for your specific application. It should be obvious that the selection of a main or rod bearing for a daily-driven small-block is going to be quite different than one for a 9,000 rpm NHRA Competition Eliminator drag race engine. But at least now it’s clear that not all bearings are the same.

 

King Bearing Oil Clearance Chart

Oil Viscosity        Rod Bearing Clearance       Main Bearing Clearance

20w / 5w20         < – 0.0021                              < 0.0020

30w / 5w30         0.0021 – 0.0026                  0.0020 – 0.0025

40w / 10w40       0.0026 – 0.0031                  0.0025 – 0.0030

50w / 20w50       0.0031 – >                           0.0030 – >

 

Often after measuring a complete set of eight rod bearings, you may find a set that is a little tighter clearance and one that’s a little looser than you’d like. We have found that merely switching the loose for the tight and measuring again will bring both tolerances in closer to the desired clearance. This can be attributed to minor housing bore differences and different crush characteristics between the rods.

Often after measuring a complete set of eight rod bearings, you may find a set that is a little tighter clearance and one that’s a little looser than you’d like. We have found that merely switching the loose for the tight and measuring again will bring both tolerances in closer to the desired clearance. This can be attributed to minor housing bore differences and different crush characteristics between the rods.

If you look closely at a high performance set of rod bearings, the inserts may be marked “upper” and “lower.” This is because the bearing insert has been narrowed on one side to clear the radius against the cheek of the crank. Be careful to always place them in their proper position to prevent the bearing from rubbing on the crank fillet radius. When inserted properly, the narrow side of the bearing will be on the same side as the chamfer on the rod and cap.

If you look closely at a high performance set of rod bearings, the inserts may be marked “upper” and “lower.” This is because the bearing insert has been narrowed on one side to clear the radius against the cheek of the crank. Be careful to always place them in their proper position to prevent the bearing from rubbing on the crank fillet radius. When inserted properly, the narrow side of the bearing will be on the same side as the chamfer on the rod and cap.

When customizing bearing clearances, you can mix half-shell thicker or thinner bearing halves to establish the desired clearance. For example, to decrease rod clearance by 0.0005-inch, use a -1 (0.001 less clearance) in the rod and then measure the clearance again. Use only the same style bearings when mixing half sizes and never mix manufacturer bearings in the same housing bore.

When customizing bearing clearances, you can mix half-shell thicker or thinner bearing halves to establish the desired clearance. For example, to decrease rod clearance by 0.0005-inch, use a -1 (0.001 less clearance) in the rod and then measure the clearance again. Use only the same style bearings when mixing half sizes and never mix manufacturer bearings in the same housing bore.

Most leading bearing companies now offer ¾ groove style main bearings. This extends the bearing’s oil groove from the upper bearing past the parting line to ensure a better supply of oil to the loaded portion of the bearing in the cap. Full groove bearings should never be used in a performance application because this reduces bearing surface area and increases the load on the remaining portion of the bearing.

Most leading bearing companies now offer ¾ groove style main bearings. This extends the bearing’s oil groove from the upper bearing past the parting line to ensure a better supply of oil to the loaded portion of the bearing in the cap. Full groove bearings should never be used in a performance application because this reduces bearing surface area and increases the load on the remaining portion of the bearing.

Only measure bearing clearance 90 degrees to the parting line. All engine bearings are designed with additional clearance closer to the bearing parting line. This taper prevents bearing movement due to cap walk that could potentially wipe oil off the crank journal.

Only measure bearing clearance 90 degrees to the parting line. All engine bearings are designed with additional clearance closer to the bearing parting line. This taper prevents bearing movement due to cap walk that could potentially wipe oil off the crank journal.

Anytime a set of rod bolts are pressed into place, the rods should be resized to accurately maintain not only the proper inside diameter but also concentricity. Changing from bolts to studs in main bearings will also change the clearance due to different load characteristics. Changing to main studs often does not require line honing, but you should measure the clearance as it could change.

Anytime a set of rod bolts are pressed into place, the rods should be resized to accurately maintain not only the proper inside diameter but also concentricity. Changing from bolts to studs in main bearings will also change the clearance due to different load characteristics. Changing to main studs often does not require line honing, but you should measure the clearance as it could change.

There are some enthusiasts who will argue this point, but we don’t consider Plastigage to be a precision measurement device. This tool can be used when nothing else is available, but in our experience, this tool is extremely inconsistent with variations of well over 0.001-inch compared to using a calibrated dial bore gauge and a micrometer.

There are some enthusiasts who will argue this point, but we don’t consider Plastigage to be a precision measurement device. This tool can be used when nothing else is available, but in our experience, this tool is extremely inconsistent with variations of well over 0.001-inch compared to using a calibrated dial bore gauge and a micrometer.

This is the aluminum main bearing we pulled from our big-block Chevy engine when it began to destroy the bearings. Note how a portion of the bearing is severely scratched. This is where the aluminum alloy was pulled away by the crank. That’s the metal we found in the oil. A softer tri-metal bearing would have been worn down but would not have dumped bearing material into the oil – unless it was severely distressed – at which point you have other problems.

This is the aluminum main bearing we pulled from our big-block Chevy engine when it began to destroy the bearings. Note how a portion of the bearing is severely scratched. This is where the aluminum alloy was pulled away by the crank. That’s the metal we found in the oil. A softer tri-metal bearing would have been worn down but would not have dumped bearing material into the oil – unless it was severely distressed – at which point you have other problems.

 

Sources

Daido Metal, USA; daidometal.com

Federal-Mogul (Speed-Pro); 248/354-7700; federal-mogul.com

King Bearings; 800/772-3670; Kingbearings.com

Mahle Clevite; 800/338-8786; mahle-aftermarket.com

Melling Automotive Products (Dura-Bond); 775/883-8998; melling.com

Scat Enterprises; 310/370-5501; scatenterprises.com

About the author

PPN Editor

Power & Performance News is the source for news, tech and products that help you get more performance from your vehicle. If powertrain performance projects and hardcore technical content are your interest, Power & Performance News is the publication designed for you. Our acclaimed editorial staff covers all aspects of engine and driveline upgrades with a mission of presenting information that is both interesting and achievable for the “average car guy”.
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