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.
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.
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.
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.
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.
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.
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.
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.
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 – >
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