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How to Select and Install High-Performance Engine Bearings

There’s a bearing type for every reason and every possible application. MAHLE Clevite has got you covered, from Mom’s grocery-getter to 11,000hp blown Nitro motors. Here’s the scoop on choosing the right type.

An internal combustion engine’s main and rod bearings must “bear-up” under tremendous stresses and loads. In a pure grocery-getter, longevity is key: Bearings are expected to last for hundreds of thousands of miles, often with minimal maintenance. In a pure race car, they “only” have to survive the race, but to do that, they must withstand extreme high rpm, high heat, and cylinder pressures that can reach 10,000 psi or more. Add to that the increasing use of thin race oil and (rules permitting) ever smaller bearings, as racers attempt to wring out just a little more power. Somewhere in-between is the dual-purpose hot rod, where the engine may make big power, but also has to last for at least many tens of thousands of miles—essentially, a blend between the needs of a grocery-getter and the high-end race car. As technology continues to evolve, we sought out the latest recommendations for selecting high-perf bearings from MAHLE Clevite’s Bill McKnight, who for over 20 years has been training, teaching, and advising engine rebuilders and leading motorsports teams. Clevite has the industry’s broadest race bearing offerings, years and years of experience (it’s been around since 1920), and constantly works with leading race teams to develop new bearing technologies.

How They Work

In a perfect world, bearings work under the principle of “hydrodynamic lubrication.” In theory, direct metal-to-metal contact between the rotating parts should never occur due to a thin protective oil film between the two surfaces that forms a hydrodynamic wedge capable of supporting large loads. Think of tires aquaplaning on a wet road: Water accumulates faster than the vehicle weight and tire tread’s pumping action can push it out of the way, causing the vehicle to slide on that thin water layer. The oil film that forms on the main and rod journals acts the same way.

But here’s the key: For aquaplaning to occur, the car must reach a critical speed. Likewise, the crank must be rotating at a certain speed before the hydrodynamic wedge can form. Additionally, hydrodynamic lubrication assumes a flex-free assembly. In the real-world. Occasional metal-to-metal contact does occur. For the daily driver, this primarily includes start-up and shut-down; for a hot rod and race-car, add to that shaft flex, bending loads, and deflection caused by high loads, g-forces, and inertia. That’s where the bearing does its job: Take the brunt of occasional high-friction, metal-to-metal, contact when the oil film momentarily breaks down so the rest of the engine can continue operating without damage. But to do this they need to be matched to the correct application, then installed with the proper clearances.

Bimetal or Trimetal?

Today, there are two primary means of bearing construction: bimetal and trimetal. Modern bimetal designs consist of two materials: a steel back plus a layer of bearing material. Since the 1990s, the overlay has consisted of a relatively hard aluminum alloy bonded to the steel. It’s what comes stock on nearly all of today’s production cars.

A bimetal bearing has two layers: a steel backing with a top layer of either babbit or (left) aluminum. Babbit bearings for daily drivers have been pretty much phased out. Most performance bearings are a trimetal configuration (right), which consists of a steel back, a copper-lead or bronze alloy intermediate layer, and a relatively thin overlay material that varies per application.

Three-layer or “trimetal” designs have a steel back, a copper-lead intermediate layer, and a top layer of varying materials. They’re the standard for high-performance, racing, and (in the OE world) a select few high-end cars like the supercharged Corvette. MAHLE Clevite’s trimetal bearings are trademarked as the “Clevite 77,” and they first started appearing in the early 1950s.

Most modern cars and light-duty trucks come stock with “hard” bimetal aluminum bearings like Clevite’s A-series (left). The first step up is a three-layer, P-series Clevite-77 “trimetal” (right), that offers a little more shock resistance, but has a slightly shorter service life.

The trade-off between bimetal and trimetal is the expected service length versus resistance to extreme loads. The modern aluminum bimetal bearing is hard, so it wears slowly for a lifetime of moderate duty service. But on high-power engines subject to lots of horsepower and twisting cranks, you don’t want too hard a bearing. That’s where the softer, more forgiving, trimetal comes in. After all, most high-perf cars don’t go hundreds of thousands of miles on the original engine, but they do need to withstand metal-to-metal contact more often.

Modern OE cracked-cap rods are made in one piece. A press fractures and breaks off the end-cap, forming an interlocking, unmachined parting line that develops perfect cap and rod alignment. But they can’t be rebuilt by grinding down the two unmachined mating surfaces. Instead, bore an assembled rod’s big-end slightly oversize and use Clevite’s thicker P-series rod bearing (A, compared to original thin bearing, B).

Race Bearings

“On the street, step up from P- to H-series Clevite-77 trimetal bearings when horsepower output is roughly 50-percent more than the stock power.” —Bill McKnight

Today, Clevite offers four different overlay configurations for high-performance bearings, with a suffix in the part number denoting the material.

Clevite’s racing bearings, from front: Originally developed for NASCAR, H-series trimetal is suitable for most high-perf and racing apps. Don’t polish them: Their “ugly,” dull, top coat is normal. V-series trimetals have a more conformable overlay with better shock resistance. M-series is a severe-duty bimetal with a thick babbit overlay that’s still used by some early Hemi racers.

P-series: A stock replacement trimetal bearing for most older engines with about a 0.001-inch babbit (lead-tin-copper) overlay electroplated onto the bronze. A “P” might be all that’s offered for old-school engines made prior to the early 1960s.

H-series: The H-series Clevite-77 high-performance bearing 90 percent of Clevite’s performance bearing sales. It uses the same babbit formula as a P, but at about half the thickness to hold up better under heavy loads typically encountered in high-performance engines. “HN” variants are slightly narrower to clear enlarged fillets typical of most high-perf aftermarket cranks.

V-series: Originally popularized by the Vandervell bearing company (now owned by MAHLE), it uses a somewhat softer lead-Indium overlay. 98 percent of alcohol and nitromethane racers are said to use this type of bearing. They need fairly soft bearings to withstand the high shocks and loads; it also seems to work well with engines running larger journals. In general, if you are making from 2,500 to 11,000 hp this bearing might be for you. McKnight says supercharged big-block Chevys in the 1,800–2,000hp range might also benefit from this bearing.

M-series: Although classified with Clevite’s modern performance bearings, this is really a legacy bimetal design, with about a 0.020-inch thick babbit layer over a steel backing. Today it’s used primarily by Pro Mods, alcohol dragsters, and pulling tractors. As McKnight puts it, “It offers controlled wear until it’s worn away. Four out of five old 1950s first-gen Hemis use this bearing in tractor-pull applications. It’s effective up to 2,500 hp.”

To a large measure performance bearing selection closely dovetails with power output. “It’s basically a horsepower thing,” explains McKnight. “Zero to 700 hp will often run on stock A- or P-series bearings. 700 to 3,000 hp definitely requires a Clevite H or V series. 3,000–11,000 hp: You better be calling me!”

Journal Size

Although not common on the street or at the Sportsman level, Pro racers have long experimented with smaller-than-stock (for a particular engine) journal sizes—which of course means smaller bearings. There are a number of perceived advantages to smaller journals, including reduced bearing speeds and less rotating and reciprocating mass. With less friction, power improves. Other benefits include reduced inertia problems and an easier-to-balance engine. In NASCAR cup racing, rod bearings got as small as 1.770 inches before NASCAR standardized on the so-called “Honda” 1.850-inch-od. Small bearings are also popular in NHRA Pro Stock.

The downside? A smaller bearing is subject to higher unit loading per square inch of surface area, even as horsepower levels continue climbing. That these engines even survive is, McKnight explains, “due to very good fit-up, very good oil, tight clearances, and frequent replacement (we love that part). NASCAR engine life is about 600 miles; Pro Stock motors, about 8–10 miles. That’s not acceptable to most HOT ROD readers, but that’s what racing is all about!”

For over 10 years, Clevite has offered this indented-lug design for the very narrow shells used in NASCAR. This provides a smooth shell-half parting-line that reduces the narrow bearing’s tendency to fracture at a standard lug’s indentation point.

But for every rule there’s an exception: In the extremes of Funny Car and Top Fuel drag racing, unit loading is so crushing that some competitors use 3-inch mains. Even some blown small-block Chevy stroker “street” engines now make so much power that for long-term durability it’s increasingly common to order aftermarket blocks with larger 400ci main journals. Such larger than “normal” journals add strength to a big stroker crank by increasing the rod and main journal overlap.

Clearance

“Tighter bearing clearance is better than looser; thin oil is better than thick!” —Bill McKnight

What do you do if you’re walking on a frozen lake and hit a patch of thin ice (besides pray)? Lie down and spread your body’s load over as wide a surface area as possible. The same holds for bearing clearances. When the journal and main or rod bores are close to the same diameter (low clearance), the oil film contact area gets wider, spreading the load over a greater area.

On an engine prone to bearing failures, the old-school fix was a high-volume oil pump, thicker viscosity oil, looser bearing clearances, and maybe even fully-grooved main bearings. Current thinking considers these crutches counterproductive in most cases. They’re like a dog chasing its tail: It takes more power to spin a high-volume pump with thick, heavy oil, which is needed because of those big bearing clearances. But big clearances actually make it harder to establish and maintain the hydrodynamic oil film, as explained by Clevite’s technical literature: “Tighter clearances are desirable because they cause the curvature of the shaft and bearing to be more closely matched. This results in a broader oil film that spreads the load over more of the bearing surface, thus reducing the pressure within the oil film and on the bearing surface. This will in turn improve bearing life and performance.”

Bearings aren’t perfectly round or of uniform thickness. Each shell-end is about 0.005-inch longer than a half-circle, and their wall thickness is slightly tapered. This causes an assembled bearing to bulge slightly outward, generating a spring-like “crush,” which is what really holds them in position. It also compensates for bore distortion and aids heat transfer. Taper and eccentricity amounts vary by series and application.

Locating tangs are really just a hand-assembly aid. In fact, some modern rod bearings such as this one for a Chrysler 4.7L OHC V8 have no locator. It was a midstream change for this engine, but the no-tab bearing even replaces the old tanged version on the first-design tanged rods. Eliminating the locators saves up to 26 machining “cuts” per motor. That’s huge cost savings for the OEs.

Such tighter clearances must correlate closely with engine oil viscosity. Thinner oils that reduce friction (whether for more power in racing or better gas mileage on the street) work best with and are intended for tight-clearance operating environments. Even old-school engines can tighten up their clearances if they’re running modern thin oils. “I’m sold on full synthetic oil in my engines,” maintains McKnight.

How tight should the bearing clearances be in a high-perf or racing application? When running SAE 30 or thinner straight viscosity oil as well as modern multiviscosity oils, McKnight recommends a minimum 0.001-inch clearance for every 1-inch of shaft diameter:

Minimum Clearance = Shaft Diameter × 0.001

For example, McKnight says an “LS Chevy has a 2.100-inch rod journal, so around 0.0025 would be a very good clearance to strive for.” And on an old-school, big-journal engine like a 460 Ford with its 3-inch mains? “That calls for 0.003-inch minimum clearance—which seems loose, but it’s still tight relative to such a large journal. If you are running the engine hard and are relatively new at this, add 0.0005-inch more for starters. As you learn from experience and by observing the bearing condition when you teardown the engine, you can start reducing clearance.”

The eccentric bearing shells are thickest at 90 degrees from the parting line split; therefore, measure clearance vertically, 90 degrees from the shells’ parting line. The rule of thumb for performance engines is to allow 0.001-inch of vertical clearance for every 1-inch of shaft diameter.

Photo By: Jeff Smith

Once you get down to 0.002 or less—not atypical with Pro racing engines using small bearings or today’s OE engines designed from the ground up for tight clearances—oil may have difficulty getting into the journals to establish the wedge. That’s where the really thin new-tech synthetic oils come in: Some Pro Stock engines run 0W-7 or 0W-10 viscosities—but they preheat a cold engine before startup.

Many hard-core engine builders are so exacting they use slightly different-size upper and lower shells on a given journal to “split the hair” on clearances. “I’ve recently been working with some NHRA motorcycle teams,” relates McKnight. “They use very small rod journals, make 200 hp per cylinder, and use 0W-10 oil, which means we must have the clearance as tight as possible.” It’s OK to mix bearing sizes if less than 0.001-inch clearance adjustment is needed. Mixing standard with 0.001-inch oversize (denoted by an “X” in the part number suffix) or undersize shells respectively decreases or increases clearance about 0.0005 inch.

Aluminum rods use bearings with a dowel-hole in the lower shells. In a pinch, dowel-hole bearings also can be used in a steel rod. But if you think you need to “pin” a steel rod to prevent spun bearings, something else is wrong: clearance, crush, and/or oil starvation. Tangs and pins are really just assembly location aids only.

On most engines, only the top main bearing is grooved. McKnight: “That bearing should be ‘hydroplaning’ on the oil film. Which hydroplanes easier: slicks or treaded tires? A bearing groove is just like tire tread.” Upper main shells, which see less loads than the lowers, have retained a groove to supply the connecting rods with oil.

Another old-school “trick” was a main-bearing set with a 360-degree groove. Most main bearings are a half-groove, 180-degree design, with a smooth bottom shell and a grooved upper shell to supply oil to the rods. A 360-degree configuration has a full groove on both the upper and lower shells. Although still offered for some engines to appease traditionalists, they’re pretty much obsolete. McKnight explains, “Grooving the bottom shell takes away 20 percent of the bearing’s load-bearing area. Forces are trying to drive the crank down out of the engine, so that’s critical! Today we’re smarter. The oil is better. We know how to improve the oiling characteristics of marginal stock oil systems. Because bearings taper out from the center towards the parting line, grooves actually gives you an avenue to loose pressure. Some of today’s stockers now only have just a 120-degree upper groove.”

Although still offered for some legacy engines, fully-grooved main bearings usually aren’t the way to go. As groove length increases so does horsepower loss and peak oil film pressure, both of which are transmitted directly to the bearing. This fully-grooved small-block Chevy main set goes for $89.97 at Summit Racing. Summit Racing

Traditionally, the top main shell’s groove has always been a “180-degree” style (the top has a full groove, left). However, as technology continues to evolve, some new OE motors as well as high-end motors are reducing the groove to 140 degrees (right) or even 120-degrees

Coatings

“Use a TriArmor coated K-series bearing any time you can get one.” —Bill McKnight

Clevite’s TriArmor coating is a proprietary dry film applied to a trimetal Clevite-77 bearing’s top surface, but not to its parting lines. It adds low-load start-up protection, and serves as a high-pressure, high-load dry film antiwear agent that adds protection across a broad range of temperatures, especially when oil flow is marginal.

A relatively recent innovation, Clevite has offered coated bearings for about 14 years. A blend of polytetrafluoroethylene, graphite, and molybdenum disulfide, its “TriArmor” coating protects the bearing in marginal oiling-film situations that may occur in circle-track, Sportsman racing, Pro Stock drag racing, and even enthusiast street vehicles.

Clevite dark gray TriArmor coating (denoted by a “K” in the part number suffix) protects the bearing under extreme stress. The bearing shown here (PN CB-663HNK-1) is a 0.0001-inch undersize, H-series, narrowed, and coated small-block Chevy rod bearing. Note that on a coated (K) bearing, the K may not be embossed on the shell, but will be present on the packaging. Even on a street car, McKnight says a coated bearing is “really, really, good insurance if your car’s parked all winter.” TriArmor coating increases the bearing thickness about 0.0003-inch, not significant unless you’re running very tight clearances.

Coated bearings couldn’t be used at first in alcohol- or nitro-fueled boats, pulling tractors, and drag-racing engines; their extreme cylinder pressures could wipe the standard coating off in one pass. To solve the problem, Clevite developed its “Nitro V” coating for these specialized Hemi-engine main and rod bearings. Now used by most Pro teams, they may not necessarily last any longer, but when things do go bad, they don’t blacken the crank, saving thousands of dollars.

Some factory stockers now or soon will have coated bearings. While these cars’ bimetal aluminum bearings have a long service life, they don’t have the lubricity of the old babbit bearings they replaced. This can become a problem as fuel-saving traffic-light “start-stop” technology becomes more common. Startup is where a lot of bearing wear occurs, and now it’s happening dozens of times a day in city driving. Small diesels with bimetal aluminum bearings also can experience problems due to a diesel’s higher loads.

Resolving these issues requires a new generation of coatings. Performance-oriented coatings were meant to improve bearing performance under extreme loads, but for relatively short durations. For a stocker, the loads are less extreme, but the bearings need to last 300,000 miles. MAHLE Clevite is now testing a new polyamide polymer coating with exceptional thermal and wear properties to solve these issues. Stay tuned.

Cleanliness

“Some builders used to wipe the dull gray coating off a trimetal H-series bearing. It just means you need more bearings quicker. They’re supposed to be ugly!” —Bill McKnight

Choosing the right bearing, oil, and clearances will all be for naught if care is not exercised during assembly. McKnight says, “40 percent of bearing failures are caused by dirt, most often left in when you build the engine. Don’t polish new bearings with a rag; clean them in acetone with a thin sheet of newsprint paper. Another 40 percent of wear is caused by lack of good lubrication. That starts with assembly lube on the bearings that offers protection until the engine picks up oil and starts pumping it around.” Follow these guidelines and your engine will be loaded for bearings. And if you have any more questions, Clevite has a toll-free tech phone line and its tech advisors also respond to email inquiries.

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