Mopar Rod and Main Bearing Guide - Basic Bearings 101

Although performance bearings aren't the first thing you think of when defining the specifications for a high-performance engine, and they probably won't do much to increase engine output, they are one of the most important items necessary for ensuring the reliability you'll expect and need when other engine modifications are made.

The right bearings are an all-important factor in a performance engine-in fact, they're vital. Bearings, of course, keep the crankshaft-which is turning somewhere around 7-8,000 rpm-spinning smoothly as the rods transmit combustion force to drive it.

According to Bill McKnight, Training and Motorsports Management for Mahle Clevite, the world's largest supplier of engine bearings, performance crankshafts and rods often feature lighter weight, and operate under much higher engine speeds and loads than those in everyday engines.

Bearing Characteristics
Because of bore-shape variables, the most desirable bearing shape is provided with a slightly oval inside diameter. As a result, bearings are manufactured with an eccentric wall. In most cases, the bearing wall thickness is greatest at the top and bottom (90 degrees from the parting line). The bearing ID tapers slightly at the parting line area.

Connecting rods are subjected to especially high inertia loads at the top of the exhaust stroke when the weight of the piston, rings, and wrist pin pulls at the rod big end. This dynamic force also tries to pull the rod's big end out-of-round, which causes the bearing housing bore to become tighter at the parting line area. In order to address this, the slight eccentricity is designed into the bearings. This prevents the bearing from contacting the crank journal when bore distortion occurs. Eccentricity is also important for oil film formation when the engine is first started.

In addition to a range of bearing eccentricity (low, medium, and high), performance bearings are offered with a host of additional design features to properly function in the high-load and high-speed environment of a high-performance/competition engine.

Designed for Interference Fit
Main and rod bearings are designed to provide interference fit with their housings. In half-shell bearings, this is called "crush." Each bearing shell (upper and lower) is made with a length that results in slightly more than a true half-circle, so that the ends of the bearing shell, when installed into a housing, protrude slightly beyond the parting line of the housing,

McKnight says, "When the cap is installed and the cap bolts are tightened, these protruding ends of the bearing shells are forced against each other and push the bearings against the housing, compressing the bearing shells. The outward force of the bearings, as they squeeze into place, causes a slight change in the size and shape of the housing. As you might expect, different housing materials (such as steel versus aluminum) provide different levels of bore distortion. Bore distortion is a natural by-product of the assembly. Compensating for this bore distortion can be tricky-a multitude of variables can directly affect main bore and the connecting rod's big end bore shape."

Engine blocks and connecting rods feature irregular shapes that surround the bearing's housing-bore. For example, connecting rods usually feature a cast-in beam at the top of their bore, notches for bolt heads and/or nuts at each side of the housing bore, and/or thin or thick ribs at the bottom of the cap and so on. In addition, dynamic loads (when the engine runs at various speeds and loads) change in both magnitude and direction. All of these factors combine to cause the bearing housings to develop an out-of-round shape under many operating conditions. Depending on the surrounding metal shape and mass, some housings will go out-of-round in either the horizontal plane or the vertical plane. This varies with each engine, and quality performance engine bearings are designed to compensate for these geometric changes.

Main Bearings Depend on Good Oil Film
All bearings depend on a film of oil to provide support for the journals, and as we all know, the crankshaft should never actually come into contact with the bearing surface. The oil film is developed as the crankshaft rotates and oil is pulled into the loaded area of the bearing, allowing the journal to "ride" on this film similar to the way a tire rides on a film of water when it hydroplanes. Many early engines used full-grooved main bearings (a groove in both upper and lower shells) and some even used multiple grooves. As engine and bearing technology advanced, bearing grooves were removed from most modern lower main bearings. The result is a thicker film of oil for the crankshaft to ride on. This provides improved bearing life.

In order to develop the best possible main bearing designs for high-performance engines, Clevite has invested a considerable amount of time and research into studying the effects of main bearing grooving. This research has shown that the best overall design for Mopars and most other engines is one that features a simple 180-degree groove in the upper main bearing shells.

Selecting The Right Bearings
Tri-Armor
Clevite77 offers a line of bearings called TriArmor coated engine bearings. They feature a proprietary moly/graphite treatment applied to rod and main bearings. TriArmor provides extra insurance in severe use without compromising any existing bearing characteristics. These engine bearings are treated with a patented blend of molybdenum disulfide and graphite. Engine builders have long known that even modest reductions in friction can lead to measurable increases in power, and coated bearings deliver that.

H-Series
The H-series is a popular design, suitable for a wide variety of high-performance and racing engines. In a nutshell, if you're unsure about which type of performance bearing to use, the H-series is an excellent candidate. These bearings were developed specifically for use in NASCAR-type applications, but are suitable for all types of competition and high-performance engines.

H-series rod and main bearings provide maximum crush and a medium level of eccentricity with a hardened steel back. Rod bearings also have a thin overlay. One very notable feature of the H-series bearings is a narrowed width to provide greater crankshaft fillet clearance. If your crankshaft features large fillets (common with high-performance crankshafts), and if the engine will run in the medium- to high-rpm range, the H-series is an outstanding choice.

V-Series
The V-series rod bearings typically feature a low to medium eccentricity and provides a hardened steel back. For applications involving crankshafts with large fillets, narrowed bearings are available (under a VN suffix) to accommodate increased crankshaft fillet clearance. The primary difference between the V-series and other Clevite 77 TriMetal bearings is the use of a lead-indium overlay. A lead indium overlay offers a slightly better conformability than a lead/tin/copper overlay, along with slightly reduced wear resistance.

Fitting The Bearings
Since bearing thickness is greatest at the top center and bottom center (90 degrees from the parting line), always measure a bearing at 90 degrees to the parting line, in order to determine what your minimum oil clearance will be. When measuring bearing wall thickness, use a micrometer that features a ball-shaped anvil (use of a flat-anvil mic will result in an inaccurate reading).

A good way to measure bearing clearance is to measure the bearing inner diameter with the upper and lower bearing shells installed in their bores, and with the cap bolts fully tightened to the specified torque value. Once the bearings are installed and the cap is fully tightened, measure the installed bearing ID with a dial bore gauge.

Next, measure the crankshaft journal (for that specific bearing location) with a micrometer. Subtract the journal diameter from the installed bearing ID to determine bearing oil clearance. A good rule of thumb dictates that you should have about .001 inch of clearance per inch of shaft diameter. For instance, if a main journal outer diameter measures 2.375 inches, then you should have about .0024-inch clearance. In order to provide a slight margin of safety (especially for rod bearing locations), you can add an additional .0005-inch clearance.

Let's say you're having a hard time obtaining the desired clearance. "Extra Clearance Bearings" (Designated by the letter "X" in the part number suffix) are available, but only for standard size journals. A word of caution: Do not attempt to increase bearing clearance by polishing the journal. Polishing is not a precision operation, and excessive polishing is likely to destroy journal geometry (straightness and roundness). If clearance is too loose, you can choose an undersize-ID bearing. According to Clevite, it is permissible to mix bearing sizes if the required mixing results in less than .001-inch difference of wall thickness. When mixing bearing sizes for select fitting, never mix parts that have more than .0005-inch difference in wall thickness. Also, always install the thickest wall bearing shell in the upper position (for rod bearing application) or the lower bearing shell position (for main bearing application).

Checking Clearances with Plastigage
Bearing clearance can also be checked with the use of Plastigage, which is a fragile, soft, and compressible plastic wire strand. This soft material is inserted between the crankshaft's journal and the bearing. Once compressed, the plastic wire retains its new crushed width and can be used as a reference to determine bearing clearance by comparing the crushed width to the gauge printed on the package.

Plastigage was designed to allow anyone-even those lacking sophisticated tools-to measure total vertical oil clearance during engine assembly. In order to check bearing clearance using Plastigage, clean the block saddles and caps, and install the upper main bearings in the block saddles and the lower main bearings in their respective caps. Install the upper bearings using prelube just as if you're building the engine.

Position the dry crankshaft carefully onto the installed upper main bearings.

Place a strand of Plastigage lengthwise onto each main journal (the strand should be positioned front-to-rear and should be parallel with the crankshaft), and simply rest the Plastigage on top of the journal.

Carefully install each main cap and tighten the main cap bolts to specified torque value. DO NOT rotate the crankshaft while the Plastigage is in place. This will smear the Plastigage, rendering it useless.

Carefully remove the main caps and measure the crushed width of the Plastigage strand, using the graduations printed on the Plastigage package envelope. Measure the entire length of the strand. Note all dimensions on a piece of paper. Proceed to the next cap and check strand width and so on. Once all main journal locations have been checked, apply prelube and install the caps, tightening to spec. Plastiguage is oil soluable so there is no reason to try removing it.

When using Plastigage to check rod-bearing clearance, install rod bearings onto the rod and rod cap. Install the rod onto the crank's rod journal (make sure rods are in correct order) with fillet chamfer sides facing journal fillets. Following the procedure we've already discussed, lay a piece of Plastigage onto the exposed rod journal, install the rod cap and tighten, then remove the cap and measure the Plastigage. Again-and we need to stress this point-do not allow the crankshaft to rotate when Plastigage is in place. The crank must remain stationary.

Plastigage is available in four different sizes for checking vertical oil clearances on main and rod bearing locations. Each package features a handy measuring scale printed in both inches and millimeters. Strips are also color coded for easy size range identification.

Recommended bearing clearances have been determined as a result of extensive R&D by bearing makers and engine manufacturers. Optimal clearances for specific engines can be further refined after running and teardown/inspection (such as would be performed by professional race engine builders).

Also note: If your measured clearance is too tight, DO NOT attempt to polish the bearing running surface with any type of abrasive pad or paper. Bearing overlays are extremely soft and thin and can easily be damaged or even removed by abrasive media.

Engine Assembly Lubricant
Using a quality engine-assembly lubricant is a smart thing to do. Bearing assembly lube is specially formulated to protect the engine bearing and crank upon initial start-up. In addition to all the 1/2 shell bearings, both rods and mains, be sure to apply this lubricant to the thrust bearing faces as well. Before attempting to position the crankshaft in place, always check for cleanliness. Carefully wash the crankshaft in hot soapy water using a soft, clean brush. Rinse with clean water and blow dry with compressed air. Once the crankshaft is clean and dry, apply a thin film of clean engine oil to the journals. Even though engine assembly lube has been applied to the exposed bearing surfaces, it's still a good idea to lube the crankshaft as well before installing. Lube all main journals and rod journals, applying lube well into and beyond the fillet areas.

Carefully position the crankshaft into the main bore saddles, being careful to avoid nicking the crankshat journals. The mere weight of the crankshaft contacting an edge of a block's main bearing saddle can easily nick, gouge, or scratch a crankshaft journal, which can degrade oil flow around the journal or result in a scored bearing, leading to bearing failure.

Connected Adjustments
Choosing a main bearings is one thing, but what about the connecting rods? Many of us put the connecting rods in and hit it with a torque wrench, but is there a better, more accurate way? Even if using the best rod bolts available, measuring bolt stretch offers a much more accurate method of achieving ideal clamping loads as compared with the use of a torque wrench. To use the bolt-stretch method, fasteners must be through bolts (not cap screws) and have flat ground ends that permit accurate measurement of overall length.

To measure rod bolt stretch, first measure the total rod bolt length (from the head surface to the tip of the shank) in the bolt's relaxed state (when installed onto the rod but without the nut). Then measure the bolt again after the nut has been tightened. The difference in length indicates the amount of stretch the bolt experiences in its installed state. For the majority of production rod bolts, stretch should be in the .006-inch range. Check with the supplier to obtain the proper stretch versus load specification. If the stretch is less, the bolt is probably experiencing too much friction that is preventing the proper stretch (requiring lubricant on the threads). If stretch is excessive, the bolt may have been pulled beyond its yield point and is no longer serviceable.

Clevite's Bill McKnight says that while a micrometer may be used to measure the rod-bolt length, the most accurate method is to use a specialty fixture that is outfitted with a dial indicator. This is referred to as a rod-bolt stretch gauge and is available from several specialty tool sources.

Connecting rod bolts should be regarded as high-tensile springs. The bolt must be stretched short of its yield point in order to achieve accurate, and, most importantly, repeatable, clamping of the rod cap to the rod. Improper or unequal bolt clamping force can easily result in an out-of-round rod bore.

Stock, or production, rod bolts typically offer a tensile strength of approximately 150,000 to 160,000 psi. However, due to variances in bolt production, tolerances can be quite extreme, with peak bolt stretch travel occurring anywhere from, say .003 to .006 inch. If you use only a torque value in the attempt to achieve bolt stretch, you run the risk of achieving unequal rod bolt clamping loads. High-performance rod bolts are manufactured to meet much tighter tensile strength tolerances.