When a customer asks for a set of engine bearings or an engine kit, what do you do first? Identifying the engine application is the logical place to start. You’ll need the year, make and model of the vehicle, the engine displacement in cubic inches or liters and possibly the vehicle identification number (VIN) or other information to accurately determine which engine is in the vehicle. Once that has been accomplished, you can look up the bearings for the engine to see what’s available.
In most cases, you’ll find a variety of bearings listed. There are standard sized and undersized bearings. There may be a choice of bearing types, such as stock or performance, a choice of bearing materials such as aluminum, tri-metal or other materials, and, of course, a choice of brands. In other words, there will be a lot of options from which a customer can choose to select a set of engine bearings.
Main bearings and rod bearings are typically available in standard size and .010,” .020” and .030” undersizes for passenger car and light truck engines. Undersize means the inside diameter (I.D.) of the bearing is smaller to accommodate a crankshaft that has been reconditioned by grinding the journals to a slightly smaller size.
For bearings to last, crankshaft journals must be smooth (no grooves or roughness), round (no flat spots or eccentricity) and flat (no barrel or taper wear). By the time most engines need a new set of bearings, the crankshaft journals are also worn. This requires removing the crankshaft and regrinding the journals to undersize, or replacing the original crankshaft with a crankshaft kit that includes a reground crankshaft and bearings. Sometimes the original crankshaft can’t be reconditioned because the journals have too much wear, a bearing has seized and damaged a journal or because the crankshaft is cracked or broken.
When a crankshaft is reconditioned, it is always checked for cracks using magnetic particle detection (Magnaflux) and wear. If the journals can be reconditioned without removing too much metal, the crank will be ground and the machinist will stamp numbers on one or more counterweights to indicate how much the journals have been ground (10-, 20- or 30-thousandths of an inch undersize).
The main and rod bearings must have the same undersize to fit the crankshaft properly. If standard size bearings are accidentally installed on an undersize crankshaft, the bearings will have way too much clearance, causing a severe drop in oil pressure, excessive bearing noise and rapid bearing failure. Likewise, if undersize bearings are accidentally installed on a crankshaft with standard-size journals, there won’t be enough bearing clearance and the bearings will bind causing an immediate failure when an attempt is made to crank or start the engine.
To reduce confusion regarding bearing sizes, most bearings have a number on the back of the shell to indicate the bearing size (standard or undersize). Some bearing manufacturers also add color codes for easier identification. One bearing manufacturer marks its standard sized bearings with a brown color code, .010” undersize bearings with a black color code, .020” undersize bearings with a pink color code and .030” inch undersize bearing with a yellow color code.
CHECKING BEARING CLEARANCES
The best way to make sure the bearings fit properly is to measure the installed clearances by one of two methods. The easiest method is to use “Plastigage.” This is a soft plastic material that is placed between the bearing and journal when the bearing is assembled. A strip of the plastic is positioned lengthwise across the journal below the center of the bearing. The main cap or rod cap is then installed and the fasteners are torqued to specifications to crush the plastic between the bearing and journal. The joint is then disassembled to remove the plastic strip. The crushed width of the plastic strip is then compared to a chart to determine bearing clearance.
The other method requires special measuring equipment. Calipers are used to measure the outside diameter of the crankshaft journal. The inside diameter of the bearings are measured with a dial bore gauge. Rod bearings are measured with the bearings installed in a connecting rod, and main bearings are measured with the bearings installed in a main bore. The inside bore diameter is measured vertically at the center of the bearing shell, not at the bearing parting line (because bearings are slightly eccentric to compensate for crush and loading). The outside diameter of the crankshaft journal is then subtracted from the inside diameter of the bearings to determine the clearance for each set of bearings.
Experienced engine builders know that checking each and every bearing clearance can prevent misassembly problems. Crankshaft journals may not always be ground to the exact same undersize, so checking bearing clearances is insurance against these kinds of mistakes.
Recommended oil clearances vary, so the installer should follow the engine manufacturer’s recommendations. Some engines today run very tight clearances of .0015” or less. High-performance applications may require different bearing clearances than unmodified engines. Many engine builders target a clearance range between .0022” and .0027.” Clearances greater than .003” are not normally recommended. Large bearing clearances will lower oil pressure, and may require a high volume oil pump. An engine should maintain a minimum oil pressure of 10 lbs. per 1,000 rpm.
Some bearings are also available with an oversized outside diameter for reconditioned connecting rods. Many late-model engines have powder metal connecting rods where the parting line between the rod and cap is formed by literally cracking the rod apart rather than cutting and machining the rod and cap. The advantage with this approach is that it creates a better fit between the cap and rod for better cap alignment. The disadvantage is that a cracked connecting rod cannot be reconditioned by recutting the cap and honing the bore back to standard size. If the bore is distorted, it must be honed to oversize to accept a bearing with a larger outside diameter. Or, the connecting rod must be replaced with a new one.
CHOOSING A BEARING MATERIAL
Bearing materials have changed a lot in recent years, and are improving all the time. Most late-model domestic and Asian engines are factory equipped with aluminum alloy bearings. The type of alloy will vary depending on the supplier that manufactures the bearings and the application. Without getting into a Brand A versus Brand B debate, suffice it to say that all bearing suppliers use a variety of alloys and some aluminum alloys are tougher than others depending on how much silicon, tin, copper and other elements are in the mix. Aluminum bearings are very corrosion resistant and long lasting, and are even used in many diesel engines. Some also have an overplate layer to enhance embedability and conformability.
Tri-metal copper/lead bearings were the industry standard for many years, and are still used as original equipment in many European engines. Tri-metal continues to be popular with many aftermarket engine builders and performance engine builders because of its high embedability, conformability and load-carrying properties. Like aluminum bearings, there are various alloys from which to choose. A typical tri-metal bearing will have a top layer of lead/tin/copper over a layer of copper/lead on a steel backing plate. The bearing may also have a bright tin flash plating on the outside to enhance its cosmetic appearance.
High-performance tri-metal bearings for racing applications typically use a stronger copper/lead alloy in the base layer, a stronger steel shell and may not have a tin flash surface plating because the tin plating can migrate across a bearing’s steel backing under race conditions, causing high spots on the inside diameter. These high spots may distort the bearing and have an adverse effect on the oil clearance. It can also concentrate loads and increase the risk of fatigue failure. For supercharged drag racing, many racers use babbit or other special alloy bearings.
Some high-performance bearings also incorporate additional design features such as extended oil grooves to improve lubrication, side chamfers for additional clearance on crankshafts with larger radius fillets on the journals, and special thrust-bearing surfaces that improve lubrication and durability.
Some customers have very strong brand preferences or prefer a certain type of bearing material. Some say the best approach is to replace same with same (aluminum with aluminum, and tri-metal with tri-metal) unless the engine is being built for a performance application, in which case the bearings may have to be upgraded to a stronger high-performance material. The best advice is to follow the bearing supplier’s recommendations.
If a customer is overhauling an engine, he’ll need more than a set of bearings. He’ll need piston rings and gaskets, maybe pistons if the original pistons are worn (oversized pistons if the cylinders are worn and have to be bored or honed to oversize), new cam bearings, likely a timing chain and gear set (or timing belt), freeze plugs and probably an oil pump. Most of these parts are included in a basic overhaul kit.
Engine kits are a great way to sell engine parts because they include most of the parts that may be needed to repair an engine with the convenience of a single part number. It saves a lot of time looking up parts and reduces the risk of mismatched parts.
If the crankshaft is worn, a crankshaft kit will provide a reconditioned crank with undersized bearings. Stroker crankshaft kits, such as those that increase the displacement of a stock 350 Chevy V8 to 383 cubic inches, are a popular upgrade. The longer 3.750” crank increases low-end torque and horsepower roughly 10 percent over stock. Most such stroker kits include pistons and connecting rods with the crank.
Which parts are ultimately replaced will depend on the condition of the engine, how much the customer is willing to spend and any upgrades or performance modifications to the engine. Upgrades would include things like choosing chrome or moly rings instead of cast-iron rings, choosing stronger hypereutectic or forged pistons over cast pistons, stronger or lighter connecting rods, etc.
Most master and deluxe overhaul kits also include things like a new camshaft and lifters, pushrods, valve springs and retainers, valve guides and new exhaust valves. Most experts recommend replacing all of these items on high-mileage engines if you want the engine to last.
A new oil pump is especially important because the oil pump is the heart of the engine’s lubrication system. Reusing an old pump that is worn or contains debris can doom a newly rebuilt engine to premature failure.
Regardless of which parts are being replaced, the bearings and cylinders must be thoroughly lubricated when the engine is assembled. Assembly lube or motor oil is fine, but oil tends to drain off bearing surfaces if the engine sits for a period of time. A dry start can be very damaging, so some bearings are now available with a protective anti-scuff coating. The same coating can also help protect the bearings and crankshaft journals if oil pressure is lost because of a lubrication problem (at least temporarily).
Prelubing the engine is always recommended before it is started following bearing replacement or overhaul.
Other items that may be needed when replacing bearings or overhauling an engine include motor oil, antifreeze, filters (oil, air and fuel), spark plugs, plug wires, belts, hoses, thermostat, water pump, motor mounts, head bolts and fasteners, and possibly some special tools such as a piston-ring expander, ring compressor, torque wrench, engine hoist and engine stand.