Building a high-performance engine demands careful consideration of every component, especially the bearings that support the crankshaft. Main Bearings And Rod Bearings play a vital role in managing the engine’s power output and ensuring its longevity. Choosing the right bearings, ensuring proper fit, and meticulous installation are paramount to a successful engine build. This article delves into the critical aspects of main and rod bearings, exploring their types, common failures, design differences, and crucial installation considerations.
Bearing Selection: Matching Bearings to Application
Selecting the correct bearings for your engine build is crucial. Always consult the bearing manufacturer’s recommendations for your specific application. Different bearing types cater to various performance levels, from stock engines to high-horsepower racing applications. Sticking with a proven brand and type is advisable if you’ve had prior success. However, if you’ve encountered failures or fitment issues not attributable to machining or assembly errors, consider exploring alternative bearing grades or materials.
Modern production engines often utilize bi-metal aluminum bearings due to their lead-free composition, cost-effectiveness, and impressive durability in stock applications. These bearings possess good seizure resistance and contain silicon for enhanced hardness. The silicon content also aids in polishing microscopic imperfections on cast iron crankshaft surfaces.
Bearing manufacturers utilize various aluminum alloys, each formulated to optimize fatigue strength, wear resistance, seizure resistance, and conformability. High-performance aluminum alloys often incorporate tin, copper, or other elements to boost strength for racing applications, enabling some to handle loads up to 10,000 PSI (550 to 600 hp). While many crate engines utilize bi-metal aluminum bearings, remember that these are designed for specific horsepower levels. Exceeding these limits can lead to bearing failure.
Tri-metal lead-copper bearings offer higher load-carrying capacity (up to 12,000 PSI or higher) due to their copper-lead layer. To counteract copper’s lack of seizure resistance, a thin babbitt overlay (typically 87% lead, 10% tin, and 3% copper) provides lubricity, seizure resistance, and embeddability. This babbitt layer can deform to accommodate minor journal irregularities, including crankshaft deflections in high-output engines. It also acts as a sacrificial layer, preventing fretting and failure under extreme loading.
Race bearings often feature a thinner babbitt layer for increased load capacity, but at the cost of reduced bearing life. Polymer-coated bearings (bi-metal or tri-metal) provide additional protection. These coatings, often containing graphite, molybdenum disulfide, and/or Teflon, offer lubricity during dry starts and mitigate seizure risk in case of oil pressure loss.
Bearing Failures: Causes and Prevention
Contamination is the leading cause of bearing failure, even in high-performance engines. Cleanliness is paramount; metal shot blast, honing residue, grinding debris, or metal chips in oil passageways can be catastrophic for bearings. Even with efficient oil filtration, microscopic debris can pose a threat, especially under extreme racing conditions where oil films can be incredibly thin.
Bearing Design: Optimized for Performance
High-performance bearings often incorporate design modifications to enhance load-carrying capacity. These include chamfered oil holes, narrower but deeper oil grooves, and partial grooves to distribute load across a wider surface area. Eccentricity, the variation in bearing thickness, also plays a crucial role in oil film formation and maintenance, particularly at high RPMs. Racing cranks often have larger fillet radii, requiring chamfered bearings for clearance.
Bearing Installation: Precision and Care
Quality control is essential throughout the manufacturing, machining, and assembly processes. Bearing manufacturers strive for precise sizing and consistent shell thicknesses. However, always verify bearing sizes and markings before installation.
Accurate machining of crankshaft journals, main bores, and rod bores is critical for proper bearing fit. Always measure bearing clearances with a dial bore gauge, not just a plastic gauge. Insufficient clearance can lead to binding or seizure, while excessive clearance causes oil pressure drops, noise, and potential bearing damage.
Bearing crush, the slight deformation of the bearing upon installation, ensures a secure fit and promotes heat transfer. Aluminum rods require special consideration due to their higher thermal expansion rate. Rod bore diameter, bearing crush fit, and bearing oil clearances should be rechecked after a period of operation with aluminum rods.
Bearing Oil Clearances: Finding the Right Balance
Optimal oil clearances depend on oil viscosity and application. Traditional engine builders often prefer looser clearances with heavier oils for increased crank flex tolerance, particularly in drag racing or dirt track environments. However, NASCAR engines often employ tighter clearances with low-viscosity synthetic oils.
A common guideline is .0007˝ to .001˝ of clearance per inch of journal diameter, but this varies with oil viscosity. Oil pressure should be sufficient to maintain the oil film; 10 PSI per 1,000 RPM is a general rule, but some builders opt for lower pressures to reduce parasitic losses. Oil flow requirements depend on factors like engine material, piston oilers, and supplemental oil feed lines.
Always install bearings dry in the bores, then lubricate journal surfaces with assembly lube or oil. Prime the oil system before initial startup to prevent dry start damage. Address any unusual noises immediately after startup, as they may indicate loose components or excessive clearances. Careful attention to these details will help ensure the longevity and performance of your engine build.
