MB-2687P - Clevite Main Bearings
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Part #: MB-2687P
Notes: Individual Main Bearings
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Various forms of main bearing grooving have been used over the years. We are frequently asked what difference grooving makes.
First, it’s essential to understand that bearings depend on a film of oil to keep them separated from the shaft surface. This oil film is developed by shaft rotation. as the shaft rotates it pulls oil into the loaded area of the bearing and rides up on this film much like a tire hydroplaning on wet pavement. Grooving in a bearing acts like tread in a tire to break up the oil film. While you want your tires to grip the road, you don’t want your bearings to grip the shaft.
The primary reason for having any grooving in a main bearing is to provide oil to the connecting rods. Without rod bearings to feed, a simple oil hole would be sufficient to lubricate a main bearing. Many early engines used full grooved bearings and some even used multiple grooves. As engine and bearing technology developed, bearing grooving was removed from modern lower main bearings. The result is in a thicker film of oil for the shaft to ride on. This provides a greater safety margin and improved bearing life. Upper main shells, which see lower loads than the lowers, have retained a groove to supply the
connecting rods with oil.
In an effort to develop the best possible main bearing designs for performance engines, we’ve investigated the effects of main bearing grooving on bearing performance. The graphs on the next page illustrate that a simple 180° groove in the upper main shell is still the best overall design.
While a slightly shorter groove of 140° provides a marginal gain, most of the benefit is to the upper shell, which doesn’t need improvement. On the other hand, extending the groove into the lower half, even as little as 20° at each parting line (220° in total), takes away from upper bearing performance without providing any benefit to the lower half. It’s also interesting to note that as groove length increases so do horsepower loss and Peak Oil Film Pressure which is transmitted directly to the bearing.
These are not to be confused with the standard passenger car and light truck parts for the same retention applications which also have a “P” suffix letter. These high performance parts have unique core part numbers different from the standard parts for the same application. P-Series parts are the oldest series of Clevite high performance bearings. The rod bearings in this series typically have the greatest amount of eccentricity. Most rod bearings are available either with or without dowel holes for use in aluminum rods. Most P-Series main sets are full grooved to maximize oil flow to the rod bearings. Both rods and mains have high crush for maximum retention, and a reduced overlay thickness to prevent overlay fatigue, sometimes referred to as hen tracking.
Rod bearings use a hardened steel back for added strength and resistance to fretting. Extra clearance rod bearings are available for .001” additional clearance and select fitting. Use the P-Series rods where extremely high RPM’s cause severe rod bore close-in. This is typically indicated by nearly full parting line to parting line shaft contact with bearings having less eccentricity. Use P-Series mains where higher eccentricity is desired to narrow bearing contact patterns and to provide increased oiling to rod earings. Rod bearing oil starvation is typically indicated by polishing and smearing of the bearing surface, possibly accompanied by discoloration predominantly concentrated at the axial center of the bearings.
These bearings are identified by a letter H in the part number suffix. Part numbering is based on the same core number as the standard passenger car parts for the same application. These bearings were developed primarily for use in NASCAR type racing, but are suitable for all types of competition engines.
H-Series bearings have a medium level of eccentricity, high crush, and rod bearings have a hardened steel back and thin overlay. These bearings also have enlarged chamfers for greater crankshaft fillet clearance and are made without flash plating for better seating. Bearings with .001” extra clearance are available for standard size shafts and carry the suffix HX” (X = extra clearance). Rod bearings are available with or without dowel holes (HD = with, H = without), main bearings are available with standard 180 degrees upper half grooving and with full 360 degrees grooving (H = 180 degrees, HG 360 degrees). Use H-Series bearings with crankshafts that have oversize fillets and where engines run in the medium to high RPM range. H-Series bearings should be used if contact patterns obtained with P-Series parts are too narrow. Contact patterns should ideally cover 2/3 to 3/4 of the bearing surface. See accompanying contact pattern diagrams. If you aren’t sure which type of performance bearing to start with, the H-Series bearing will be your best choice.
These bearings are identified by a letter K in the part number suffix. Part numbering is based on the same core number as the high performance part and will service the same application. These bearings were developed primarily for high performance applications and all types of competition engines. K-Series bearings have a proprietary moly/graphite treatment applied to the bearings surface, but not the bearing parting lines. The PTFE carrier material gives good low load start-up protection. The moly serves as a high pressure, high load dry film anti-wear agent. Graphite provides additional protection across the broad range of temperatures, especially when oil flow is marginal and is especially slippery with an oil film. These bearings, which are also referred to as TriArmor, still offer the strength and durability of the legendary Clevite TriMetal bearing construction coupled with the latest in coating technology.
These parts essentially duplicate the former Vandervell parts under the Clevite part numbering system. (Same core part no. as standard passenger car parts but with a suffix letter “V”).
V-Series rod bearings typically have low to medium eccentricity and a hardened steel back. All V-Series main sets use a single piece thrust bearing rather than the former Vandervell assembled type of construction. V-Series parts are not available with oversize chamfers. Extra clearance parts are available with a suffix VX (.001” extra clearance), and VXX (.002” extra clearance) for some applications. V-Series bearings do not have flash plating on the steel back. Narrowed parts are available with a VN suffix for some applications. These are made to accommodate increased crankshaft fillet clearance.
The chief difference between the V-Series and other Clevite TriMetal bearings is the use of lead-indium overlay. Use V-Series bearings if prior experience has shown a preference for the lead-indium type of overlay. Lead indium overlay offers somewhat better conformability than leadtin-copper overlay with slightly reduced wear resistance.
Clevite “Micro” bearings make up the M-Series. These are special purpose bearings having a nominal .006” thick babbitt lining on a hardened steel back. M-Series rod bearings have been slightly narrowed at one end to provide extra fillet clearance without the need of a large chamfer. The lower rod shells have a dowel hole for use in aluminum rods with dowel pins. M-Series mains have enlarged chamfers and, for certain applications, oil holes and oil grooves have also been enlarged.
Use M-Series parts to take advantage of the high degree of conformability offered by the babbitt lining. These parts are intended mainly for engines where severe crankshaft deflections cause edge loading of the bearings. Under these operating conditions bearing service life will be very short.
Frequent inspections are recommended and bearings should be replaced at the first signs of distress.