Current bearings fall into two basic construction designs, bimetal and trimetal. Bimetal designs feature a steel backing with a top layer of aluminum alloy. Trimetal bearings have a steel backing like the bimetal but contain a copper lead center layer with various materials being used for the top layer depending upon the usage application of the engine. A bimetal bearing is widely used in most OEM applications because it is a harder material and can last for many thousands of miles to provide a longer service life for the engine, also a bimetal is more cost effective to produce. In regard to the crankshaft, street driven vehicles don’t have issues to contend with such as crankshaft deflection (high HP street engines being the exception) or operating in harsh environments so a harder bearing is a better choice. When moving to higher power outputs and RPM levels, the trimetal design is able to take the additional abuse that this type of engine can encounter without suffering the same level of damage that a bimetal design would. By no means however, is that last statement intended to infer that a bimetal design can’t be used in performance applications. In engines that are producing 500-550hp they can work very well and give a long operating life. Both types of bearings essentially possess the same load bearing capacities with the difference being in the construction. They are engineered to perform specific duties in varying engine applications. If we look at a couple of popular HP street builds like the SBC 383 and the SBF 347 which are built in large numbers using cast steel cranks and generally producing anywhere from 400-500hp, a bimetal bearing would be a good choice for these type engines. For the most part, the sustained RPM levels are lower and peak below 6200 in these applications. Since the bimetal is a harder bearing, it can adequately support the crank while giving a longer service length. In comparison, a rule restricted small block engine built for dirt track competition but making similar power levels while operating at higher RPM’s and in a harsh environment should use a trimetal type. This will give a higher level of embedability (the bearing materials ability to absorb small foreign particles in the oil) to prevent damage to the journals and withstand crank deflection better due to the softer design of the bearing. In engines that are producing higher power levels and full competition builds, the trimetal will be the bearing of choice. Another way of summing it up in regards to engines that are achieving high HP, high RPM’s and twisting the crank is to remember that you don’t want to use a bearing that’s too hard. The other aspect of utilizing a softer bearing in HP builds is the ability to withstand crank deflection without peeling away portions of the bearing surface because the softer material will wear off or deflect under the load.
In recent years there has been a move toward tightening up clearances. It has occurred in OEM’s and the aftermarket both. This is partly due to advancements in bearing designs but also has much to do with modern engine oils. Given that the crankshaft is spinning on a hydrodynamic layer of lubricating oil between the main journal and the bearing, it’s easy to see how the viscosity of the lubricant that’s used can play a major role. Many modern performance oils have a much thinner operating viscosity than what was widely used in the past. The chemical compounds of these oils are also designed to take higher levels of heat and promote friction reduction. Granted, another huge component in this equation is the strength factor of the crankshafts available to us today but power can be found in setting clearances tighter to take full advantage of the oils. Consider that when the main journals of the crank and the internal dimensions of the bearings are closer to the same diameter, the oil film has a wider contact area in which to distribute the load. This provides a reduction within the oil film and on the surface of the bearing. The thinner viscosity oil simply flows much easier than a heavier, thicker oil. We will pause at this point to reiterate the fact that you must have a strong crankshaft in order to make the engine work with tighter clearances in performance applications otherwise the bearings will be history. If we look at the highest HP producing engines on the planet, which would be blown nitromethane, it gives us a clear picture of what happens when the crank is flexing. The cylinder pressures and massive blower boost place astounding loads on the bottom end, which is why you see the teams dropping the pan and pulling caps to check the bearings after each pass. This example is definitely from the extreme end of the competition scale but again, it shows how quick you can kill a bearing when crank flex takes up the clearance. Now that you’ve heard a portion of the case for the benefits of tighter clearances combined with thinner oils, we’ll make a 180 degree turn with the caveat that not every application can utilize this set up. If we refer back to our previous example pertaining to crank flex with Top fuel engines, they wouldn’t make it through the burnout without knocking out the bearings if the clearances were tight. In these type engines main bearing clearance is generally around .005 - .006 with lubrication duties being handled by synthetic oil that has a viscosity in the 70-90 range. These engines are running on the verge of hydraulic lock due to the amount of nitro filling the cylinders so you can imagine the loads that the bearings are under. We can also look at older or vintage engines in the same manner, not that they are operating under such extremes but in the fact that better cranks and aftermarket blocks are not available when building an engine of this type to make more power than it was originally intended to produce. There can still be a reduction in the clearance on older applications though if utilizing modern synthetic oils. Just to clarify, we’re not talking about massive dimensional differences. Something in the range of .0005 - .001 can make a difference in the oiling efficiency and film pressure of the engine. Initially, the practice of increasing the bearing clearance came about when builders were looking for ways to keep their motors alive while making more power out of factory engines without the benefit of the technology available today. Looser clearances could help you go more rounds or make more laps with cranks that were moving around in OEM blocks that suffered everything from cap walk to poor oiling characteristics under elevated loads or RPM’s. Along with the additional clearance, higher viscosity oils were used in the effort to stave off bearing problems in conjunction with higher volume pumps to move the lubricant through the engine. A lot of this came from the pioneers of the hot rod industry but there has also been a great deal of advancements made by bearing manufacturers in terms of the technology we were using in the not so distant past too. We assemble our sprint car engines today differently than we did in the early 90’s and with the current technology bearing failures are fairly rare. The industry accepted standard for clearance is .001 for every inch of journal diameter which would mean if you had a 2.500 main journal then .0025 clearance would be the number to aim for. One manufacturer gives a suggested range of .0007 - .001 per inch with a builder option of adding an additional .0005 for performance engines. A friend of mine at one of the performance oil companies put it into some simple terms by stating that “If standard is .0025, then loose is .003 and tight is .002” regarding clearances. Another area that is affected by the clearance is oil temperature. Tighter clearances can produce higher oil temps but with less loading the engine will operate smoother. Additional clearance will have more oil flowing through the bearing which will give better oil cooling but the peak load for the bearing will be higher.
Cam bearings, like the mains, are manufactured with various materials but the majority of bearings that are used in the aftermarket are bimetal designs composed of a steel backing with an aluminum alloy lining. This provides greater fatigue strength, conformability and better wear characteristics than babbit. As with other engine bearings, advancements in technology and manufacturing processes have produced a higher quality product available today. Rather than cover the lineage of the cam bearing we’ll discuss some of the installation and design strategies. One of the biggest culprits in cam bearing issues has to do with misalignment. This can occur from improper installation or be the result of block distortion. Use of a centering cone while driving in the bearings will result in achieving the correct alignment in the cam bore and prevent damaging the steel backing. Precautions should also be taken to prevent any nicks or gouges in the bearing material during installation and especially when test fitting the cam for proper rotation. Block distortion happens gradually over time and the camshaft wears the bearings to match the amount of distortion through continual engine operation. Upon disassembly of the engine, the camshaft would rotate freely in the worn bearings but produce tight spots or make installation of the new cam impossible after the bearings have been replaced with a new set due to the distortion of the housing bores. Larger OD bearings are available for many mainstream engines which allows the cam tunnel to be re-bored in order to eliminate the distortion of the block. The general consensus of bearing manufacturers suggests an optimum operating clearance of .002 with an upper range of .004 in order to provide the best oil layer. When installing cam bearings, the positioning of the oil hole can be placed in a preferential location in order to aid the oil in achieving a more efficient hydrodynamic layer upon start up and operation if the housing bores in the block are fully grooved or the bearing itself has an oil grove made into it. The best position for the oil hole when viewed from the front of the block would be at the 2 o’clock (60 degrees) location when using a standard rotation camshaft. This location enables the oil layer to begin to form quicker at the lower part of the bearing plus the rotational forces of the cam can assist in pulling oil through the feed hole. Not all blocks have this option and there may not be a grooved cam bearing available, at which point the feed hole should be centered with the oiling hole in the housing bore to insure a full flow path for lubrication. We’ve really just scratched the surface on the subject of bearings as there are several other areas to discuss such as roller cam bearings and coatings that we’ll have to save for later. A quick note regarding coatings though would be to simply say that if a coated bearing is available for your particular application it will always be beneficial to utilize it if it fits into your build budget. Matching your bearing material to the application in conjunction with setting clearances and oil selection will go a long way in keeping the engine performing smoothly.