I'll copy/paste the info since most prolly won't click to read it:
[font=Geneva,Verdana,Arial]Trimetal or Aluminum Bimetal Bearings?[/font] [font=Geneva,Verdana,Arial]There is a lot more at stake than a few pennies price difference. The right bearing choice can significantly improve your rebuild success rate.[/font]
[font=Geneva,Verdana,Arial]In the real world, building engines for the aftermarket takes place is less than perfect environment. Chances are you're working on a variety of power plants that have operated under a broad range of conditions. An original equipment manufacturer; on the other hand, assembles new engines which are all the same from uniform new parts in a pristine plant. You just don't have those advantages! No matter how clean the shop and excellent the technicians, you still can use all the forgiveness you can get from your bearings. A close look at aluminum bimetal versus cast copper/lead Trimetal leads to only one conclusion. Aluminum may work well for OEM builders, but Trimetal is the sensible choice for the aftermarket.[/font]
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Babbitt, a tin or lead based alloy with varying amounts of antimony, copper or arsenic, is the original and still the best bearing material available. It has great seizure resistance, embedability to trap destructive abrasive particles, good corrosion resistance and conformability to assure good fit. So why use anything else?[/font]
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With all this going for it, babbitt has one problem. It lacks fatigue strength. It breaks down under load. The common solution is to apply a thin layer of babbitt over a supporting layer of stronger material.[/font]
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Figure 1[/font]</td> </tr> </tbody> </table> [font=Geneva,Verdana,Arial] [/font] [font=Geneva,Verdana,Arial]
A look at figure 1 points out an interesting characteristic of babbitt. As the graph shows, the durability of babbit greatly increases as the material decreases in thickness. This seeming contradiction is due to the greater support provided by the backing material. Simply put, the less babbitt, the fewer the problems with fatigue and deterioration. [/font]
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In order to maximize its durability, the babbitt must be kept to the range of .001" or less.[/font]
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Cast copper/lead alloys, while still forgiving, make much stronger bearing materials than aluminum or babbitt.[/font]
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By combining the surface properties of a babbitt overlay with the strength of cast copper/lead, the best overall combination of properties is provided in a single composite. <table align="right" border="0" width="150"> <tbody><tr> <td align="center" valign="middle" width="150">
</td> </tr> <tr> <td align="center" valign="middle" width="100%">[font=Geneva,Verdana,Arial]Figure 2[/font]</td> </tr> </tbody></table> [font=Geneva,Verdana,Arial] [/font][/font]
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[font=Geneva,Verdana,Arial]As shown in figure 2, Trimetal consists of a strong steel shell lined with this cast copper/lead, babbitt composite.[/font][/font]
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[font=Geneva,Verdana,Arial]This combination has many attractive aftermarket attributes: [/font][/font]
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[*]Conformability to adjust for less than perfect tolerances and geometry
[*]Durability to assure long life
[*]Embedability to trap abrasive particles
[*]The best bearing surface to minimize friction and heat generation[/font][/font]
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[font=Geneva,Verdana,Arial] Leave off the babbitt coating, and there is no embedability to trap abrasive foreign particles which can mean big trouble. <table align="left" border="0" width="150"> <tbody><tr> <td align="center" valign="middle" width="150">
</td> </tr> <tr> <td align="center" valign="middle" width="100%">[font=Geneva,Verdana,Arial]Figure 3[/font]</td> </tr> </tbody></table> [font=Geneva,Verdana,Arial] [/font][/font][/font] [font=Geneva,Verdana,Arial]
[font=Geneva,Verdana,Arial][font=Geneva,Verdana,Arial]Leave that copper/lead cushioning layer out, as in an aluminum bimetal bearing, and you have a very unforgiving product. A slight misalignment or tolerance variation can quickly lead to failure.[/font][/font][/font]
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[font=Geneva,Verdana,Arial][font=Geneva,Verdana,Arial]The obvious questions seems to be why would anyone pick aluminum bimetal? Only one reason, under the carefully controlled contamination and tolerance conditions of mass production, OEMs are able to gain an incremental cost reduction.[/font][/font][/font]
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[font=Geneva,Verdana,Arial][font=Geneva,Verdana,Arial] <table align="right" border="0" width="150"> <tbody><tr> <td align="center" valign="middle" width="150">
</td> </tr> <tr> <td align="center" valign="middle" width="100%">[font=Geneva,Verdana,Arial]Figure 4[/font]</td> </tr> </tbody></table> [font=Geneva,Verdana,Arial] As shown in figure 4, the actual construction of a Trimetal bearing is a bit more complex. A nickel barrier plating prevents interaction between the babbitt and copper/lead. Flash lead/tin plating protects and provides a cosmetically more attractive finish.
What all this means to an engine rebuilder is that Trimetal bearings, products such as those found in the Clevite 77 line, will provide the conformability, durability, embedability and forgiveness required in aftermarket conditions. In fact, Clevite Engine Parts supplies over 90% of its aftermarket bearings in this Trimetal configuration.
Years of fine-tuning the complex variables in this product have maximized its performance to the point where there is no better choice for the engine builder who is serious about his work and reputation.
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