These are coated bearing shells....
These are coated bearing shells. Note the coated gray/blue surface asopposed to the uncoated silver/gray of standard bearings. Clevite deemsit unnecessary to coat the thrust faces of bearings since they do notcarry reciprocating loads. The parting line surfaces of these bearingsare not coated so as not to disturb the critical crush dimension.Image courtesy of Clevite
Many years ago, I began having some engine parts coated with variousfriction-reducing materials. This process was considered a secret bysome, and there was some hocus pocus about the process. The coating ofengine parts has matured over the years and so has the knowledge of itseffectiveness.
Usually an engine builder buys the bearings of his choice and sends themto a specialty coating shop. The shop's experience and opinionsdetermine the type of coatings that are applied to the bearings. Somecoaters do a better job than others. The engine builder must then verifythat the coatings and the process are to his own specifications. It hasstill been a custom process.
Until now, the one thing that hasn't changed has been the availabilityof coated bearings to the individual or to the small engine shop.Clevite, the supplier of most performance engine bearings, has longprovided the standard performance bearing with its Clevite 77(R).Clevite has added to its line a production coated bearing that can bepurchased at many auto parts stores and through speed shops.
This bearing is based on the familiar Clevite 77 TriMetal(R) bearing.What has been added is a moly-graphite/PTFE-based coating. This resultsin the Clevite TriArmor(TM) bearing. The Clevite TriArmor(TM) bearing isthe result of several years of laboratory research and field testing inrace car engines. This was necessary to establish processes andprocedures that resulted in bearing coatings that were both performanceenhancing and quality controlled.
For a bit of history on coated bearings, I dialed up Bill McKnight ofClevite. Now the director of training and marketing for Clevite,McKnight was the long-time director of the company's engine machine shopschool. At this facility, he had access to the best inspection, testing,and machining equipment.
All this begs the question: Why coated bearings, and what will they dofor me? At this point, I am going to let the experience of Bill McKnightanswer.
"For a long time, bearings were so good that there was little need forimprovement," he says. "The Clevite 77 TriMetal(R) bearing line was thestandard of the industry in its various configurations. Performanceengine builders specified preferred clearances to their crank grinder,double-checked them, and focused their attention and creativityelsewhere. But as other components improved, such as low-tension rings,which approached the limits of reduced friction, engine builders beganto look at the crankshaft/bearing interface. This became another area tobe explored for further reducing friction.
"We all know that in an ideal world, the bearing never actually touchesthe crankshaft journal. Theoretically, it always rides on a thin film ofoil. In the real world, of course, bearings do contact crank journals,especially during start-up.
"As excess capacity in the aerospace industry became common, thesehigh-tech companies began looking to performance engine builders forbusiness. That is when racing engine builders started sending enginebearings out to specialty coating companies. Soon, Clevite began its owncoated engine bearing program."
Notes McKnight, "We found some coatings were very slippery but not verydurable. The best performance was found in a blend of moly (molybdenumdisulfide) and graphite. It was so slippery it wouldn't stay on thebearing. Once we found a way to suspend this blend in an inert polymer(PTFE) substrate, we had good adhesion to the bearing withoutcompromising lubricity.
"The benefits of all this are several fold: The reduction of frictionresults in a power increase. Longer engine life is due to the lesserfriction. Also, protection during start-up is no small issue.
"The TriArmor(TM) bearing coating is sacrificial. It gives itself awayin the interest of lubricity. Even when the coating is gone, there isstill an unmodified Clevite 77(R) performance bearing underneath."
Bearing crush is what holds...
Bearing crush is what holds the bearing in place. The tang on the shellthat fits the saddle is only for locating the bearing during assembly.This crush, as shown on a connecting rod, is critical to bearinginstallation. With the Clevite TriArmor(TM) bearing, the crush is notdisturbed by virtue of the parting surfaces not being coated. Rememberthat when measuring bearing clearances, always measure perpendicular (90degrees) to the parting surfaces. The bearing will always have somerelief at the parting line. Measurement at this point will be less thanaccurate. Image Courtesy of Clevite
A question comes to mind about bearing clearances. If you apply acoating, won't that decrease bearing clearance? The TriArmor(TM)bearing's coating is 0.0003-inch thick. This reduces the clearance byabout half of one thousandth of an inch. McKnight explained that mostengine builders find this slightly tighter clearance will not affectperformance or engine life, especially in view of the reduced friction.Of course, if the engine builder has a particular preference, he can askthe crank grinder to make any size adjustment he feels is necessary.
McKnight says that no special procedures are necessary when installingthe Clevite coated bearings. One thing to note is that the coatedsurfaces are compatible with petroleum-based oils, as well assynthetics, along with the various additives. He does advise givingspecial attention to bearing parting surfaces. The parting surfaces ofthe Clevite coated bearings are not coated. A buildup here wouldincrease the bearing crush to a point of deforming the bearing shell.Crush is critical because it holds the bearing in place. Some customcoaters coat the parting surfaces along with the bearing surface. Then,the engine builder has to sand the bearing shell parting surfaces tocorrect the crush. I don't like an abrasive anywhere near the bearings.Fine particles easily embed in the bearing. Besides, if a bearingparting surface must be sanded, and crush is critical, exactly how muchwould you sand? With the Clevite coated bearing, this would beunnecessary.
McKnight also said that it was not necessary to coat cam bearings andthrust surfaces on main bearings. This is because there are only radialloads, not reciprocating loads.
The bottom line of all this is that now you can go to a good parts storeand buy a set of coated bearings to put right into your race engine.