Grn14 is, in substance, correct. To summarize (having built race engines since 1971...), the trick is to cause the ring load (and the cam lobes) to "lap" succesfully to the mating surfaces. Many theories, many anecdotes, many "this works" practices, etc. But the fact of the matter is the two primary part materials that "rub" require a certain temperature and lubrication regime that allows the materials to "smear" without spalling, and by consequence, require only lubrication to otherwise stay in close contact under which they operate.
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Assuming oils are equal, and the ring to wall pressures (and lobe to rocker assemblies...) can withstand the results of the heat and kinetic energy (translated into the increasing friction) can be controlled by rpm levels, then these components can be "mated" to withstand the much higher pressures and destructive friction potential when the motor is operated at peak levels.
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The alloys used in the cylinder walls, rings, cam lobes, valve stem assemblies, and rocker arm assemblies undergo a surface change under heat and pressure. The metals, all of which have a crystalline structure when produced, are inherently "friable", and can abnormally wear, unless a gradual process of heat and pressure changes the structure with minimal destructive deformation. Analogy? Take a knife blade, and heat it, beat it, quench and repeat the process a few hundred times, and it will result in a molecular structure on the surface area (and down several layers...) that is incredibly "hard".
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That's all fine and dandy... But the second part of the "break-in" is the "polish" that results. The finer the surface at the micron level, the easier it is for the oil to maintain the necessary film thickness to prevent further "polishing" or "lapping". We still need some porosity at the micron level to provide a residual lubricant level (that's what rings need to survive...), but also is why after 100,000 miles, we need to replace rings, valves, guides, camshafts, etc.
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When we break in a motor, we are applying pressure (and frictional temperature) in opposite directions and variably over surfaces. The purpose of the break in is actually two-fold. First, we want to have the frictional surfaces "mate" evenly - not just under accelerational, or any particular sustained rpm level operations. Second, we want to give the metals time to "mate", and thus change their micron level alloy characteristics to a level that minimizes friction and the consequential wear under both acceleration, sustained level rpms, and deceleration.
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The manufacturers are conservative (for liability and warranty reasons...). Varying the rpm (and the load) during the first 500 miles is "de-rigeur" for bikes and cars. Follow the manufacturer's instructions? Pretty much guarantees a good break in. Can we "accelerate" the process? Of course!! As long as we know what we are really doing.
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The key is to vary the rpm, the accelerational and decelerational loads, and vary the sustained rpm levels accordingly. Heat and pressure will do it's work.
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Caveat... I was "edumacated" as a mechanical engineer, with a focus on metals and alloy structures. Not that I worked in the field worth a crap after all that "schoolin", but it has applied to my hobby of building race engines, and making sure my hot rod and bike engines stayed together and were reliable.
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But that is, of course, anecdotal, LOL!!!!
* Last updated by: groomlake on 10/8/2014 @ 8:59 AM *