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Crank Polishing

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What is the point? Is there 6 feet of snow where you are?

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No... I'm making the crank more reliable by removing any burs or microscopic cracks that could potentially cause a failure.

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No... I'm making the crank more reliable by removing any burs or microscopic cracks that could potentially cause a failure.

I thought the reason to polish a crank was to minimize parasitic loss in wet sump engines?

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Further, I've read that you don't really want to polish a crank because the small imperfections in the metal surface trap oil and keep a sheen of oil between the rod bearing & crank. Shot peening actually puts small dents into the surface (picture a golf ball) that hold oil for the same purpose. The shot peening also relieves stress within the metal making it (*supposedly*) stronger and less likely to crack or fracture.

 

I supposed you could use fine grit sand paper (1000 grit or higher) and knock off any imperfections from the surface without actually altering the surface shape.. But why? Why do you want to do this?

 

A crank is a disposable item. It will wear out. You will have to replace, or rebuild, it after "X" number of hours whether you treat it or not. Don't waste time doing anything special to the crank because it is doubtful that you will see any performance, or longevity, gain from doing so.

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Polishing a crank does reduce crank windage, therefore reducing parasitic loss within the crankcase, but only minimally.

 

Shot peening strengthens the surface of a part.  Shot peening actually, may cause microscopic cracks in the metal's surface that may lead to a failure, which is why a shot peened part should be vibratory deburred or micro-polished after the shot peening process.  I don't think shot peening helps retain oil on a metal surface, but WPC, a similar peening process, definitely helps with oil retention.     

 

I think polishing the crank halves is fine, since they don't actually come in contact with any other metal, and that's all I would be polishing.  Crank halves can last for a long time, and only need a rod replacement every "X" amount of hours.  Yes, I could buy a ready-to-go stock crank every "X" amount of hours, but for roughly the same price, I can rebuild my crank with a much stronger rod, making a catastrophic failure less likely.  Since I have my crank apart now, I mine as well polish the halves and throw a Carillo or CWI rod on it. 

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I understand what you are saying now.. I was thinking journal surfaces is what you were wanting to polish. Instead, you are wanting to polish the "stuffers" or "tins" as we call them on 2 stroke cranks.

 

I doubt you would ever notice a performance difference between a polished, and a non-polished, crank but if that floats your boat-GO FOR IT!

 

While you are in there polishing, have you heard about knife-edging the crank counterweights? Read up on that.

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The crank webs are the only thing that you could polish. These are not the weakest point of the crankshaft on a motocross bike, and are not a typical failure point in crankshaft failures, so there isn't really a point. Any burr on the outside of the crank web would not create a stress point anyways by the way the forces are loaded on the crank. An area of concern would be around the machined holes where the big end pin is pressed in, but these are already machined and deburred from the factory in these areas.

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According to Mickey Goldmill Polishing your crank makes your knees weak...

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