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Piston / cylinder letter codes


monty348
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Greg, I'm not sure this is relevant for the earlier Beta's, but here is what the engine service manual for the Beta Evo series lists for the difference in size. In each case they are in increments of 0.01 mm (0.0004").

image.png.e52da91810dbf06d1ed0cf4f6c98f062.png

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Its all about limits and fits. Mass produced components have only a finite tolerance acceptable for mass assembly, so depending on quantities say 100 cylinder barrels bored to the specifications may be plus+ or minus- but still acceptable to be married up to a piston again + or - in specification. Thats the beauty of having a new piston and a rebore, the engineer will match everything closely to the specs, more accurately than at the factory. 

These limits and fits are the reasons some engines are noisy mechanically and others are quiet when new.  

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