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Replacement piston sizing


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I have a Polini Thor 100 which has nearly 200 hours on it and as part of the 200 hour servicing it is recommended to replace the piston and rings. I currently have an 'A' size piston and am trying to establish what the most suitable size to order for replacement is. I contacted Polini who suggested measuring the bore to find the best fit but have as yet been unable to supply me with dimensional info. I have access to bore micrometers at my workplace so that's not a problem but I need to know the recommended clearance and pistons sizes form A to D.

Does anyone have this information or any suggestions.

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The different sizes aren’t what you think there for ,  A. B. C. D , etc are for manufacturing tolerances , so the same motor as yours from same production process will probably  have a slightly different bore or maybe slightly thicker cylinder coating the chap selecting the piston will have a choice of pistons to select from ,  A. B. C. D parts bin , where the pistons are kept, hope it helps .

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