B25S and SS pistons

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Cocker Cox
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B25S and SS pistons

Post by Cocker Cox » Thu Jul 02, 2020 7:16 am

Hi, All,

Continuing my earlier post about refurbishing the cylinder head and rocker box of my 25SS (now sorted), I’m interested in the difference in compression ratios of the S (10:1) and SS (9.5:1). Where is the difference made, with the piston or cylinder head? Both have different parts numbers, although the SS head has 5/16” rocker box studs as opposed to 1/4” for the S, which may account for their different parts numbers. But the pistons look the same, dome-wise....

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Re: B25S and SS pistons

Post by minetymenace » Thu Jul 02, 2020 8:22 am

The difference between 10:1 and 9.5:1 is about 1.3 cc in compressed volume or a little under 0.5 mm in crown height, with my eyesight the crown heights would look the same as well! If you start with a high compression piston (assuming there is enough meat) you can measure your compression ratio and skim the crown down to achieve your desired CR.

Check out EdV's pistons if you are looking for a new one, modern rings...nice.
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