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the pistons have reliefs ,total 10 cc ,the head gskt bore is 4.41

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Unitred Machine(KB)on line calculator says 10.12 to 1 comp. ratio Good, safe pump gas combination

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Yep! Thats what I used, but @ 5cc 10.174

But a 5cc difference should be more then a .05 comp change. I,ll run the numbers again @ 10cc


EDIT, Yep, with 10cc Cab, I get 9.74 comp.

84cc head
5cc piston
4.375 bore
.008 deck
.039 gasket ht.
4.41 gasket diameter
3.75 stroke
= 10.174

add 5cc piston for a total of 10 cc = 9.74 comp @ United Machine KB.