DIDN'T KNOW YOU WERE IN USA !
RON FARTHING'S THE MAN TO TALK TO:-
www.ronfarthing.co.uk or:-
e-mail info@ronfarthing.co.uk
THE MARSTON SUNBEAM REGISTER (PRE 1940 SUNBEAMS ONLY) info about club from:-
trevor@daviesnewport.f9.co.uk
They have copies of spec: sheets,spares etc.
and members that have links to the original factory.
(sunbeamland in wolverhampton)
Model 8 std bore is 70 mm , stroke 90mm.
I doubt if your piston is original so your rings will be different to sunbeam's own.