75 post up your casting number and year if you know it- those here can give you a worst case low cr high cc guess
of course if they have been milled along the way cc's would be less, cr higher
real original ccs may be 5 ccs greater than published numbers - which turn out to be the MINIMUM that race sanctioning bodies will accept
for example 74cc adv BBM 915's are closer to 80 and open chambers vary from 86 to 90
gasket thickness- almost no one builds with .020 or under steel shims unless you are a class racer .028 and around .040 are much more common
pistons vary in compression height and valve pocket cc's
taken together they can add up to quite a difference
