As mentioned, you can not know EXACTLY what your combustion chamber size is, without measuring the chambers. Can't go by online info, or what was advertised years ago. And it don't take a really big cut to reduce the chamber size of a low '70's cc head, down to an upper 60's cc chamber size.
I would strongly agree with this. I've built lots of engines (from many manufacturers) and advertised numbers are rarely what you actually find when get out your tools and measure. You're using 45+ year old parts, you rarely know what has been done to them in that time, as well as the fact that the manufacturing process was not that precise to start with.
As someone said earlier, too many variables. You need to measure chamber size, piston deck height, valve reliefs, head gasket thickness, even cylinder eyebrows can add to chamber size.
-=>Lee<=- Due to budget constraints the "light at the end of the tunnel" has been turned off!
