I have only done a careful measurement of one Vincent cylinder head and the result was 103.5 ccs. That was done with a calibrated pipette and a thin piece of Perspex.. Books on tuning will explain what to do if it is not clear. That, of course, needs the head to be off. What is it that you really want to know? If it is the compression ratio then compression testers are now only a few pounds. If you can gain access to a calibrated pipette, or similar, then you can use thin (5 grade oil) and put it down the plug hole ensuring that the valves are shut and the engine/bike is leant in the correct direction to ensure that the plug hole is exactly at the top of the combustion chamber. Fill to half way up the plug thread. Given the above figure of 103.5 ccs and the swept volume of 499 ccs you can see that a flat topped piston that did not protrude into the cylinder head would give about 5.8:1 compression ratio. That is why pistons have 'bumps'. The Terry Prince heads have a much smaller cylinder head volume.