I know that it's not entering the debate but I have measured, faily accurately, the displacement of old OHV engines using a fluid.

The fluid used being SAE 50 oil (To minimise leakage past the rings). The oil can be warmed a little to aid pouring and measuring, it will soon cool in the engine cylinder. This is not practical when the engine is in the bike but ok on a bench.

With the rockers, cam drive or push rods removed, remove the plug and tilt the engine until the plug hole is as near vetical as possible.

Ensure that the piston is at TDC

Using a laboratory burette, measure the amont of oil to fill the combustion space to about midway up the plug hole and record.

Move piston to BDC and slowly refill the cylinder to the same level in the plug hole and record.

The second reading minus the first, being the swept volume of that cylinder.

The effective compression ratio can also now be determined by dividing the total volume (combustion space volume plus swept volume) by the combustion space volume.

This process can be fairly accurate and I seem to remember that the late and great Phil Irving mentioning something similar in his "Tuning for Speed" masterpiece.

However the big disadvantage is getting rid of all the excess oil before running the engine.


PS For the Green or Eco minded out there, use old engine oil