I'm trying to find the cc,s of the dish on a .040 over 383 low compression dish piston in a 1968 engine.The pistons are in a assembled engine.

You can use the downfill method where you run the piston down the cylinder a known amount and calculate the difference between what the volume should be and what it really is on account of the dish. Lots of engine books go into detail.

Of course, you didn't say if the heads are installed or not but I assume they are not.

If you cannot find the listed spec & the heads are off, .7854 X dia of dish squared X depth X 16.387. Put something straight across the dish (steel scale,popsickle stick etc) & measure the dia and slip feeler gauges under it to get the depth. i.e. a 4" dish w a .040" depth would be 8.2CC's.