It's an oft-repeated story -- but it still is worthy of another telling.
Before you are awarded with a record your engine size must be verified as within the min/max limits for your chosen class. Measurement using dial indicator and micrometer/ruler is the basic and a very accurate way to measure. There are other ways, though -- for instance, if the sparking plug (hey - he lives in Britain
) is dead center in the head the measuring official can use a measuring stick -- graduated length for depth, and with a ruler that's on a center pivot so it can swing from vertical (to get into the cylinder) to horizontal (to measure the bore). That's a pretty accurate way to go.
There are a couple of hydraulic methods of measurement -- one using air and one with a fluid such as brake fluid. There the valve train is disconnected so all valves are closed and oil is introduced into the cylinder from a graduated column. When the cylinder is 100% full the crank is rotated 180 degrees, and the oil/fluid that comes out is measured. that gives you the displacement. Air can be used as well -- but since I've never seen it done and only heard about it -- I won't try to describe how it's used.
The fluid volume methods are not a precise as the physical measurements, and are therefore more commonly used when there's little chance the the displacement is very close to the edge of the class. That is -- if you're in a 500 - 750 cc class and your engine is (according to you, who built it, about 745 cc, more or less, you'll probably be asked to allow the officials to do a direct measurement. If you've got a motor in the same class and it's a 685 cc -- pumping it with oil will likely be all that's required.
Remember, of course, that I'm giving you the story as one who's been measured a good few times. I'm not the guy doing the measuring, and if he wants you to tear it apart - you don't have lots of choices remaining. You could, I suppose, whine and moan and try to have the chief inspector tell the not-so-chief that he doesn't need to be that precise. Remember, of course, that the inspector is the guy that's holding you at arm's length from your record - don't be too snippy