Answers to questions like

How is 1.9 mm of squish gap equal to 18% of the volume of the combustion chamber ?

And how is the stock compression ratio 7.1 : 1 with a 0.9mm squish gap ?!

fig 1.5

Here is how I got the figures for squish gap volume and compression ratio for different squish gaps. These figures are for the 'high compression' Y2 head, which is by far the most common, and a stock bore. The lower-compression Y1 head has a squish band that is not the same thickness throughout, which makes it harder to measure up.

The picture on the left shows the piston at the top, or TDC (Top Dead Center), as they say in the trade. I have divided the compressed volume into three parts: