Just to clarify, the raised D profile pistons ' reduce' the combustion chamber volume by 2.16cc compared to a flat top ?
Yes, a reduction in chamber volume.
In a loose way you could say it'll have less total chamber volume than a flat top, but to be accurate rather than assume, you need to measure the Pin to Crown heights of both as they are unlikely to be the same. The D Copper Pistons have a nominal height of 1.366" from the Pin Centre to the lower part of the Crown (with the D added on top of that).
