Originally Posted by frog
Here's how I understand it...
In the picture above, the decomp. plate only replaces the same amount of material that was removed by the skim(s).
However, because the walls of the plate are vertical, the squish area has changed shape.
Over to you Stu...
The issue is frog, that the decompression plate will usually have circular bores rather than the same shape as the chamber face with the two D-shaped flat areas (which are in fact the squish areas), this leaves a gap the thicknes of the plate between those flat areas of the head and the piston crown raised areas, hence the squish clearance increased by the plate thickness.