Before selecting your pistons, you will have to know the required compression height so you can get to the actual compression ratio for the engine. Compression height is the distance between the centerline of the wrist pin bore and the top of the piston. When using a flat top piston, this can be simple math. But, add valve notches or a “pop up” dome, and now you need to do some math. The dome volume is comprised of the dome (material above the piston’s deck), minus the valve notches. This final number will either be a negative of positive. If the volume of the dome is greater than the volume of the notches (typical), then you have a positive dome height.