On the left is the top side of a 2.5 piston. The top side is the rebound side, and the rebound shim stack will sit against the six holes found close to the center. On the right is the bottom side of the same 2.5 piston. This is the compression side, and it faces downward. The compression shim stack sits against the three smaller round holes. You will also notice a tiny hole close to the outer edge. This hole is never covered by a shim, and is called a free bleed hole. Oil flows freely though this tiny hole, whose function is to smooth out the tiny bumps.