These NEW cylinder heads from S3 are an affordable and easy way for customers to change and personally setup the compression of the GG PRO125. They are superbly engineered and come in a kit with FOUR different inserts for different conditions. VERY HIGH, HIGH, STANDARD and a 'blank' insert so you can make your own for even more personal set up. The basic rule is this.....if you want a fairly flat power for beginers or really slippy conditions than you would set the bike up with a lower (std) compression insert. If you wanted a more aggresive power setup or you are going high in altitude then you could experiment with higher compressions...