This Phathead set fits Honda CR250 1997-99. Phathead sets are CNC machined from 6160 Billet Aluminum. They feature interchangeable domes (combustion chambers) with different compression ratios. Phathead sets include a Phathead body, your choice of two domes, o-rings, all necessary hardware and installation instructions. The standard two domes for this set are the H22 (16.4cc) the dome closest to stock, and one step higher compression H23 (15.5cc). The higher compression domes available are the H24 (14.4cc) and H25 (13.3cc) domes. When selecting domes, the smaller the cc the higher the compression. To order a Phathead set choose the color of head body with the drop down menu below. Enter the 2 dome sizes you want with the set. By entering no dome selection defaults to the standard set of domes. WARNING. When using the higher compression domes you must run high octane fuel. Running low octane fuel in a high compression engine could cause serious engine damage. Experienced motor builders, to view a list of additional dome options available for this head set go to the H2 and N2 series domes category