In 1948–69 Big Twin motors, the clearance between the front exhaust lobe and the ignition drive gear must be checked and modified as required. In most cases the ignition drive gear must be turned down approximately 7 mm. Big Twin motors from 1948–57 will also require the clearance between the front intake lobe and the engine case to be checked and modified if necessary. If using one of our cams in Panhead engines, the cylinder head must be modified (except J-Grind cam). It is highly recommended to check all clearances, especially valve collar to guide distance, everytime you change a cam.