The casting limits width increases. Most engine builders increase the timing by either shortening the skirt on the intake side and/or lowering the base of the port. The standard has been to take .040" (1 mm) off the skirt depending on how much the base of the cylinder is lowered. The combination of lowering the cylinder to reduce the squish and trimming the piston often lowers the port by .060" (1.5 mm).

Lohring Miller