back the lower edge of the piston skirt, as that extends the intake timing - to permit backflow as the piston descends - without producing any measurable improvement in flow coefficient. It is, however, possible to improve flow with a down-turned lip at the top edge of the intake port window. But the primary thing one must remember when carving away at an intake port is that ripples in the port walls, or any sudden change in cross section, have a far more damaging effect on flow-coefficient than a slightly rough finish in the port. Therefore, it is vastly more important to smooth the port than to give it a mirror-finish. And it should be obvious that the port face, the gaskets and heat-block (if any) and carburetor should all align very neatly, without any steps between parts - even if that means doweling everything in place to assure alignment.