difference()
{
  cylinder(h=23, r=30/2, center=true, $fn=50);
  translate([0,0,3.01])
  cylinder(h=23, r1=24.5/2, r2=25.5/2, center=true, $fn=50);
  cylinder(h=100, r=2.5, center=true, $fn=50);
}