We have these bushings custom made to meet our exact tollerances. They are cnc machined and then surface ground to .0002" tollerance, an extra step done for precision press fit that you will not find with others.  We recommend freezing these over night and installing with green loc-tire with a driver and heavy blow hammer. These will not spin in the head like others or crack HLA bores when installing.  Lifter Bores in head must be machined down .350" before installing.