The pilot is absolutely critical to getting a concentric valve seat. It “pilots” the cutter by indicating off the same bearing surfaces that the valve stem will be riding on, and carrying that center-line up to the cutter. If the pilot is not straight and concentric, the cutter cannot form a concentric seat. Neway pilots are center-less ground, a process that enables us to hold extremely tight tolerances. 

Solid pilots have a very slight taper ground into them, so that they lock into the valve guide. The rate of taper is just .0007” per inch (7 microns/cm). This very slight taper means that the pilot will be hanging onto enough of the length of the guide bore to accurately capture the center-line. The flip side of having such a slight amount of taper is that a single pilot can only accommodate a small amount of variance between guide bore sizes, so you will usually require more than one solid pilot for any given nominal size.