What is the difference between full-ball and half-ball small hole bore gages? Small hole bore gages come in two main types: full-ball and half-ball bore gages. The terms full- and half-ball refer to the end of the bore gage that is inserted into the bore to complete the measurement. This end is typically opposite to the knurled knob used for setting the anvils. Full-ball small hole gages are generally simpler to set the anvils on and lock into place. More often than not, these bore gages will provide a more accurate and precise measurement of a bore. Half-ball gages are more prone to springing during measurement and require a more experienced user. Half-ball bore gages are more likely to result in an inaccurate measurement. However, some machinists prefer half-ball bore gages because they allow the user more control and leave room for adjustment in unusual measurement circumstances.