Build a Bore Gage
Bore gages are comparative instruments designed to check hole diameter and condition. Dorsey dial bore gages use the principle of two point gaging and three point centralization. The gaging contacts are diametrically opposed which allows the most accurate reading of a bore. The reference contact is constructed to allow range extensions and adapters to be added to change the measuring size of the gage without loss of accuracy or balance. During use, the gage is inserted on an angle for easy entry into the bore. The centralizer ensures locating on the true diameter. The gage is "rocked" and the indicator is observed to find the highest measurement to obtain a true diameter reading.