References
NAVSEA 0900-LP-060-2010, Technical Manual for Electrical Machinery Repair: Electric Motor Repair Shop Procedures Manual (Series), Chapter 2, "Test Equipment and Measuring Instruments"
NAVSEA S9086-KC-STM-010/CH 300, Naval Ships' Technical Manual (Series), Chapter 300, "Electric Plant, General"

Background

The bore gage is an instrument that is used to precisely measure bearing housings to an accuracy of within one ten-thousandth inch.

One advantage of the bore gage is that it is a dial indicating instrument that can be read directly. Determination of the proper amount of pressure inside the bore does not depend on a sense of feel like using an inside micrometer does.

Setting the bore gage requires the use of standards (ring masters) before commencing measurements. These standards correspond to the exact outside diameter of most of the ball bearings that you will find in electric motors.

Any deviation (plus or minus) in the bearing housing bore is readily indicated when the bore gage is properly set and inserted. The readings obtained are then compared to the allowable minimum and maximum measurements of the size and type of bearing installed.

Major parts of a typical bore gage

• An insulated handle that reduces heat transfer from the hands to the instrument, which minimizes the possibility of inaccurate readings caused by the thermal expansion of the metal
• A dial with digits indicating thousandths (0.001) and incremental lines indicating ten-thousandths (0.0001) inch
• A bezel that allows zeroing the dial
• A bezel clamp that locks the bezel in place
• A head, consisting of a sensitive contact, two centralizing contacts, an extension piece, and the internal mechanism that transmits sensitive contact movement to the dial. The sensitive contact detects surface variations.
• Two spring-loaded centralizing contacts that position the sensitive contact in the bore
• A set of removable extension pieces that adapt the gage to read different sizes of bores. Adjustment and locking of these pieces is done to zero the dial indicator in the standard