How to Use Dial Bore Gage?

Dial bore gauge kit consists of a dial indicator, an insulated handle with a slotted lockable collet assembly on one end and a spring loaded guide on the opposite end. For reference in these instructions the spring loaded guide is the "bottom" and the slotted collet end is the "top" of your dial bore gauge. The dial indicator features a rotatable bezel, lockscrew and adjustable external indicator "points". The spring loaded guide features a total of four "contact" points. These contact points consist of one end with a bore and threaded knurled locknut. This bore accommodates the various length anvils and spacers included with the kit. The opposite end of the spring loaded guide features three points. The two outside evenly spaced stationary points allow accurate centering of the tool when placed inside a bore. The center point is spring loaded to permit a variation of range.

Using a micrometer held in a stationary position, adjust the micrometer to the preferred size and lock the barrel. Place the dial indicator in the collet side hole in the insulated handle and gently snug the lock. Always hold the insulated handle by the insulation and not by the tubular structure. This will insure consistent thermal stability of your dial bore gauge settings. Do not finish tighten the dial indicator lock just yet because to fine tune your bore gauge to various diameter settings you may want to raise or lower the dial indicator in the bore of the insulated handle. Choose a combination of anvils and spacers to be placed in the bore side of the spring loaded guide at the bottom of the insulated handle. What is required here is sufficient length of anvils and spacers to provide enough spring tension "load" of the dial indicator to be able to produce both PLUS and MINUS indicator readings. To fine tune these anvil and spacer lengths use the thin spacers and the adjusting of the dial indicator up and down in the insulated handle.

Gently rock the dial bore gauge assembly in between the two anvil faces of the micrometer. Knowing that the shortest distance between two points is a straight line you will find this shortest point displayed on your dial indictor while it sweeps across the micrometer anvil faces. If your bore gauge assembly feels like it "bottoms out" hard you have too much (too long) of spacer or anvils. The indicator needle will rotate or "fall" in the "plus" direction on anything other than the shortest distance set on your micrometer. Keep in mind that you also want sufficient travel in your assembly to allow the dial indicator needle also to be able to go in the "minus" direction of the dial indicator. Finish tightening the top locknut of the dial indicator to the insulated handle. Once doing so recheck the assembly in your micrometer and fine tune the indicator reading of what you want to call "zero" by rotating the bezel face of the indicator and tighten the lock screw to hold that position. Slide the external indicator points to whatever you feel is required for the type of bore your are measuring. Typically the external indicator points would be set at acceptable plus or minus tolerances of your choosing.

Holding the insulated part of your bore gauge and keeping in mind that, when your bore gauge is not touching anything it is spring loaded extended to a larger dimension than your bore is, gently lower the gauge into the bore you are measuring at a slight angle to the bore and then hold the bore gauge perpendicular to the bore and check that the two stationary points of the spring loaded guide are sitting evenly in the side of the bore. Gently rock the bore gauge assembly front to back to determine the shortest distance between the sides of the bore. Take readings top and bottom to check for taper in your bores.

It is recommended that you keep your "standard" (micrometer, caliper or ring gage) set up in case you have any doubt of any given reading. That way you'll always be able to go back and check against your known standard.