Bore gauges

[kvom]

I was reading through Bog's thread on restoring the model steam engine, and saw his use of the spring-loaded gauges for measuring internal diameters.  I recently acquired a good quality Japanese set through another forum.  I had suspected that using my digital caliper's inside measuring jaws resulted in inaccurate figures.  When I used a gauge to measure the bore of an engine I'm working on, I found that that caliper was .003" off (hence the too-loose pistons I had made a few times).

My instructor gave me the following technique for getting the best accuracy.  With the gauge inside the bore, hold the shaft at a slight angle from vertical and tighten the lock just slightly.  Then raise the shaft to the center and finish closing the lock.

[bogstandard]

Kvom,

It is the little flats on the nibs that cause the problem with a digivern, they can't sit correctly on the curved surface of the bore.
But as you most probably know digiverns aren't classed as a precision instrument, and can only be relied upon to 0.002" (0.05mm). Even then they have to be held correctly to the job otherwise it can be a lot less accurate.
You are quite right on the use of the bore gauge, and the more feel you can get in your fingers, the more accurate they become. This is true of all hand held measuring, even the trusty mic can give the wrong readings if not handled and used correctly.

Bogs