Quote Originally Posted by thehomeengineer View Post
This is OK as long as the bore/hole is deep enough or a through hole to allow the adjustable parallel to be located in place to take the reading. I like the use of the plug gauges as this can have an upper and lower tolerance turned/machined on any old piece of bar big enough to suit the application.
I do have some Moore & Wright bore parallels, which have a crown on the top of them, to measure across, but find the same issue with them. Great when they fit but very limited in their use.
For me, first choice (if you have one) internal bore mic as the bore can be checked in several positions and along its length so any taper that can occur can also be identified.
The Home Engineer
I too appreciate the sliding crowned parallels, despite limits of range, great on narrow bores. Depending on accuracy requirements, tapered leaves work well also. A well known brand can differentiate .001's, the user might be able to interpolate .0005. But none of those reveal combined circularity, cylindricity, and represent the axis.
Just as many reasons as there are methods of 'hole measurement' need to exist. Often, two or more used to determine what tolerances are.
Even a 3-point bore gauge, which is an essentially perfect geometric solution, in several 90° readings, might still get affirmed by a dial sled.