DoN. Nichols wrote:

Well ... the ones which I have are quite old. These days, most
people buy either dial or digital calipers. Dial is nice if you can
keep chips out of the rack, but Digital (if good) is far more
convenient, including the ability to zero at any point (such as on what
you are fitting to) and then measure how much more needs to be removed
to reach the proper dimensions. Also, measuring the distance between
the centers of two holes of identical size by zeroing it while measuring
the ID of one hole, then shift to measuring the distance between two
opposite sides of the two holes in question. This subtracts the
diameter of one hole from the final measurement which is the sum of the
center distance and half of each hole's diameter, giving the actual
center-to-center distance.

I chose the vernier calipers because I wanted both imperial and metric
on the same tool, and I wanted them to last. I hate replacing tools.

O.K. But it misses three-lobed bores, which can be in a shape
which actually measures the same between any two opposite points, but
still is potentially seriously out of round. (This is more likely to be
formed by certain machining operations -- including centerless grinding
for OD shapes, and in a used automobile engine, the ovality is more
likely, and for that your bore gauge is probably quite adequate.

I've not seen a three-point bore gauge intended for measuring engine
cylinders. All the gauges I found on the market were two-point with a
centring device. I picked the one I mentioned because it's complete with
all the anvils, locking nuts and the spanner, and is in good condition.
There were more accurate gauges available (measured down to 0.0001"
instead of 0.0005"), but they were much more worn and had missing parts.
Also, I figured that for investigating wear as opposed to reboring,
having a 40 thousandths range on the dial might be more useful than a 10
thousandths range. If people are interested, I can take a picture of the
bore gauge and calipers.

Best wishes,

Chris