On Mon, 22 Feb 2010 03:58:39 +0000, the infamous Christopher Tidy
scrawled the following:

Ed Huntress wrote:

Hi Ed,

The reason is thermal expansion of the caliper. If you're pushing for the
best accuracy, and particularly if the room is cold, holding it in your hand
for a few minutes can make a difference. If you warm a three-inch-long piece
of steel by 30 deg. F, from your 40 deg. shop to 70 deg., it will expand by
almost 0.001". If the piece you're measuring is the same temperature as the
caliper, and if they're both steel, the actual temperature matters little,
but a difference in their temperatures, if it's more than a few degrees, can
result in inaccurate measurement.

Right. I rarely wear gloves except for particularly dirty jobs. I prefer
barrier cream.

Now that I'm no longer doing greasy mechanical work, I tend to wear
gloves more often, to keep my finnernails clean. Barrier cream is
great, except for that.


But it's time to back up. I suppose you realize that a slide caliper is not
a high-accuracy gage. It's usually used for moderate-accuracy work. You were
asking what accuracy you can achieve with it, and the answer is, using a
good caliper, cotton gloves, and calibration with gage blocks, you probably
can achieve +/- 0.001". But not everyone does. Some people have the touch
and some don't.

Thanks. That's the figure I wanted. These are good calipers and I think
my touch is fair, but I'm not being careful about temperature and the
scale on the calipers cannot be moved to calibrate them. Sounds like my
guess of +/- 0.002" is probably fair.

Fair to high. 2 thou to start, 1 thou after you get used to it, 5
tenths once you're good and comfortable with your dial calipers. But
for critical measurements, micrometers are the way to go.


But it depends on whether you're measuring *relative* dimensions or
*absolute* dimensions. If it's the former, you don't need a well-calibrated
gage. If it's the latter, you'll never know for sure how accurately your
gages measure unless you check them from time to time against a gage block
(or a stack) that's somewhere in the middle of the gage's range, and another
one near its largest opening. For a 3" - 4" mike, I'd want at least a 3"
block (or a stack to make that dimension) and a 4" block or stack. I'd like
to have a 1/2" block, too, to wring with the 3" block to test the midrange.

What's a stack?

When a gage block isn't thick enough, you stack another on top of it.


If any of this is unclear, ask, and I or someone else will explain. FWIW,
most home-shop work doesn't require accurate measurement of absolute
dimensions. Usually we're trying to make two things fit together, and what
you need to know for that is their *relative* dimensions. If that's the
case, forget the gage blocks for now.

In this case, I think it's the absolute dimensions. The engine's piston
wobbles noticeably in the bore. It's an aluminium piston in a cast iron
bore, so I suspect there could be much more wear on the piston than the
bore (the bore looks good, without a ridge at the top). So I want to
know how well a brand new piston will fit in the existing bore, before I
buy one.

Remember that the COE is different for steel and aluminum, with steel
expanding less by half. Aluminum pistons are wobblier by nature.

Don't go by the piston, go by the bore. It sounds like your engine is
in fairly good shape, but check for elongation perpendicular to the
crankshaft. Bores become oval from wear, where the connecting rod
pushes 'em up and drags 'em down the opposite sides. If you bore it
out, you'll need all new pistons by default.

--
"Just think of the tragedy of teaching children not to doubt."
-- Clarence Darrow