You can insert a ring and square it with a piston inverted, then 
measure on that plane.
         Peter C.
===
At 09:18 AM 2/16/2007, derf wrote:
>Hi,
>Yes, but when the tool is out of the ideal plane it will give you a slightly
>larger reading.  Take several measurements and go with the smallest you
>get.  You can eyeball a telescoping gauge and get pretty close.  The problem
>with measuring cylinders is that they are bigger in the middle than at the
>top or bottom, if there is any wear at all.  There will be a ridge at the
>top.  Pistons, too, are barrel shaped with a larger diameter in the middle
>than at the top or bottom.
>
>You will probably get the best reading with an internal mic.  Hold one end
>of the mic against the cylinder wall and open it up until it touches the
>other side without free play.  You kinda get a feel for it.  Do this several
>times and your lowest reading will probably be your best reading.  Again,
>anything out of the ideal plane will measure larger.  Make sure your inside
>mic has round tips.
>
>Don't be surprised if your cylinders aren't perfectly circular.
>Cheers,
>Derf
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