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From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Bore gauge Q?
Date: 27 Mar 1999 18:52:24 GMT

>From: Jan Ben <>
>Is there another way to set-up a 3-prong bore dial gauge, other than the
>I need to measure a 4" bore to under .0005.  The bore dial gauge range
>is about .020 if that matters.
>Do you think I stand a chance of making a 4.000" bore ring standard?
>How to proceed?

For measuring bores of that size, like engine bores, I find a dial gauge bore
comparator works best. Most engine reconditioners will use this type of gauge.
It has one prong connected to the dial gauge, the other prong screws in
diametrically opposite and is replaceable depending on bore size and two "legs"
hold the gauge true to the bore diameter. You just rock the gauge until you get
the lowest reading which means you are perpendicular to the bore and set the
gauge on any micrometer.

I was lucky enough to drop on a reconditioned second hand Mercer gauge which
has extensions to cover 2" up to 6" and it reads to 1/10th of a thou. It is a
bit frightening to use when you are honing a bore because it shows up every
damn tiny fault which the 1 thou reading gauges wouldn't even notice. Very
repeatable though and I really would rate it to a couple of tenths if the
micrometer is calibrated accurately. It's amazing how much a bore shrinks in
size when you measure it just after honing and then leave it overnight and
measure again. The temperature generated by the honing op stays in the block
for a hell of a long time. It's worth the dicking around though when I'm trying
to get a race engine spot bollock on size.

I've also got telescopic gauges but they a pig to use, especially on soft
materials and I wouldn't rate them closer than half to 3/4 of a thou. Best way
to use them is nip them up gently while they are at an angle in the bore and
then rotate them over centre. They will then compress down to the smallest size
which should be the true diameter. Colleagues reckon that a good rule of thumb
when using them is that they will normally measure about 1/2 a thou oversize.

The three prong gauges are good but it's a pain having to use ring gauges to
set them up and you can't find out in which direction the ovality lies in a
bore like you can with the Mercer.

My 2 pence worth.

Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

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