Subject: Bore gages
From: Mike Graham
Date: 12/07/03 23:36 GMT Daylight Time
Message-id:

I've never used a bore gage (dial bore gage, whatever).

Good god man, you've never lived.

I've only used
inside mics (of the tubular variety) and telescopic gages. Are bore gages
sufficiently snazzy to make them worth the relatively upper-crusty price
tag? A set of intramics is absolutely out of the question. 8-)

Bore gauges are a must have if you do a lot of accurate i/d work. I have three
sets. The main one for engine bores is a Mercer 2" - 6" which reads to a tenth
of a thou. It does that fairly repeatably too. It's actually fairly scary
putting it down most production bored and honed blocks and watching the needle
move so far round the dial with taper and ovality. Most engine builders use a 1
thou gauge which makes a badly bored block look a lot less wrong. That gauge
made me get my own boring and honing perfect though because it exaggerates
small variations so much you feel like you've cocked a job up if the gauge
moves at all.

The other two sets get used a lot less. One runs 0.75" to 2" and the other goes
down to 0.5". They are both pretty old sets and read to a thou. The Mercer
looked brand new and still does because I've taken care of it. I picked the lot
up several years ago for 200 quid at a metrology establishment with a 0-1" dial
gauge and a 3" to 4" Moore and Wright mike thrown in for free. I only went
there for the Mercer but he kept bringing out stuff I thought would "come in
handy sometime" and he made me an offer I couldn't refuse for the job lot
Those places often have good second hand stuff for sale which they've rebuilt
and calibrated. What my lot would cost new these days I shudder to think.

I only use the telescoping gauges for measuring valve seat insert bores in
cylinder heads when I'm fitting unleaded inserts or bigger inserts. With a
shallow hole bored down to a flat base there often isn't enough depth to get a
bore gauge in and still be able to rock the pins over centre. I hate using the
telescoping ones though. Slow and fiddly and you get a different measurement
every time you use them. A colleague reckons you'll always measure slightly big
with those and he tends to knock half a thou off whatever the micrometer says
when you measure the gauge. Especially in soft metals like ally where the ends
dig in a bit as you rock them over centre. One of these days I must have a play
with a through bore and see how a T gauge reads compared to the Mercer.

For through bores like engines and especially long bores the dial gauges are
the only way. You can take a reading in a couple of seconds and know it's
right. You can map out a bore from top to bottom measuring taper and ovality to
a tenth or so in no time. With T gauges it'd take forever and not be accurate
anyway.

The three pronged gauges are very expensive and supposedly very accurate but no
good for meauring ovality of course as you aren't just reading across a
diameter. I've never found the need for one. I couldn't manage at all without
the Mercer though.


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
You find somebody to love in this world you'd better hang on tooth and nail -
the wolf is always at the door. (Don Henley - In A New York Minute)