Measuring an inside bore with an inside mic takes a bit of a "feel". I
worked in the machine tool industry for all of my career and there the
tolerances are many times in 10,000ths of an inch. Not easy to find "a
tenth". Having worked on the assembly floor quite a bit with the
inspectors, I really got an appreciation of what it took to measure
something even to a thousandth, especially an inside bore. Being an
amatuer, I would only trust my measurement to a thousandth on an inside bore
with a three (3) point bore gauge. Even that takes a bit of a feel. I
think I'd be lucky to get within 0.002 or 0.003 with an inside mic.
I once had the task of getting five (5) parts machined on one of the
precision machining centers and then getting various dimensions on the parts
measured, multiple times for repeatability and accuracy. As I recall, I
took the parts to two (2) different CMM's and to a precison plate and height
gauge run by our best inspector and still couldn't get acceptable
repeatability of the measurements to a thousandth. I finally had to take
the parts over to Detroit to a CMM manufacturer who had a million dollar CMM
in a temperature and humidity controlled room to get accurate, repeatable
measurements. And that was 1968 million dollars.
I once rebuilt a Honda 4 cylinder engine and the bearings came in graded
sizes of 0.1 MM or about 0.0004 Inch. I used plastigauge and kept running
back to the dealer for bearings to get the correct ones on each journal and
rod (a very accomadating dealer!). The engine ran for another 100,000 miles
and three kids learning/driving/abusing it. Must have been close.
Tim
Murphy's Law Racing TR-4 #317
----- Original Message -----
From: "Bill Babcock" <Billb@bnj.com>
To: "riverside" <riverside@Cedar-Rapids.net>
Cc: <fot@autox.team.net>
Sent: Friday, February 01, 2008 2:30 PM
Subject: Re: [Fot] checking bearing clearances
> I'd say you did the right thing but came to the wrong conclusion.
> First, you can and should use plastiguage across the bearing parting
> lines, I generally put it there and in the middle of the top bearing.
> Second--most folks wouldn't have found the problem any quicker with an
> inside mike--everyone checks the middle anyway. It's not the right
> thing to do, but I've seen even good machinists measure only there.
> Third, all measurements can lie--if your crank doesn't turn completely
> freely there is something wrong that WON'T GET BETTER by itself.
>
> On Feb 1, 2008, at 12:15 PM, riverside wrote:
>
>> Once upon a time, I laid -check that- installed a
>> reground crank with nice new standard bearings that checked
>> out fine with plastigauge if a bit on the loose side.
>> you could barely turn the crank by hand.
>> bought a set of bearings from another manufacturer (.001 undersize)
>> and installed same. plastigauged snugger as one would expect,
>> but crank turned freely now.
>> i bought a set of modestly priced micrometers and bore gauges from
>> Enco and measured everything. The first set of bearings were
>> junk across the parting lines where the plastic could not go.
>> Haven't used the stuff since and don't plan on it.
>>
>> art de armond
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>
> Bill Babcock
> Babcock & Jenkins
> Billb@bnj.com
> 503.936.7660
> www.bnj.com
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