<VV> Micrometers vs. Plastiguage

[RoboMan91324 at aol.com]

Excellent Point Bob,
 
Yes, the Plastigage measures the clearance between the  rotating member and 
the bearing while the micrometers measure and include the  clearance on 
both sides.
 
I am a little confused about your comment regarding the use of  two 
measuring devices.  The micrometers I use have both the ID and OD  measurement 
capability on the same device so you don't need to worry about  measurement 
inaccuracies due to compounding variances on two measuring  tools.  If you use a 
telescoping ID measuring device, you still use the  same micrometer to 
measure the OD and the telescoping device.  However, I  do agree with your 
comment about human error.  I have seen surprisingly bad  results coming from a 
person's bad reading of the tool.
 
I always use Plastigage for these kind of  measurements.
 
A cautionary note; You should take more than one measurement  on at least 
one main and one connecting rod journal.  I do it to  each.  Plastigage is a 
lot cheaper than rebuilding your motor a second  time.  It is rare but I 
have seen slightly different diameters from one  (axial) side to the other on a 
particular journal.  I don't know how bad  the machine or the machine 
operator must be for this to happen but it does  occur.  Also, this should go 
without saying but you should take at least  one measurement on every journal.  
If there can be a slight variation on a  single journal, there could be a 
significant variation on the OD of the journals  from one end of the 
crankshaft to the other.
 
In addition, even though it is in the directions .... how many  of us read 
the directions? .... be careful to prevent any turning of the crank  in 
respect to the bearings.  This also goes for the connecting rod  journals.  It 
can be difficult to control this while tightening and  removing nuts and 
bolts, separating the two parts of the crankcase and  connecting rod caps or 
removing the crankshaft ........ it is  heavy.
 
Doc
 
1960 Corvette, 1961 Rampside, 1962 Rampside, 1964 Spyder  coupe, 1965 
Greenbrier, 1966 Canadian Corsa turbo coupe, 1967 Nova SS, 1968  Camaro ragtop

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
In a message dated 4/3/2012 9:43:31 P.M. Pacific Daylight Time,  
virtualvairs-request at corvair.org writes:

Message:  6
Date: Tue, 3 Apr 2012 16:42:18 -0400 (EDT)
From:  BobHelt at aol.com
Subject: Re: <VV> Micrometers vs. Plastiguage
To:  matthew_lockwood at hotmail.com, virtualvairs at corvair.org
Message-ID:  <4b6b0.3a7fc40.3cacbaaa at aol.com>
Content-Type: text/plain;  charset="US-ASCII"

Hi Matt,
I'd suggest that you be careful using  the same specs for mic vs plastigage 
measurements. The Shop Manuals all  refer to making Plastigage measurements 
and  their specs refer to that  method. When Plastigaging, you are 
measuring the  clearance of the crank  journal to the bearing . That is the running 
clearance. The SMs specs all  are in the 0.001 to 0.003 inch range. 

But by using micrometers to make  these measurements you are measuring the 
clearance on both sides of the  journal and thus your clearance measurements 
will  be twice the specs for  any bearing. You are measuring the jourmal 
diameter to  the bearing  diameter. In addition, the Mics cause an increased 
measurement error   since you must use TWO different measuring devices that 
may or may not  be  calibrated correctly. All that plus any human error in 
maiing  two  measurements.

Regards,
Bob  Helt