On Wed, 08 Oct 1997 18:07:50 -0700 joe kleifges <hardchrome(at)netvalue.net>
I will be the first to admit perhaps its a waste of bandwidth to send to
the list as well as the message originator in replies, so here goes the
>i have a question regarding bearing sizes and clearances.
> are clearances measured "per-side", as a total on one side, as per
>silly string, or as the difference between bearing ID and journal OD.
> i have a crank with 2.095 crank journals, and a bearing set, that in
>rods, measures 2.099-2.100.
> the publisheed clearances in the book of rootes reads..
> .001-.0025 shall be the rod bearing clearance.
> there is no apparent statement of the measuring method, but i will
> is shimming of bearings an acceptable, reliable, and safe practice???
>.001 shim per side will give me a neat .001-.0015 clearance.
> since my rods are not "assembly clean" i may have bumped up the size
>some grit, but 2.099-2.100 is a correct undersize according to the
>catalogs and other materials
Joe, based upon the dimensions that you spec, it would appear that you
30 under crank journals, but 35 under "relative" bearings. Rootes did
35 under bearings, so most likely you have a machine shop special.
That, or you have small block chevy rods and bearings laying around.
Also, verify that your big ends are not egg shaped or distorted due to
a spun bearing.
Have the rods resized and shot peened whether you think they need it
or not. The engine it saves may be your own.
The bearings are measured with a known round big end, with bearings
installed, and with the rod bolts tightened to spec, an inside micrometer
is installed on the inside of the bearing surface.
This is subtracted by the similar reading of an outside micrometer on the
journal. You should strive toward the 1.5 thou clearance, unless you are
building a race spec engine.
Plastigage is a very rough method for measuring the clearance, as
is often shows lesser than real clearances, particularly on main
Shiming was performed years ago, but you really should not consider this
practice as your clearances are way out there. Shimming is not a
particularly good thing to do anyway as the OD of the shells should
already match the ID of the big end, and putting in 1 thou of shim
material will actually put additional pressure on the split ends of
the shells. This distorts the bearings, and while it may get a
sloppy machinist out of a dog house, it does nothing for the longevity
of the motor.
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