Posted 14 September 2012 - 10:03 PM
THis??
how much clearance do your bearings
need?
how much clearance do I need for my rod,
main or camshaft bearings? This is one of the
most frequently asked questions we receive.
Unfortunately there isn’t one simple answer
that suits every case. This is because engine
application, lubricant selection and operating
conditions will dictate different clearance levels.
This isn’t to say we can’t generalize on at least
a starting point.
First, let’s define how and where clearance
should be measured. half shell rod and main
bearings do not have a uniform wall. The wall
is thickest at 90 degrees from the split and
drops off a prescribed amount toward each
parting line, depending on the bearings intended
application. This drop off is called “eccentricity.”
In addition, there is a relief at the parting lines.
eccentricity is used to tailor the bearing shell to
its mating hardware and to provide for hardware
deflections in operation. eccentricity also helps
to promote oil film formation by providing a
wedge shape in the clearance space. The relief
at each parting line insures that there will not be
a step at the split line due to bearing cap shift
or the mating of bearing shells that differ slightly
in thickness within allowed tolerance limits. (See
figure 1.)
For these reasons, bearing clearances are
specified as “Vertical clearance” and must be
measured at 90 degrees to the split line. The
best method of measurement is with a dial bore
gage that measures the bearing Inside Diameter
when the bearings are installed at the specified
torque without the shaft in place. measurements
should be taken at front, center and rear of
each bearing position. another common method
of checking clearance is through the use of
Clevite® Plastigage®
.
For most applications .00075 to .0010” (three
quarters to one thousandth of an inch) of
clearance per inch of shaft diameter is a
reasonable starting point. For example a 2.000”
shaft diameter would require .0015 to .0020”
bearing clearance. (.00075 X 2.000” = .0015”
and .0010 X 2.000” = .0020”) Using this formula
will provide a safe starting point for most
applications. For high Performance engines
it is recommended that .0005” be added to
the maximum value determined by the above
Centerline
wall
Bearing
half
shells
Eccentricity =
amount of change
in wall at this point,
from centerline
Parting line
Parting line
relief
JOURNAL
1/4"
3/8"
Figure 16 | © mahle 2007/2008
Bearing clearance
calculation. The recommendation for our 2.000”
shaft would be .0025” of clearance. (See figure
2.)
Remember however, that the above are only
recommended starting points. The engine
and its application will tell us where to go
from these starting points. For example, a
passenger car engine assembled at .0010”
per inch of shaft diameter might turn out to be
noisy on start-up, especially if the engine has an
aluminum block. most passenger car engines
are originally assembled by “Select Fitting” to
achieve clearances that are less than what would
result from random selection of mating parts.
This is because the stack-up of manufacturing
tolerances on the mating parts may exceed
the acceptable level for control of noise and
vibration. In addition, most new passenger car
engines are now designed to use 5W-30 weight
oils to reduce hP loss and conserve energy.
These lighter weight oils are capable of flowing
more freely through tighter clearances.
let’s pick some typical manufacturing tolerances
and look at the potential clearance range that
results. a tolerance range (from min. to max.
sizes) of .0010” is typical for most crankshaft
journals as well as both rod and main bearing
housing bores. If the engine uses bimetal
bearings the wall tolerance is .0003” per shell or
.0006” in total. adding these up we get .0010”
for the housing + .0010” for the shaft + .0006”
for the bearings = .0026” total clearance variation
possible due to mating part manufacturing
tolerances. If our minimum assembled clearance
is just .0005” this makes the maximum possible
.0031.” (.0005” min. + .0026 tolerance range
= .0031” max.) For normal passenger car
application .0031” of bearing clearance would
generally be too much. however, if we take the
same engine, let’s say a small V-8, and put it in
a truck used to pull a camping trailer and use a
heavier weight oil, the larger clearance would be
more acceptable.
Clearance is also somewhat of a safety
factor when imperfections in alignment and
component geometry creep in. as surfaces are
more perfectly machined and finished, sensitivity
to oil film break down is reduced and tighter
clearances can be tolerated. Tighter clearances
are desirable because they cause the curvature
of the shaft and bearing to be more closely
matched. This results in a broader oil film that
spreads the load over more of the bearing
surface thus reducing the pressure within the oil
film and on the bearing surface. This will in turn
improve bearing life and performance. Typically
a used bearing should exhibit signs of use over
2/3 to 3/4 of its ID surface in the most heavily
loaded half. (lower main and upper rod halves)
Illustrations depicting these typical wear patterns
are shown at the front of the Clevite® engine
bearing catalog.
Clearance is just one of many variables that
effects bearing performance. In addition things
like oil viscosity, which is determined by oil
type and grade selection, engine operating
temperature, oil pressure, engine RPm, oil hole
drillings in both the block and crankshaft, bearing
grooving and other bearing design features
all interrelate in the function of an engines
lubricating system.
Figure 2© mahle 2007/2008 | 7
Bearing clearance
lighter weight oils have less resistance to flow,
consequently their use will result in greater oil
flow and possibly less oil pressure, especially at
larger clearances. all oils thin out as they heat
up; multi-grade oils, however, don’t thin out as
rapidly as straight grades. Original equipment
clearance specifications are necessarily tight
due to the use of energy conserving light-weight
oils, relatively high operating temperatures and
a concern for control of noise and vibration,
especially in aluminum blocks.
high Performance engines on the other hand,
typically employ greater bearing clearances for a
number of reasons. Their higher operating speeds
result in considerably higher oil temperatures
and an accompanying loss in oil viscosity due to
fluid film friction that increases with shaft speed.
Increased clearance provides less sensitivity to
shaft, block and connecting rod deflections and
the resulting misalignments that result from the
higher levels of loading in these engines. Use of
synthetic oils with their better flow properties can
help to reduce fluid film friction.
Friction and horse Power loss are prime
concerns in high Performance engines for
obvious reasons. as a result, the coating of
various engine components with friction reducing
compounds has become common practice.
Clevite® has announced the introduction of their
line of Triarmor™
coated bearings for selected
high Performance applications. Clevite® wants
to provide high Performance engine builders
with Clevite® performance series bearings
already coated with a friction reducing surface
treatment. Use of these coated bearings may
result in slightly less clearance than the uncoated
Clevite® high performance parts for the same
application. This will typically be in the range of
.0005.” This is because the coating, although
expected to remain in place during service, is
considered to be somewhat of a sacrificial layer.
Some amount of the coating will be removed
during break-in and operation resulting in a
slight increase in clearance. This is the reason
no adjustment in bearing machining dimensions
was made to allow for coating application.
So as you can surmise from reading the above
notes, bearing clearance is not a subject that
can be addressed without taking into account
numerous variables including; geometry of the
parts, oil viscosity, oil temperature, engine load,
shaft diameter, bearing coatings and one’s own
ability to accurately measure and assess these
variables.
Didnt have time to read it all, will do tomorow, off to bed now. Looks good the first half i read though.
Cheers.
...... 
