How much clearance do I need for my rod,
main or camshaft bearings? This is one of the
most frequently asked questions. Unfortunately,
there isn’t one simple answer that suits every
case. 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.)
Attachment 9388
| 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
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
calculation. The recommendation for our 2.000”
shaft would be .0025” of clearance.
Attachment 9389 |
