OCTOBER 2019 II CHEVY HIGH PERFORMANCE 23
In this story, we’re going to look at
the basics of bearing tolerances and
the theories that can help you decide
whether to build an engine with tighter
or looser clearances. But no matter
what you decide, it’s guaranteed that
someone else will have his or her own
opinions on the topic. At least you’ll be
armed with facts to help make a more
informed decision.
THE RULE OF THUMB
Generally speaking, bearing
clearance should be 0.001-inch for
every inch of journal diameter. Let’s
take the traditional 350 small-block as
an example. The factory specs include
2.450-inch main bearing journals and
2.100-inch rod journals, which means
the target bearing clearance should
be about 0.0024-inch (0.0025 is the
common spec) on main bearings
and 0.0021-inch on the rod bearings.
Again, in general terms, it’s better to
have the clearance slightly larger to
ensure optimal performance and oil
temperature, particularly if the engine
is built for higher performance.
The idea that a slightly larger
bearing clearance is better for overall
engine performance, however, is
another generality and is at the core of
the age-old tolerance debate. Some
builders will force a tighter bearing
clearance with oversized bearings
to achieve a horsepower advantage,
while others will maintain the factory’s
target spec and accept a slightly larger
clearance to foster engine durability.LOOSE VS. TIGHT
The advantages of a “loose” bearing
clearance—one that is on the high side
of the recommended specification
range or even higher—is lower friction,
particularly at start-up, which translates
into lower oil temperature. Ultimately,
that can mean greater engine longevity.
The flip side of that is excessively largeclearances will push excess oil out of
the sides of the bearings, which can
enhance power-robbing windage and
lower oil pressure.
A “tight” bearing clearance, with
a smaller hydrodynamic wedge
between the journals and bearing
surfaces has advantages, too. It
can provide more even and uniform
loads on the bearings, as well as
more uniform oil pressure on them.
Additionally, less oil is required to
maintain the hydrodynamic wedge,
which translates into a reduced load
on the oil pump that can result in more
horsepower.
That all sounds great, right? But
there’s a downside to tighter bearing
clearances: heat. A thinner film of oil
between the bearing and journal is
going to heat up faster and rise to a
higher temperature than an engine
with a looser clearance. It’s less of a
concern in a dedicated racing engine,
but for an engine that’s driven daily, it
can increase wear and make it easier
to spin a bearing, which will totally ruin
your day.
As a rule of thumb, main bearing
clearance should be targeted between
0.0025- and 0.0020-inch for most
street and street/strip engines.THE SAFETY FACTOR
Having a looser bearing clearance
is also a safety factor for the less-
than-perfect manufacturing standards
of stock-type engines and their
machining and surface finishes. In
other words, in a generally stock
engine rebuilt to stock specifications,
the journals and/or the backs of the
rods or mains may not be perfectly
round. Extra bearing clearance helps
ensure everything keeps rotating as
intended without excessive friction
and heat that will ultimately break
down the oil film.The low-tech Plastigauge method for measuring
bearing clearance provides good accuracy for
homebuilders who don’t have a micrometer
or dial bore gauge; and regardless of the tool,
don’t overlook this vital step in engine building.
Assumptions, as they say, are the mother of all ...
well, you know.
When installing a bearing—whether standard,
oversized, or undersized—it will “snap” into place.
When it does, make sure the oil feed hole from the
block is lined up precisely with the oil hole in the
bearing.
With the bearings in place and either the
crankshaft or connecting rods installed, the main
caps are torqued to spec. If the micrometer/dial
bore gauge method was used, there’s no reason to
remove the caps again, but if the bearing clearance
is being checked with Plastigauge, the caps will
need to come off to measure the clearance.
A slightly “loose” bearing clearance can ensure
engine longevity with regular-production engines
built to assembly line standards, providing a slightly
larger oil film cushion to stave off excessive friction
and heat.Aftermarket
machining isn’t
always as accurate
as assumed, either.
Minor imperfections
in crankshaft turning
can generate nearly
imperceptible
variations that can
manifest themselves
as too-close-for-
comfort clearance
between the journal
and bearing surface.
Again, erring on the
“loose” side of the
bearing clearance
helps prevent
disaster.