24 CHEVY HIGH PERFORMANCE II OCTOBER 2019
Preaching Tolerance
The opposite is true, too. A more
precisely machined engine block
and rotating components enable the
builder to run tighter tolerances and
leverage the inherent performance
advantages because the components
are more closely matched in
roundness, and that promotes the
more even oil support on the bearing
surface, as discussed above. Again,
there’s a fine line to walk with how
much tighter to go, but more precise
machining affords that leeway.DON’T SKIMP ON
MACHINING
When considering building an
engine with tighter bearing tolerances,
the main and rod bores must be as
round as possible—no more than
0.001-inch variation on a stock-
type rebuild and 0.0005-inch on
a performance engine. Main bore
alignment is also very important, with
perhaps no more than 0.0005-inch
misalignment between the adjacent
bores and only 0.001-inch overall,
when using conventional tri-metal
bearings. That overall tolerance should
be no more than 0.002-inch when
using aluminum bi-metal (lead-free)
bearings.
Such precise machining not only
ensures optimal bearing clearance, it
accommodates minute yet importantmanufacturing variances with the
bearings themselves. In short, if you’re
going to run tighter clearances, spend
the extra time and money to make
sure the block, main caps, and rods
are as true as possible.KEEP IT LOOSER WITH
POWER-ADDERS
Nitrous oxide, supercharging, and
turbocharging exert tremendous
loads on an engine that results in high
cylinder pressures and, consequently,
much higher temperatures. They also
typically come with tremendous and
immediate increases in engine speed,
all of which can be a challenge for oil
control.
The two big factors here are
heat and crankshaft flex. The heat,
of course, comes from the higher
temperatures, while the fast run-up
in engine speed can cause flexing of
the crankshaft, both of which can lead
to bearing clearance problems. It’s
not necessary to dial in excessively
loose clearances when running a
power-adder, but don’t tighten them
up. Aim for the high side of the normal
range, and add 0.0005-inch if the
measurement comes out on the lower
side of the range.WHAT ABOUT OIL
VISCOSITY?
Regardless of their weight, the
formulations of motor oils today
deliver greater load-carrying capacity
than a couple of decades ago. That
means they’re able to spread out that
load with less pressure per square
inch. That means lower friction and
heat, allowing a lighter-weight oil
(thinner viscosity) to accomplish theThe material removed from a line-bored block will
require oversized main bearings, which have the same
material thickness on the inside of the bearing surface
that faces the crankshaft journal, but more material on
the outside that is seated against the bore.
The higher engine
load and resulting
heat that comes with
a supercharger or
turbocharger can
effectively thin out the
motor oil, which can affect
bearing clearance. In an
otherwise stock engine,
options including bumping
up to a higher-viscosity
oil or using a premium,
performance-type shear-
stable oil that maintains its
viscosity under high loads.
Bearing clearance for
nitrous applications don’t
necessarily change over
conventional build specs,
but aiming for the looser
end of the range helps
provide more insurance
if the “hit” is strong
enough to induce a bit of
crankshaft flex.^ Oil viscosity plays a crucial role in bearing
clearances. Thinner oils support tighter tolerances,
while thicker-viscosity oils should be matched with
looser clearances. Modern engines such as the LS
and LT variants generally have tighter tolerances
and use thinner oils than earlier small- and big-
block engines.
When building an engine with the intention
of running tighter bearing clearances, precise
machining is an absolute must. Align-honing the
block, for example, provides optimal dimensional
accuracy, while also enabling more accurate
crankshaft location that can reduce crank flex
at high rpm. Flex can quickly erase the bearing
clearance and burn up the bearings.