22 CHEVY HIGH PERFORMANCE II OCTOBER 2019
PREACHING
TOLERANCE
Ten factors to consider when it comes
to engine bearing clearance
✜ TEXT & PHOTOS: Barry Kluczyk
B
earing clearance is one of the fundamental aspects of engine building and
one that continues to spark debate and divisive opinions—and as with
everything from nitrous systems to eating chocolate, there’s a balance to strike
between just right and regret.
In an engine, the critical bearing clearances we’re talking about are for the
mains and connecting rods; and the clearance is the amount of space between
the shaft and bearing surface that’s filled with the vital, lubricating cushion of oil,
known as the hydrodynamic wedge.
And it ain’t much of a cushion. Even if the installed gap between the bearing
and shaft is only 0.0015-inch, the oil is displaced with the loaded bearings. The
oil wedge lifts the shaft when it starts rotating to keep it moving with minimal
friction, but that oil film can be as thin as 0.00002-inch.
It doesn’t take much in the way of excess heat, inaccurate machining, or
other factors to affect that critical oil wedge. Bearing clearances play the most
important role in maintaining the optimal balance of that oil wedge, ensuring the
engine’s performance and longevity.
WRENCH
For those with the correct tools, determining the
bearing clearance starts with measuring the main
and rod journals with a micrometer and subtracting
the numbers from measurements taken with a
dial bore gauge on the installed bearings. For the
greatest accuracy, the dial bore gauge should be
“zeroed” on the inside of the micrometer before
measuring the bearing to ensure the measurement
is the exact difference between the two.