Chevy High Performance – October 2019

28 CHEVY HIGH PERFORMANCE II OCTOBER 2019

Preaching Tolerance

oil. It will help move the oil through the
engine to maintain optimal bearing
temperatures.
On engines with looser clearances,
a strong oil pump is also essential to
ensure the bearings aren’t starved
for lubrication, as more oil to them
is required than a tighter-clearance
engine.

RACING ENGINE MYTHS
Don’t be swayed by tales of
extreme clearances—tight or loose—
for racing engines. If you’re building
an engine primarily for the street, with
occasional or even regular blasts
down the quarter-mile, stick to the
rules of thumb described in the tips
above. Racing engines come in all
forms and are built for a variety of

special operating conditions, and the

urban legends you’ve heard thirdhand

from the cousin of a guy who used to

sweep the floors in a NASCAR shop

don’t mean much when it comes to

building an engine for your vintage

Chevelle.

In fact, whether it’s a NASCAR

engine, a Pro Mod drag racing engine,

or even a Late Model circle track

engine, each has its own parameters;

and each is built typically with

specialized bearings, which are used

with very specific oils that are often

blended with antiwear additives such

as ZDDP (zinc dialkyldithiophosphate).

Then there’s the whole question

of whether the engine is naturally

aspirated or fed with a power-adder.

A NASCAR engine may run super-thin

The assembly tolerances and oiling requirements of a dedicated racing engine are different than street and
street/strip engines. Don’t use tales from the track as the basis for the bearing clearances for a street engine.

When mixing bearings, all the half-shells of one

size should be on the same side of the bore—either

upper or lower—and the half-shells from the other

size on the opposite side of the bore.

Mixing standard and over- or undersized bearing sizes allows the engine builder to tailor clearances more
precisely, but it requires purchasing more than one set of bearings to accomplish it.

oil and have tighter tolerances, while

a Top Fuel engine runs very loose

clearances and very heavy oil to help

keep the monster alive.

Bottom line: The variables for racing

engines and how they’re built are

endless, so don’t get bogged down by

what you’ve heard on cruise night or

in the pits at the track. Unless you’re

building a Top Fuel engine or jumping

into NASCAR, ignore those tall tales

and stick to the basics.

MIX AND MATCH FOR

OPTIMAL CLEARANCES

Generally speaking, if you want

to go a little loose, subtract 0.0005-

inch and if you want to tighten up the

clearance, add 0.0005-inch. The twist

to that is bearings aren’t typically sold

in such increments. They’re offered in

standard size, undersized by 0.001-

inch or oversized by 0.001-inch.

You go with the undersized bearing

to loosen the clearance and the

oversized bearing to tighten it.

Start with the standard bearings

and if you find a clearance adjustment

is required, move up or down, as

necessary. And because 0.001-inch in

difference can be more than desired,

you can mix bearing sets to achieve

the 0.0005-inch desired adjustment.

Simply mix one of the shell halves of a

standard bearing with a shell half from

an over- or undersized bearing. Yes,

it requires the purchase of two sets

of bearing sets, but that’s the price to

pay for clearance optimization.

One more thing: When mixing the

bearings, make sure all shell halves

are aligned. That means install all the

standard halves on the block side and

all the undersized halves on the cap

side or vice versa. It doesn’t matter

which side they go on, only that the

same sizes are on the same sides of

the components. CHP