Small block Chevy engines long ago became mainstays in both the
traditional and high performance marketplace. So many of them
have been rebuilt over the years, and so much has been written
about the rebuilding process, it would appear that nothing more
need be said.
But the small block engine has changed over the years and so have
consumer preferences and orientation. Consequently, many long
standing rebuilding techniques are due for a change, or at least
some refinement. That is, assuming a "high performance rebuild"
is more than a standard short block with high performance heads
and a high lift cam.
Points to keep in mind are that many customers for high performance
engines are much more informed than their counterparts of 10 or
20 years ago. As opposed to a still wet-behind-the-ears teenager,
the current high performance customer will likely be 30 to 50
years old with some racing background and basic knowledge of proper
It’s also probable that he or she has been down the road before,
may have had a bad experience with a previous shop and is a bit
gun shy. The flood of questions gushing forth from the mouths
of many high performance customers today is often a consequence
of previous experience; they’re looking for some verbal reassurance
before spending money.
Being the most popular engine in the world, the small block Chevy
presents a number of high performance opportunities. Originally
introduced with a displacement of 265 cubic inches, the small
block Chevy has grown over the years, ultimately reaching 400
cubic inches. Two economy versions, one displacing 262 cubic inches,
the other a "whopping" 267 cubic inches were also produced,
but these are entirely unsuitable for performance use.
|Small Block Specifications|
Since the 1950s, all bore/stroke combinations have been rebuilt
in high performance form. However, at this late date, blocks with
4.00" bores constitute the lion’s share of the performance
business. There’s also a sizable demand for 4-1/8" bore blocks
from which 400+ CID small blocks are built.
As with any rebuild, one that will produce a high performance
engine starts with the block. In the overall scheme of things,
only two types of cylinder blocks exist – those with two-bolt
main caps and those with four. But over the years, dip stick position
has changed, rear main seal configuration has been updated, and
a variety of alloys have been used.
Dip stick position isn’t much of an issue, unless you’ve ordered
the wrong oil pan. Then you wind up with the dipstick on one side,
the notch in the oil pan on the other, and an engine with a severe
oil leak. Alloy content is a somewhat different matter. Thousands
of high performance small blocks based on a standard alloy block
casting have run successfully for years. But for maximum strength
and longevity, a "high tin" block is preferable.
A block’s alloy content is denoted by two figures cast into the
front face, just above the main bearing bore, in the area normally
concealed by the timing cover. Many production small blocks have
the numbers "010," "020" or both cast into
their front face, just above the main bearing bore. If both numbers
are present, one above the other, it indicates that the block
alloy contains 10% tin and 20% nickel. A single number, either
a "010" or "020" represents the amount of
nickel and indicates negligible amounts of tin.
No numbers, other than the casting numbers that are typically
found beneath the timing cover, translates to only minor amounts
of tin and nickel being present in the block alloy. (Tin and nickel
are two metals that are commonly alloyed with cast iron to improve
durability, hardness and heat dissipation.)
Although a "010"/"020" block is most desirable,
it’s not always possible to find one that’s suitable for high
performance use. Alloy composition aside, cylinder wall thickness
is the overriding consideration in block selection, and one with
no tin or nickel and thick cylinder walls is generally preferable
to a high-nickel block with thin walls. Truck and older Chevy
II blocks are reputed to have thicker than average cylinder walls,
but there are no guarantees; sonic testing is the only way to
be certain that wall thickness is adequate.
Beginning with the 1986 model year, Chevrolet began producing
blocks with a one-piece rear main seal. There’s enough difference
between 1985 and earlier and 1986 and later blocks that oil pans
and crankshafts are not interchangeable unless an adapter is fitted
to the block. Most commonly, a crankshaft and oil pan designed
for the older-style, two-piece seal is installed in a late model
block with one-piece seal. Adapters allowing this to occur are
available from a variety of aftermarket companies and through
GM Performance Parts as p/n 10051118.
The introduction of hydraulic roller lifters for the 1987 model
year brought about other cylinder block changes. To accommodate
original equipment hydraulic rollers – which are of a different
design than aftermarket types – the tops of the lifter bores were
raised and machined flat. The tapped bosses were also added in
the lifter valley so the sheet metal "spider" that holds
the lifter link bars in place could be attached.
Standard hydraulic or mechanical lifters can be installed in a
"hydraulic roller" block, but original equipment roller
lifters cannot be installed in a "non-hydraulic roller"
block. "Hydraulic roller" blocks also have a tapped
hole on either side of the camshaft hole for attaching the retaining
plate that’s installed to prevent the camshaft from "walking"
Another variation that can ruin an otherwise well-planned engine
building party is main bearing diameter. Beginning with the 1968
model year, main journal diameter was increased from 2.30"
to 2.45". On the other hand, all 4-1/8"-bore production
blocks are machined for a 2.65" main journal diameter. Consequently,
it’s advisable to verify main journal, bearing and bearing saddle
diameters to assure proper fit.
It’s also advisable to disregard model year when determining block
characteristics. Considering that new cars are typically introduced
in September or October of the previous calendar year, it’s not
at all unusual for a casting date to disagree with the model year
of the vehicle in which it was originally installed. Prior engine
swaps can also confuse the issue, so accurate measurements should
always be made.
For the 1992 model year, Chevrolet introduced a Second Generation
small block known as the LT1. (Installed in 1992 and later Corvettes
and 1993 and later Camaros, Firebirds and 1994-’96 GM "B"
and "D" bodied full-sized cars). Within the Second Generation
family, most major components are interchangeable. However, a
265 CID version of the engine was also produced, (the base Caprice
engine) so don’t be surprised if you come across an LT1 block
with 3-3/4" cylinder dimensions. With the LT1’s reverse flow
cooling system, neither the block nor heads are interchangeable
with a First Generation small block.
Irrespective of the block selected, a performance rebuild should
include align honing. Many machinists either overlook or disregard
the importance of align honing. But every critical block dimension
is taken off main bearing saddle alignment, so align boring and/or
honing should be the first machining operation and it must be
When a block is align honed, you absolutely must have the oil
pump installed and the bearing caps tightened to the required
torque, using the same type of fasteners (either studs or bolts)
that will be installed when the engine is assembled. This is critical
because when you tighten the main cap bolts or studs, or the oil
pump bolt, it distorts the cap.
It is obviously possible to build a high performance engine and
forego align honing. But if the engine is "hammered"
very often, or if the owner installs a nitrous oxide system, you
may very well end up with an unhappy customer.
Of course, the best choice for a high performance engine is a
forged crankshaft, but these aren’t readily available at low cost.
In truth, small block Chevy cast cranks are more than adequate
for most high performance applications. From 1969 until 1986,
when Chevrolet converted to a one-piece rear main seal, c/n 3932442
was installed in virtually every 350 small block not equipped
with a forged crank.
But the casting number doesn’t tell the whole story. The same
crank casting is used as the basis for 305 crankshafts. Although
a 305 crank can physically be bolted into a 350 block, it’s best
to avoid doing do. The 305’s lighter reciprocating assembly weight
translates to a considerable difference in the balance factor.
Unless a 305 crankshaft is completely rebalanced with the appropriate
bob weight, it will cause severe vibration if installed in a 350.
If there’s any question as to a crank’s identity, it should be
checked so it can be used in the appropriate engine assembly.
It’s also advisable to check any cast crankshaft for cracks. As
a general rule, a crankshaft should pass magnaflux inspection
before it’s installed in a high performance engine.
Pistons and rings
The best deal in town on small block pistons can be found in the
Keith Black and Speed-Pro catalogs. Both companies offer hypereutectic
pistons which are ideal for high performance street (and some
race) engines. These pistons are typically cheaper than their
forged counterparts and are actually better suited for long term
operation in a high performance street engine.
The hypereutectic material is extremely hard and has a very low
expansion rate so it can stand considerable abuse. Since it is
installed with .001" to .002" piston-to-wall clearance,
it can handle the abuse over a long period of time without the
clatter associated with most forged pistons. Both flat top and
domed varieties are available so just about any compression ratio
can be achieved.
Most seasoned performance and race engine builders have very strong
opinions regarding brand and type of piston ring and the required
cylinder wall finish. However, for long term durability in any
type of engine, a Total Seal ring set with a plasma moly top ring,
Gapless second and stainless steel low tension oil ring
is tough to beat.
Cylinder wall preparation can also be a hotly debated topic with
various preferences for honing stones, and final surfacing procedures
involving specific plateau finishing specifications, etc. However,
at many shops, the standard cylinder preparation for the ring
combination cited above includes boring the block to within .005"
of desired finished bore size then traveling the rest of the way
with a hone. The typical procedure involves removing the first
.0035" with 220 grit (500 series) stones, then removing another
.001" with the 280 grit stones (600 series). A final finish
is then achieved by removing the last .0005" with 400 grit
(800 series) stones.Although some engine builders use a super-slick
cylinder wall finish, many others do the final hone with 400 stones,
which knocks the peaks off the ridges left by the coarser stones.
Many rebuilders feel this type of finish is best for quick ring
seating and long term ring seal.For optimum sealing, rings should
be fit to the individual cylinders and end gaps filed to fit.
In lieu of manufacturers’ recommendations otherwise, the top ring
should be given .020" to .022" end gap with forged pistons
and .026" to .028" with hypereutectic pistons.A 5/64",
5/64", 3/16" ring configuration is often preferred for
street and recreational marine engines. (Wider rings deliver better
long-term durability.) Although a 1/16", 1/16", 3/16"
ring combination will provide improved ring seal at high rpm,
such considerations are unwarranted in a street or recreational
marine engine because the engine doesn’t spend enough time in
the tachometer’s "Twilight Zone" to justify the trade-off
of reduced ring life. Another consideration is that with a 1/16",
1/16", 3/16" ring package, oil consumption tends to
be higher than with wider rings.
The latest trend in oil rings is low tension. The oil rings are
the most significant contributors to ring drag, so reducing tension
significantly lowers internal friction. In a low tension oil ring,
improved ring conformability (the ability of the ring to stay
in contact with the cylinder wall) is achieved by manufacturing
the oil rails from material with reduced radial thickness. Some
companies are also experimenting with rails that are .015"
thick rather than .024" in thickness.
From the time the small block was introduced, Chevrolet has offered
a variety of cylinder heads. Most of the pre-emissions era high
performance heads have 64 cc combustion chambers. Note that this
is a nominal engineering dimension; in real life, most "64
cc chambers" actually measure 67 or 68 cc. Head milling is
usually required to achieve a combustion chamber that actually
measures 64 cc.
For a typical, lower-cost performance engine, 186, 462 or 492
castings are the most commonly used heads. These are the tried-and-true
"double-hump" castings of the type originally installed
on fuel injected Corvette and ’60s era Z/28 engines. Nothing has
changed much in this area of small block Chevy high performance.
However, amongst owners of late model fuel injected engines, Corvette
aluminum heads have taken the spotlight.
In stock form, the Corvette aluminum head (c/n 10088113, p/n 10185087)
has good air flow characteristics which are sufficient to support
the needs of an engine producing a maximum of about 330 hp. Properly
ported, however, these heads are suitable for 400+ hp engines.
Another consideration is that these heads were designed for use
on fuel-injected engines. As such, they have no heat riser passages
to bring heat to the bottom of the intake manifold, which can
cause cold start problems if an engine is equipped with a carburetor.
Strange as it may seem, there is quite a demand for CNC-ported
Corvette aluminum heads for installation on street-driven small
block engines. In fact, some shops specializing in late model
performance engines install CNC-ported heads on virtually every
engine they sell. With a price of more than $1,200 per pair, CNC
modifications are obviously targeted at the high end of the market.
But the strong demand for this type of porting indicates the diverse
nature of consumers who spend money on small block Chevy rebuilding
Along with aluminum heads usually goes a tuned port or LT1 aluminum
intake manifold. For all intents and purposes, an intake manifold
should be an intake manifold and the procedures used for installation
should be the same. But that doesn’t seem to hold true for late
model fuel injection manifolds. Every time one of these manifolds
is removed from an engine, the cylinder head mating surfaces should
be checked for warpage and angularity. For some reason, these
manifolds are extremely prone to distort, thereby causing sealing
problems.Many engine builders who specialize in tuned port and
LT1 engines will not install an intake manifold unless its condition
has been verified. They’ve been burned too many times by oil consumption
problems caused by internal vacuum leaks which allow manifold
vacuum to pull oil in between the manifold and head surfaces.
Prior to the advent of electronic engine controls, a high performance
engine just had to have the type of camshaft that rattled the
fenders and scared small children. These types of cams are not
compatible with a stock ECM (electronic control module, also known
as a powertrain control module and vehicle control module, depending
on year and model). Consequently, a more conservative approach
is required to ensure reasonable idle quality and driveability
– while remaining emissions legal.
Emissions legality has become a major consideration in performance
engine building. While acceptable exhaust emissions and high performance
may seem mutually exclusive, they can co-habitate successfully
in the same engine. The key to this harmony is proper camshaft
selection and as luck would have it, newer designs are much more
appropriate for current performance requirements.
The best choice is an hydraulic roller camshaft, which is the
reason that since 1987, they have been factory installed in an
ever increasing number of small blocks. Roller profiles are capable
of opening valves at a much faster rate and lifting them higher
than a flat tappet cam, and this is precisely the requirement
for not only keeping emissions in check, but for achieving maximum
power while maintaining compatibility with computerized engine
The faster opening rate and higher lift translates to more effective
use of duration, so cams with comparatively short duration (which
keeps the computer happy) produce excellent horsepower and torque
over a wide rpm range. The most aggressive production hydraulic
roller cam is the one installed in 1994 and later Camaro and Corvette
LT1s. It features intake and exhaust durations of 203 and 208
degrees respectively for intake and exhaust (measured at .050"
lift) and raises the intake valves .450" and the exhaust
valves .460". Aftermarket performance cam-
shafts with similar duration will have up to .500" lift.
These lift specs make checking retainer-to-valve guide clearance
essential. They also require that proper valve springs be selected
so that the possibility of coil bind is eliminated.
Another consideration is camshaft retention. Late model blocks
which were originally equipped for an hydraulic roller cam incorporate
a retainer on the front of the block to prevent the cam from walking
forward. When retrofitting an hydraulic roller cam in an older
block, some form of retainer must be added. Most performance camshaft
manufacturers offer such a component.