Thirty-seven years after the birth of the small block Chevy
V8, the Generation II engine was introduced in the 1992 Corvette
as the LT1. Although it shared many common dimensions, looked
much the same and even had a few common parts, it was totally
redesigned to provide more power with lower emissions and
better fuel economy.
Compared to the 1991 Chevy 350 L98 with TPI, the LT1 made
20% more horsepower, got better fuel mileage, and had a much
broader torque band with 90% of it’s peak torque available
from just over 1,000 rpm all the way up to nearly 6,000 rpm.
GM Powertrain accomplished all of this by reverse cooling
the engine so they could bump the compression ratio up to
10.5 to 1, tweaking the airflow in and out of the engine,
and using sophisticated electronic controls for both fuel
and ignition. This combination gave the LT1 300 hp in 1992
and ultimately led to the 1996 LT4 that used better heads,
more cam timing, roller rockers and sequential fuel injection
to make 330 hp .
Although the LT1 was only around for five years, there were
two-bolt and four-bolt blocks, aluminum and cast iron heads,
regular and H.O. cams that came with long and short dowels,
and three different front covers. There was also the "Baby
LT1," the 265 cid version that was the standard engine
in the Caprice from 1994-‘96. With all that in mind,
let’s take a look at this family of engines and see what
goes where.
265 - There was only one block used for the 265 cid version
of the LT1. It’s a 10168588 casting that had the numbers
"4.3" cast on the side, too. It’s real easy
to spot if the heads are off because of the small 3.74"
bore.
Getting the right cam in the right engine can be a little
bit tricky because there were several variations over the
years. There are essentially two different grinds used with
two different snouts, depending on which distributor was used
on the engine.
CRANKS
350 - The crank for the LT1 looks just like the one in the
late 350 and has the same casting number 14088526, but it’s
balanced for the lightweight pistons that were installed in
the LT1. Be sure to keep these cranks separate so they don’t
end up in a regular 350, and don’t ever use a regular
350 crank in a LT1. In fact, if you are short of LT1 cranks
and don’t have a balancing machine in your shop, you
would be better off using a crank from a 305 instead of a
350 because it’s actually closer to the balance specs
for the LT1 crank.
265 - The 265 has it’s own unique crank with a 3.00"
stroke. That’s the same stroke the original 265 had back
in 1955; it’s funny how things go around and come back
full circle. It’s a 10168568 casting.
RODS
350 - The original LT1 came with regular forged 350 rods,
that were shot peened for localized hardness under the head
of the bolt and nut. Powdered metal rods were phased in for
the Corvette around 1994 and used in all of the LT1 engines
by 1995. GM made the change because the powdered metal rods
were cheaper to make and were much stronger than the GM high
performance "pink" rods. In fact, they are supposed
to be good for up to 450 hp. They are machined at the parting
line so they can be reconditioned.
265 - The 265 rods are 0.240" longer than the ones in
the 350. Both blocks are the same height, but the stroke for
the 265 is 0.480" shorter, so the rods have to be longer
to make up for half the difference. These rods can be identified
by the single, raised dot on both sides of the shank.
CAMS
1992-’95 350 WITH ALUMINUM HEADS - The 1992 Corvette
had a steel roller cam with a shallow hole in the snout that
measured .450" in the front and tapered down to .240"
at the bottom. It had a short dowel (.320") that was
used to locate the timing gear and a hole with 16 splines
in the center of the gear for the stub shaft that drove the
early distributor. The 1993-‘94 H.O. cam had a few subtle
changes, but all of the early H.O. cams are the same for all
intents and purposes. They can be identified by the number
"241" stamped on the barrel in front of the first
lobe.
1994-’96 350 WITH IRON HEADS - The distributor
drive was changed on the iron-headed motors only in 1994,
so the front of the cam and the timing gear were changed,
too. The cam had a pilot hole that was bigger and deeper (0.500"
x 1.0625") and it had a longer (.685") dowel pin
that stuck out beyond the timing gear to drive the new distributor.
This iron-headed motor was used in the Chevy Caprice, Buick
Roadmaster and Cadillac Fleetwood, so it came with a milder
cam that improved low end torque and reduced valve train noise.
These cams have the long dowel pin and either "600"
or "779" stamped on the barrel of the cam in front
of the first lobe.
1995-’97 350 WITH ALUMINUM HEADS - In 1995, the
aluminum-headed motors got the late, pin-drive distributor,
so there’s a second version of the H.O. cam with the
big pilot hole (.500" x 1.0625" ) and the long (.685"
) dowel pin. Look for a cam with the long pin and either "242"
or "705" stamped on the barrel in front of the first
lobe.
1994-’96 265-INCH MOTORS - All of the 265 engines
came with the later, pin-drive distributor, so they all had
the later style cam with the big pilot hole and the long dowel
pin. The 265 used the same mild cam that came in the iron-headed
LT1. Look for the long dowel pin and either "600"
or "779" stamped on the barrel of the cam in front
of the first lobe.
CAM GEARS
The cam gear had to match the cam and the distributor
drive, so there were two different gears used, depending on
the year and the application.
The original cam had a small, tapered hole in the center
and a short dowel pin. It was used with the cam gear that
had the small hole in the center with 16 splines in it. It
was connected to the distributor with a short drive shaft
that was splined on both ends. The cam gear is a GM p/n 10128349.
This combination was used from 1992-‘95 on the aluminum-headed
motors.
GM had some problems with the early distributor due to both
carbon tracking and moisture, so a new sealed distributor
with a vacuum port was introduced on the iron-headed 265s
and 350s in 1994 and used on all LT1s in 1995. The new distributor
was located with a pilot shaft and driven by a pin, so both
the cam and the gear were changed. The cam had a large, deep
hole in the center for the pilot shaft and a longer dowel
pin to drive the distributor.
The cam gear had a bigger hole and it didn’t have the
splines that were found in the early gear. The pilot shaft
for the distributor extends through the hole in the cam gear
and seats in the hole in the cam; the distributor is driven
by the long dowel pin that sticks up through the cam gear.
The cam gear is a GM p/n 10206039.
350 ALUMINUM HEADS - There were two versions of the
aluminum heads used on the Corvettes, Camaros and Firebirds.
The later ones have less material around the top of the intake
ports and weigh about 2- 1/2 lbs. less than the earlier ones,
but they are identical otherwise. Look for a 10128374 and
possibly a 649.
FRONT COVERS
The front covers have been changed three times, once because
of the changes that were made to the distributor and once
due to OBD II.
The original cover had three holes, one for the crank, a
small hole (@ 0.70") for the water pump drive and a second
one for the small drive shaft for the distributor. It’s
a 10128289 casting.
The second front cover still had the small hole for the water
pump shaft, but it had a much larger hole (@ 2.63") that
sealed on the outside of the distributor housing itself. It’s
a 10214196 casting.
The second cover was modified again in 1996 to accommodate
the crank position sensor that was located in the lower corner
of the cover on the passenger side. This same cover was used
for the few engines that were installed in 1997, too. It’s
a 12550032 casting.
HEADS
There were two heads used on the 350, one aluminum and one
cast iron, along with one cast iron head for the 265. GM claimed
that the original LT1 aluminum head had a 15% increase in
airflow as a result of revised port angles and higher port
ceilings when compared to the 1991 L98 head. That was impressive
in 1992, but the iron LT1 head that came out in 1994 was even
better. It flowed 20% more on the intake side and made more
horsepower on the dyno.
All LT1 heads used small combustion chambers to get the higher
compression ratios with flat top pistons; the 350s had a 10.5
to 1ratio and the 265s had a 9.8 to 1 ratio.
350 IRON HEADS - All of the full-size cars came with
iron heads. They were 10125320 or 12554290 castings.
265 IRON HEADS - The 265 had its own unique cast iron
head with a 10208890 casting number. The chambers are smaller,
so these heads cannot be interchanged with any of the 350
LT1 heads.
That’s the story on the parts and pieces for the LT1.
Chart 2 on page 35 shows how they all fit together year by
year, but there are a few more things every rebuilder should
know in order to avoid some possible problems.
HEAD GASKETS
The 350 LT1 head gaskets are not interchangeable with
regular 350 Chevy head gaskets because they have different
water passages due to the reverse cooling. The original head
gaskets on the LT1 were wider and had holes that held the
pushrods in place for assembly, but the replacement gaskets
look a lot like the ones used on a regular 350. Make certain
not to mix them up.
Use the correct key for the year of the engine. Don’t
try to get by with a regular 350 key; the hub will hit it
before it bottoms on the crank gear and it will cause the
balancer and belts to be misaligned.
DISTRIBUTOR SHAFT
The drive shaft for the early style distributor is reversible
by design, but it may have a machined lead on it that will
cause it to pump oil past the front cover seal if it is installed
backward. Unfortunately, the shop installs it, so all you
can do is wait for the phone call when the seal leaks and
they’re looking for someone to blame.
CRANK KEYS
There is no keyway in the hub for the harmonic balancer,
so GM used a special cutback key that was flush with the front
of the timing gear on the 1992-’95 engines. It’s
p/n 10128303.
When the crank position sensor was added in 1996 for OBD
II, the cutback area on the key was shortened so it stuck
out far enough (about 0.100") beyond the face of the
timing gear to index the notched disc that was used for the
crank position sensor.
EXTERNAL COOLANT LINES
There is an extra hole that goes into the water jacket
on both ends of the heads. These should be plugged when they’re
in the front, but left open when they’re on the back.
There’ s an external coolant transfer line that connects
the holes on the back side to a reservoir that vents the air
and steam vapors that would be trapped in the head and cause
hot spots.
FRONT TAPPET CUP PLUGS
Both of the cup plugs in the front of the lifter galleries
have a 0.030" hole drilled in them. These prevent air
pockets from forming in the front of the galleries and provide
added lubrication to the water pump gear drive.
WATER PUMP DRIVE SHAFT
Be sure to check the seal surface on the geared shaft that
drives the water pump. If it’s grooved, it will leak
and it will be your fault. New ones are available from GM
(p/n 10219554) for around $40. That’s cheap insurance
when you consider that the timing cover, chain and gears have
to come off to replace it.
OIL PUMP
The LT1 powerplant uses the late model 350 oil pump with
the 3/4" pickup tube. This should be fairly easy to identify.
HUB AND BALANCER
The balancer is a two-piece assembly with a pulley that bolts
onto the hub. Separating the two made it easier to install
the Optispark distributor on the assembly line and out in
the field. The holes in the hub are offset, so the balancer
only fits on it one way, but there’s no keyway in the
hub to index the hub on the crank. This shouldn’t be
a problem unless the damper was drilled at the factory to
"trim" the final engine assembly.
If it was drilled a lot to compensate for an engine that
was out of balance, you could end up with a shaker, depending
on how everything stacked up with the remanufactured engine
compared to the original engine. If you encounter a balance
problem on a remanufactured LT1, try rotating the balancer
assembly on the crank 90° at a time to see if it eliminates
the problem.
THE LT4 CHEVY
There is one more version of the LT1 out there that could
cause some confusion if you get a core and don’t know
what it is. The LT4 was the high performance version of the
LT1 that was standard in the 1996 Corvette "Grand Sport"
and optional for any 1996 Corvette with a manual transmission.
There were also 100 1997 SS Camaros built by SLP that came
with the LT4. Chances are most rebuilders will never see one,
but just in case you do, the following are a few things that
make the LT4 special.
BLOCK - Should be the same LT1 casting with the four
bolt mains.
CRANK - The LT4 has a special nodular iron crank with
undercut and rolled fillets for added strength.
RODS - Powdered metal rods are used with a "cracked
cap" instead of a machined parting line.
CAM - The LT4 cam was similar to the ones used in
the high performance LT1, but it has had considerably more
lift at the valve because it had 1.6 to 1 rockers.
HEADS - The heads had smaller 54cc chambers that gave
the LT4 a 10.8 to 1 compression ratio. The intake ports were
raised 0.100", too. The LT4 used larger intake and exhaust
valves (2.00" x 1.55" ). The intake stems were hollow
to reduce weight and the exhaust stem was sodium-filled to
improve heat transfer. The valve springs were made of oval
wire to prevent coil bind with the higher valve lift and fitted
with lightweight retainers to help the engine rev to its 6,300
rpm redline.
All of these changes enabled the LT4 to make 330 hp at 5,800
rpm instead of 300 hp at 5,000 rpm.
