Mark IV vs. Gen V
Mark IV 1965-1990, Mark V 1991-1996
The Chevrolet big-block was updated in 1991 with a redesigned block, a new
crankshaft design, and a one-piece rear seal. The revised big-block was
christened the Gen V.
The arrival of a second-generation big-block in 1991 neared the
first significant revision in the basic design of the big-block
Chevrolet V8. The substantial changes warrant a new designation
to differentiate the updated engine from its Mark IV predecessor.
The revised big-block was named "Gen V".
The Electronic fuel injection and rigid cast aluminum covers identified the
new Gen V 454ci L19 big-block introduced in 1991.
The following are the chief differences between the Mark V (1965-1990)
and Gen V (1991-up) versions of the Chevrolet big-block V8:
Mark IV vs. Gen V
1 Piece *
Main Oil Gallery Location
Mechanical Fuel Pump
* 2 piece seal in race-prepared Gen V.
The Gen V big-block retains all of the Mark IV's external dimensions.
Block height, cylinder bore centers, and lifter bore locations are identical in
the two engines. Bosses for motor mounts and accessory brackets are also unchanged,
making the Gen V a bolt-in replacement for most early-model
big-blocks. The Gen V block does not have provisions for a mechanical fuel pump.
However; an electric pump can be substituted in most installations.
Although the second-generation big-block shares its major dimensions and many
internal components with its predecessor, numerous revisions made it more
reliable and more oil tight than the Mark IV big-block. In production
versions, a new cast nodular iron crankshaft was introduced with rolled
fillets for increased strength. Hypereutectic (high silicon) aluminum
pistons with low-friction skirts and rings reduced reciprocating weight.
Most of the revisions in the Gen V design were aimed at improving reliability
and eliminating sources of potential oil leaks. A one- piece rear crankshaft
seal replaces the two-piece lip seal used on the Marl IV. This change to a
one-piece seal required a redesigned crank-
All Gen V Chevrolet big-blocks (except race-prepared Bow Tie) use a one-piece
rear crankshaft seal. The oil filter pas is flush with the oil pan rail; the
filter element spins onto a threaded connector. The water jacket plugs are
located in raised bosses, and the 1.625-inch diameter soft plugs are
interchangeable with small-block V8 and V6/90 Chevrolets. Raised numerals on
the outside of the case identify the engine displacement; Bow Tie blocks
are identified by the Bow Tie logo.
Shaft flywheel flange, rear main bearing cap, oil pan. Consequently these
components are not interchangeable between Mark IV and Gen V-engines. However,
many other components, including intake manifolds, water pumps, camshafts,
and distributors are interchangeable between the two versions.
Although the crankshaft's flywheel flange bolt pattern was unchanged in the
Gen V big-block, the revised crankshaft valance required a special counter
weighted flywheel (or flex plate) for proper engine balance. Second-generation
big-blocks do not use the same flywheels and flex plated as externally balanced
first-generation 454cu Mark IV V8s. Flywheels are also not interchangeable
A comparison between a Mark IV block and a Gen V version
has several significant changes. The main oil gallery was relocated
from the oil pan rail to the camshaft tunnel. The boltholes and pad for a
camshaft thrust plate are rotated 90-degrees to accommodate the new oil
gallery position. The bolt pattern for the timing chain cover is the same
on both blocks, but the cover itself has been changed on Gen V block to
accept a one-piece oil pan gasket. The Gen V block's front bulkhead is moved
forward to improve coolant flow around the No.1 cylinder. Water pumps are
interchangeable between the old and new blocks. (Production big-blocks with
serpentine belt systems uses reverse-rotation water pumps.)
Between second-generation cast iron and forged steel cranks. (See the
flywheel section for more information.)
The Gen V's cast aluminum rocker covers with captured gaskets and screw in
filler caps replaced the stamped steel covers used on Mark IV versions. The
new block also introduced integral oil cooler connections that eliminated
the seals, gaskets, and adapters formerly required for oil cooler plumbing.
Another significant difference between Mark IV and Gen V big-blocks in the
location and size of the coolant passages in the cylinder block deck surfaces.
Marl IV cylinder heads should not be installed on production Gen V blocks,
which have smaller holes in their decks.
The Gen V big-block's lubrication system was updated by relocation the
Main oil gallery from the driver's side oil pan rail to the camshaft tunnel.
This change reduced the possibility of oil leaks.
All Mark IV big-blocks use a two-piece rear crankshaft seal; Gen V blocks
use a one-piece seal. The rear main cap and bulkhead were redesigned to accommodate
this new seal. The Gen V engine has two tapped holes for external oil cooler
connections. Oil is routed out of the block through the rear hole, and returned
through the front hole.
The Gen V's valvetrain was revised to ensure consistent quality. Shouldered
bolts replaced the threaded rocker are studs used on previous big-blocks.
This nonadjustable system eliminated assembly line variations in lash that
were possible with the old design. (Instructions for converting Gen V head
to adjustable rocker arms for high-performance camshafts are included in
Production and heavy-duty Gen V Bow Tie big-block engine cases are machined
with similar tooling. As a result, Gen V Bow Tie big-blocks incorporate many
features of the revised engine design. Heavy -duty Gen V Bow Tie blocks have
the same external appearance, one-piece rear crankshaft seal, and revised
oiling system as production cases.
Bow Tie versions of the Gen V big-block differ from production blocks in
several important areas. Bow Tie blocks have Siamese cylinder wall. Extra-thick
deck surfaces with blind tapped head boltholes, and machined main bearing
bulkheads with four-bolt bearing caps.
A Mark IV block has provisions for a mechanical fuel pump. This mounting
pad is deleted on a Gen V block
The production Gen V gig-block V8 introduced in 1991 uses a non-adjustable
"net lash" valvetrain. This valvetrain uses shouldered bolts instead of
threaded studs and adjusting nuts to retain the rocker arms.
The Gen V "net lash" valvetrain presents a challenge to engine builders,
however. If the camshaft profile is changed significantly for off-highway
applications, the net lash rocker assembly must be replaced with Mark IV
rocker studs; rocker arms, adjusting nuts, and push rod guide plates. This
conversion will allow an engine builder to compensate for changes in the
camshafts base circle diameter and to adjust valve lash with mechanical tappets.
The rocker stud bosses on Gen V cylinder heads are machined for 3/8-inch
shouldered bolts. To convert to an adjustable valvetrain, the cylinder
heads must be disassembled and the bosses drilled and tapped for conventional
7/16-inbch rocker studs.
The Gen V big-block has a "net lash" valvetrain with non-adjustable rocker arms.
The Gen V valvetrain uses a 3/4-inch shouldered bolt to retain the rocker
arm. Conventional big-block 7/16-inch rocker studs, rocker arms, and push rod
guide plates can be installed for high-performance off-highway applications.
A nonadjustable rocker arm has a wider slot than a high-performance
rocker (right). Note the difference in the diameter of the pivot ball holes.
Due to the compound angle of the big-block's rocker studs, this operation should
be performed with a multi-axis mill or valve machine. Drill out the 3/8 -inch
threads with a .375-inch bit, then tap the bosses fir 7/16-14 NC threads, Do
not shorten or spot face the rocker stud bosses.
Special rocker arm studs are available from after market suppliers to convert
Gen V cylinder heads without machining. These studs have 3/8-inch threads that
screw into the production Gen V rocker studs bosses without modification. They
are recommended only for valve springs with less than 450 pounds open pressure.
Replace the original rocker arms, nuts, pivot balls and push rod guide plates
with the parts listed below.
To install adjustable rocker arm studs, drill and tap the rocker stud bosses
for 7/16-14 NC threads. Do not shorten the stud bosses.
To convert to adjustable rocker arms, replace the 3/8-inch diameter shouldered
bolt (right) with a conventional 7/16-inch big-block screw-in rocker arm stud
or a special after market with 3/8-inch threads.
Heavy-duty one-piece 3/8 inch or 7/16-inch push rods should be used in place
of production Gen V push rods in high performance applications.
The following parts can be used to install an adjustable valvetrainn on
Gen V big-block Chevrolet V8 engines:
Rocker arm, stamped steel(Include ball and nut)
Rocker arm, stamped steel, Long shot (without ball and nut)
Rocker arm ball
Rocker arm adjusting nut
Rocker arm stud
Push rod guide plate (for 3/8 push rods)
Push rod guide plate (for 7/16 push rod)
After market rocker studs with 3/8 threads can convert Gen V cylinder heads
to adjustable rocker arms without machining.
Cast aluminum Gen V rocker covers can be modified for use on Mark IV big-block V8s.
The Gen V-engine introduced cast aluminum rocker covers for big-block V8s.
The rigid covers are the first lines of defense against rocker cover oil
Leaks. They have captured "O" ring gaskets and screw-in filler caps
prevents oil seepage.
The cast rocker covers matte black finish adds a high-tech touch
to any engine compartment. Four self-adhesive engine displacement
enables are available for cast aluminum rocker covers.
Cast aluminum Gen V rocker covers can be installed on Mark IV cylinder
heads with only minor modifications. The cast cover's raised "O" ring
splash shield must be removed to allow the
To install Gen V cast rocker covers on Mark IV cylinder heads, machine
off the raised "O"-ring splash shield on the exhaust side of the cover
and remove the bosses under the rocker cover hold-down bolts.
Cover to seal against the rocker cover rails on early-model cylinder heads.
The small bosses beneath the rocker cover hold-down bolts should also be
removed; these bosses provide the correct compression for the production
Mark IV gaskets in place of the "O" ring seals with modified Gen V covers.
A vertical mill is recommended for this operation. Protect the top of the
cover to prevent damage to the finish when the cover is clamped to the
milling table. Support the center of the cover with shims to prevent chattering.
A modified Gen V rocker cover will seal against the rocker cover
rail on a Mark IV cylinder head. Replace the production "0" ring seal
with a flat gasket (PN14085759).
The following parts can be used to install Gen V cast aluminum rocker
covers on Mark IV Chevrolet big-block V8 engines:
Cast rocker cover without oil filler hole
Cast rocker cover with oil filler hole
rocker cover casket steel reinforced
Screw-in oil filler cap
Grommet for PCV valve
Rocker cover stud nut 1/4-20 * 1 1/4 inch Torx head(14 required)
Rocker cover stud nut 1/4-20(14 required)
"366" rocker cover emblem
"427" rocker cover emblem
"454" rocker cover emblem
"502" rocker cover emblem