| Image Image Image Image Image Image Image Image Image Image Image Image Image
Members Login
User ID: Password:   Forgot Password  

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

Mark IV Gen V
Rear Seal 2 Piece 1 Piece *
Main Oil Gallery Location Oil Pan Camshaft Tunnel
Mechanical Fuel Pump Yes No
Adjustable Valvetrain Yes No
Rocker Covers Stamped steel Cast Aluminum
* 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 this chapter.)

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:
Part Number Description
10112680 Rocker arm, stamped steel(Include ball and nut)
3959182 Rocker arm, stamped steel, Long shot (without ball and nut)
5232762 Rocker arm ball
3896648 Rocker arm adjusting nut
3921912 Rocker arm stud
3860038 Push rod guide plate (for 3/8 push rods)
3879620 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:
Part Number Description
10183745 Cast rocker cover without oil filler hole
10183746 Cast rocker cover with oil filler hole
14085759 rocker cover casket steel reinforced
10126726 Screw-in oil filler cap
10126724 Grommet for PCV valve
14082320 Rocker cover stud nut 1/4-20 * 1 1/4 inch Torx head(14 required)
14051876 Rocker cover stud nut 1/4-20(14 required)
10126786 "366" rocker cover emblem
10126787 "427" rocker cover emblem
10126790 "454" rocker cover emblem
10185091 "502" rocker cover emblem

| Image Image Image Image Image Image Image Image Image Image Image Image Image