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Chevrolet Big Block Engine Generations - On the Mark

Charting the Changes Through Three Generations of Big-Blocks

Rick Voegelin Apr 8, 1999

Step By Step

It’s not the same old big-block anymore! This GEN VI version of the venerable Rat motor has cast-aluminum rocker covers and a six-bolt aluminum front cover. What it doesn’t have is provisions for a mechanical fuel pump or a block-mounted clutch linkage.

Perhaps the most important difference between Mark IV and GEN V-VI big-blocks is the rear crankshaft seal. The Mark IV uses a two-piece seal (top); the switch to a one-piece seal in the 1991 GEN V big-block (above)required major changes in the rear main-bearing cap, crankshaft, oil pump, oil pan, and flywheel.

There are big differences in the size, shape, and location of the core holes in the big-block's deck surfaces. Mark IV blocks have round holes between the cylinders (top); GEN V and GEN VI production blocks (bottom) have irregularly shaped holes and additional openings ahead of the front cylinders.

Trouble ahead: If you bolt Mark IV cylinder heads onto a GEN V block, there's a chance that the coolant will end up in the lifter valley. There may not be enough material on the deck to seal the early-model heads properly.

Mark IV head gaskets have two openings at the front; a GEN V-VI gasket has only one opening to force coolant to flow rearward through the block and cylinder heads.

GEN V and GEN VI big-blocks have a nonadjustable valvetrain (left) that uses shouldered bolts to retain the rocker arms. Mark IV engines use conventional threaded studs that allow changes in valve lash. GEN V-VI heads can be converted to adjustable valvetrains by installing special aftermarket rocker studs or by drilling and tapping the stud bosses for Mark IV 7/16-inch diameter rocker studs.

The lifter bosses in GEN VI blocks are taller than the bosses in earlier versions. The GEN VI bosses are machined flat on top to accommodate guides used with roller lifters.

All Mark IV blocks have a mounting boss for a mechanical fuel pump. This boss was deleted in GEN V versions--but now the boss is back on GEN VI H.O. 454 and L19 service replacement blocks.

GEN V and GEN VI blocks wear rigid cast-aluminum rocker covers that resist oil leaks more effectively than the stamped-steel covers used on Mark IV engines. GEN VI big-blocks use a similar cast-aluminum front cover with an integral pointer instead of the stamped-steel cover found on Mark IV and GEN V engines. The cast cover uses a reliable O-ring seal.

A new four-quart oil pan (PN 12495360) for GEN V and GEN VI big-block Chevrolet V-8s has a smaller sump than the six-quart truck pan installed on H.O. 454 and H.O. 502 crate motors. This smaller sump provides more clearance for chassis crossmembers in early-model passenger cars.

The oil-filter pad on a Mark IV block (left) is recessed from the oil-pan rail. The filter pad on GEN V and GEN VI blocks (right) is flush, and the pan rail has threaded holes for oil-cooler lines (arrows).

Motors, like music, were much simpler in the '60s. When Chevrolet unveiled the big-block V-8 back in 1965, the only decision in a record store was whether to buy the single or the entire album on vinyl. Since then, we've witnessed the advent of four-track, eight-track, quad-sound, laser discs, and cassettes; today we've got CDs, DVDs, and digital audio files on the Internet. Like rock 'n' roll, the big-block Chevy V-8 has been transformed by evolution and revolution.

For more than 25 years, Chevy's big-block V-8 was a rock of stability. It didn't take months of research to become a Rat-motor expert because virtually everything was interchangeable. As long as you could remember the difference between heavy-duty truck and passenger-car blocks, and recall which crankshafts needed external counterweights, you could pass as an authority.

In those innocent days, confirmed Chevy fanatics could make fun of the Ford guys who had to deal with incomprehensible parts combinations. Ford parts seemed to change almost daily, and a block built in August sometimes couldn't be used with a crank forged in October of the same year. Well, what goes around comes around: Now with three generations of big-blocks in circulation--Mark IV, GEN V, and GEN VI--Chevy stalwarts have to go through the same mental gymnastics. A degree in big-block genealogy is now a prerequisite before mixing and matching heads, blocks, camshafts, and cranks.


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