Читать книгу Chevy Big-Block Engine Parts Interchange - John Baechtel - Страница 8
ОглавлениеScores of reasons exist why hot rodders, racers, and restorers diligently seek particular cylinder blocks for their engine projects. The most common reason is to find an affordable block with four-bolt mains and reasonably thick cylinder walls; this, despite the fact that beefy two-bolt blocks are capable of adequately serving most high-performance street engine requirements up to about 550 hp and more if equipped with main cap studs. In some cases a guy might already have a Mark IV block or be in a position to acquire one cheap from a friend or local source. Cost frequently plays a significant role.
Mark IV blocks with two-bolt mains were standard equipment in all lower-horsepower big-block applications. They are suitable for moderate performance upgrades up to 550 hp and are desired primarily for restorations. Aftermarket caps can be used to convert two-bolt blocks to four-bolt, but they can’t use splayed caps because they would interfere with the main oil gallery. (Photo Courtesy Vintage Big Blocks)
Another motive is the desire to locate the correct block for a vehicle restoration project. In most cases the restorer is seeking a serviceable undamaged block with the correct casting numbers and physical characteristics that match the original application. That might include the type of rear main seal (one- or two-piece), the number of main bolts and correct mounting bosses for attaching clutch linkage, and sometimes a matching VIN. This is often more difficult to achieve and some restorers resort to special stamping sources to add the correct number to their block. It also includes the correct model block that might be a Mark IV version, a Gen V, or Gen VI block.
Racers are looking for blocks that exhibit minimal core shift, thick cylinder walls as verified via sonic testing, undamaged cylinders that haven’t been overbored, and other characteristics specific to their intended application.
Today, most of these concerns have been largely eliminated with a broad variety of new factory and aftermarket blocks that have been internally and externally upgraded to resolve known shortcomings found in original stock blocks.
Original Mark IV blocks are widely sought after and it’s important to properly mate their characteristics with the performance goals of your particular build. As noted in Chapter 1, there are three primary big-block groups (Mark IV, Gen V, Gen VI) characterized by three major differences: lubrication system layout, rear main seal design, and variations in the deck surface coolant passages.
Original side-oiler Mark IV engines with two-piece rear main seals were produced from 1965 through 1990 and included most of the legendary muscle car engines, truck engines, and crate engines through the 1980s. Revised Gen V engines with priority main oiling and one-piece rear seals were built from 1991 through 1996 incorporating various new features designed to resist oil leaks and improve performance and reliability. Their coolant passages are not compatible with earlier Mark IV engine blocks and thus cylinder heads are not interchangeable. This presented a problem because most Gen V cylinder heads incorporated an undesirable non-adjustable valvetrain and generally smaller ports not conducive to performance applications.
All original Mark IV engine blocks are machined for traditional two-piece rear main seals. (Photo Courtesy Tom Dufur)
Gen V to Mark IV Cylinder Head Adapters
To accommodate Gen V block applications wishing to run incompatible Mark IV cylinder heads, Sallee Chevrolet used to offer an adapter kit consisting of specially shaped plugs that cover and convert the irregular-shaped coolant passages in Gen V blocks to accept commonly available Mark IV heads. These were sheet-metal plugs that block and redirect coolant flow to match earlier cylinders heads. Special Fel-Pro gaskets were included with this kit to ensure full compatibility. In service they proved totally reliable and provided full compatibility with earlier cylinder heads. Unfortunately, Sallee Chevrolet went out of business and these kits are nowhere to be found.
Gen V Block Coolant Passage Compatibility
A major issue of incompatibility among big-blocks is the block coolant passage revisions initiated in 1991 Gen V production and Bowtie cylinder blocks. The new irregular-shaped core holes do not match Mark IV heads. The revised casting process for Gen V production and Bowtie blocks required an additional coolant hole at the front of the block on each deck surface. This was a casting issue, not a coolant issue. The Gen V gasket blocks these holes. If you don’t use the correct gasket to block these holes, water from the pump goes immediately through the holes and back out through the front of the intake manifold without circulating through the engine to cool it. Oops!
Here’s a close-up view of the one-piece rear seal mount found on all Gen V, Gen VI, and recent Bowtie blocks. (Photo Courtesy Scoggin-Dickey Parts Center)
Gen V production engines launched in 1991 included newly designed cylinder heads, redesigned oil feed, and water passages that do not mate successfully with Mark IV engine pieces. These engines have one-piece rear main seals, different pan rails and oil pan, crankshafts, gaskets, timing covers, pushrods, non-adjustable rocker arm assemblies, and valve covers, all of which must match correctly to ensure a successful big-block build. These components do not interchange freely with Mark IV engines. Performance engines can be built from a Gen V base, but it’s not recommended because Gen VI blocks provide full interchange capability with Mark IV components as long as you use a crank with a one-piece rear main seal.
Gen VI engines launched in 1997 provided revisions to accommodate their intended truck applications and to regain compatibility with earlier Mark IV cylinder heads. Further corrections were incorporated in the last production version of the big-block (2001–2006), the RPO L18 8.1L (496-ci) tall-deck EFI truck engine (sometimes called the Gen VII). A high-performance version, the Vortec 8100 marine engine, delivers 392 hp and 503 ft-lbs of torque. It’s not currently sought much for performance use, but as the last production big-block built, that may change over time.
Technical Notes
• Gen V Bowtie blocks accept both Gen V and Mark IV heads
• To use Mark IV heads and gaskets, tap and plug the extra core holes at the front of the deck
• If you use Gen V heads on a Gen V Bowtie block, use a Gen V gasket to block the core holes
• Gen VI heads fit Mark IV and Gen V blocks
• Mark IV heads fit Gen VI blocks
• Use the appropriate head gaskets
Some minor interchange is possible among these engines, but not without a thorough understanding of the differences and what fits what. Cylinder heads are not compatible without block deck modifications that are generally beyond the scope of most casual builders.
With the exception of dedicated racing applications and some crate engines, all Chevrolet big-blocks are manufactured from cast iron. Bare blocks weigh approximately 250 pounds and up to approximately 280 pounds in special high-performance applications (Bowtie) with thicker siamesed cylinder walls, main bulkheads, and deck surfaces. Original aluminum blocks with cast-iron cylinder liners are difficult to find and are often damaged. New, improved factory and aftermarket blocks are readily available, so an original block, if you can find one, should not be raced but preserved for restoration purposes. Even some restoration applications use a new aluminum block and preserve the original for display purposes.
Locator services make it much easier to find serviceable original blocks for your restoration or performance engine build. They actively seek and stockpile original blocks and other parts with desirable casting numbers and other identifiers. (Photo Courtesy Vintage Big Blocks)
Four-bolt Mark IV blocks have four straight bolts (not splayed) on all five main caps. They were used on all performance applications and most truck applications. Original four-bolt blocks are often suitable for performance builds from 550 to more than 750 hp. (Photo Courtesy Vintage Big Blocks)
