The rotating assembly of the Chevy big-block evolved and changed over the lifetime of the engine and through the various series. When selecting the crankshaft, pistons, and rods for your particular engine build, use the best components you can afford. You need to install a rotating assembly that supports the output levels of the engine, or in other words high-performance engines demand high-performance parts.
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The crankshaft is the core component of all big-block rotating assemblies. For many applications, original cranks and companion components (such as rods and pistons) have become less desirable due primarily to the broad proliferation of new high-quality aftermarket replacements for reasonable expense. They are still sought by some restorers, but you would be surprised how many numbers-matching cars have an original block stuffed with aftermarket crank, rods, and pistons.
Demand for big-block parts has always been high and the supply became strained in the late 1980s and early 1990s. That’s when aftermarket companies picked up the slack with affordable high-quality cranks and rods that pretty much eliminated the need for original factory parts in all but the most serious restorations. There are still plenty of good original parts out there if you know how to identify them, but there’s little need for them in most applications.
Production big-block crankshafts were manufactured in two different stroke lengths. All Mark IV 366T 396-, 402-, and 427-ci engines were equipped with 3.76-inch-stroke crankshafts that are internally balanced and machined for two-piece real main seals. All subsequent 454-ci engines were built with a 4.00-inch stroke and also used two-piece rear main seals, but required external balancing to accommodate the heavier counterweights. Gen V and VI 454/502- inch engines also used the 4.00-inchstroke length, but they were machined for one-piece rear main seals; hence, the cranks are not interchangeable due to the incompatibility of the rear main seals.
The main bearing diameter on all big-block Chevy cranks is 2.7482 to 2.7492 inches except for the rear main that is 2.7478 to 2.7488 inches. All rod journals measure 2.1988 to 2.1998 inches. All cranks have a 3.58-inchdiameter six-bolt flywheel/flexplate pattern on the flywheel flange and all share the same flywheel/flexplate center register diameter. You can install Mark IV cranks with two-piece rear seals in earlier Gen V CNC race-prepared Bowtie shortdeck blocks (PN 24502500) or tall-deck versions (PN 24502502).
Among Gen V Bowtie Sportsman blocks (2013), you can put Mark IV twopiece rear seal cranks into the short-deck block (PN 19212191) or the tall-deck version (PN 19212194). The aluminum ZL1 block (PN 12370850) and both DRCE blocks (PN 24502572, 2013), 9.525- to 9.000-inch deck; and PN 25534406, 9.250- to 9.000-inch deck) accept Mark IV cranks with two-piece rear seals. Aftermarket providers still offer aluminum adapters that allow you to install a Mark IV (two-piece seal) crank into regular Gen V and VI blocks. This is often helpful and while the adapters are expensive, they may be more cost effective than switching blocks or cranks.
Rear Main Seals
The following factory part numbers are available to accommodate both oneand two-piece rear seal applications.
- PN 10101164 one-piece rear main seal
- PN 10117767 two-piece rear main seal
All Mark IV cranks with the 3.75- inch stroke are dimensionally interchangeable, so you can use a 396 crank to build a 427 or use a 402 crank to build a 396. When you mix and match these cranks to build a displacement different from the original, you must rebalance the reciprocating assembly because of the difference in counterweights and the rod and piston combination you choose. If you look at the counterweights on a 396/402 crank, they have a third counterweight that is 7/16 inch wide while a 427’s third counterweight is 7/8 inch wide.
The use of all factory parts does not guarantee proper engine balance in every case. The 454-ci engines do not present quite the same problem, but they still require rebalancing if you change anything in the assembly including rods, pistons, or even the ring package. Rebalancing is often required because changing the piston may also include a thicker or thinner ring or a taller or shorter oil ring. The difference is minimal, but in some cases it is enough to require rebalancing depending on the degree of accuracy the builder requires.
The Chevy big-block Mark IV and later-generation engines were equipped with a variety of crankshafts. The correct casting numbers for the block and external parts, such as the intake manifold, distributor, and exhaust manifolds for period-correct engine restorations are crucial. For internal parts, such as the crankshafts, new old stock (NOS) OEM parts are usually not installed. Manufacturing processes and metallurgy have vastly improved since the 1960s and 1970s, and these parts do not affect the appearance. The rotating assembly parts of today are far superior. Manufacturers offer cast, forged, and billet crankshafts for mild-modified to race-build engines. These cranks fit a variety of performance goals and budgets.
Big-block cranks are either cast or forged depending on their usage. Cast cranks generally populate trucks and passenger cars while forged cranks shore up the performance ranks. Cast cranks are generally more flexible and they provide solid reliable service for extended periods at reduced performance levels including typical street and highway driving. They are tough enough to support performance work up to about 500 hp. When we speak of cast cranks we are really referring to cranks manufactured from nodular iron. Cast cranks are more widely available and because the material is relatively hard it does not require expensive re-hardening procedures (necessary on forged cranks) even after they are turned down to smaller diameters and refinished. They’re also a good choice for heavy-duty low-speed engines such as those found in most truck applications. A quick way to identify a cast crank is the wide parting line on the crank throw arms.
Forged cranks are stiffer and tougher than cast examples and they withstand abuse better. If you’re building any type of performance or race engine you want a forged crank. Forged cranks have a very thin parting line on the throws. If you’re searching through a pile of cranks, you can quickly eliminate those that don’t fit the description. Then you can concentrate on the casting numbers and the crank’s general condition to see if it is suitable for your purpose.
Most OEM forged cranks were hammered out of 1053 carbon steel and a select few were made from higherstrength 5140 steel alloy. For the most part, forged factory cranks are suitable up to around 700 hp, maybe 800 hp if you’re adventurous. Unless you need one for a full-on restoration, you may do better with a new aftermarket crank, especially since used cranks have probably been turned down and the tuftriding ground away. Tuftriding is a factory-derived heat treatment that provides a hardened surface approximately .005 inch thick. It was used on factory-forged cranks to improve durability, but has largely been replaced by nitriding, a low-heat process that provides a thicker protective surface up to .030 inch thick. High-performance forged cranks that have been ground should be checked and re–heat-treated or nitrided to ensure their original level of performance.
Billet cranks are fully machined from a single large-diameter bar of alloy steel billet material (typically 4340). They do not have any of the residual stresses imparted when forgings are smashed into shape by massive presses exerting tons of pressure. These cranks are expensive because they take a long time to machine to exacting tolerances. They are generally reserved for maxeffort racing engines and most big-block builders are fortunate to not require one. If you’re under 1,000 hp, you definitely don’t need one.
Flywheels and Flexplates
All 366/396/402/427 Mark IV engines are internally balanced. They accept two different flywheels depending on the application. If you’re looking for an original flywheel you must match it to the engine, i.e., internal balance and correct diameter for the application. Truth be told, you can use either size flywheel or flexplate as long as you match up the starter and the appropriate clutch and pressure plate. (See the chart on page 131.) Another way to look at it is that engines originally equipped with onepiece crankshaft seals require externally balanced flywheels or flexplates (except for ZZ427, ZZ572/620, ZZ572/720R, and Anniversary Edition 427).
Like crankshafts, original big-block connecting rods are not as highly prized as they once were. This is due, in part, to the broad availability of affordable high-quality aftermarket rods. Nonetheless, stock rods in good condition are still perfectly suitable for all street applications and many race engines, particularly street/strip or bracket drag cars. Some restorers also prefer original rods in their engines. If you’re able to pick up a set of original factory rods that haven’t been abused, it is probably worth it.
Over the course of big-block evolution six different rods have emerged. All are forged-steel I-beam construction with the same center-to-center length of 6.135 inches. They all share the same-size big end and small end. Pin diameter on all big-blocks is .990 inch and the pin bores are set to .990 inch for pressed pins and .9905 to .9907 inch. The big end I.D. is 2.324 inches as required to house bearing inserts for standard 2.2025-inch I.D. rod bearings. All factory rods are interchangeable, but some are more desirable than others depending on the application.
Some of the differences among factory rods are quite subtle while others are more obvious. The biggest difference is found in the rod bolt size and the pin type. As shown in the chart on pages 64 and 65, standard- and moderateperformance applications use 3/8-inchdiameter rod bolts and high-performance versions step up to 7/16-inch bolts. Almost all factory rods use a press-fit piston pin except the high-performance L88/ZL1 application.
Press-fit pins have an interference fit in the small end of the rod, typically about .001 to .0015 inch. Once installed in the rod they do not move. Floating pins are free to rotate within the piston pin bosses and the small end of the rod. They are typically installed with a clearance of approximately .008 to .0012 inch. All factory high-performance rods have the characteristic bumps on both sides of the upper rod beam and are commonly called “dot rods,” although some people erroneously refer to them as dimples. All performance rods are Magnafluxed and shot-peened. Magnafluxing is a crack-detection process; shot peening compresses surface imperfections in the metal to make it tougher and more crack resistant.
Factory rods have several identifying characteristics to help you determine what you’re buying. Color-coding was used for quick identification during the original assembly process at the factory. The paint may have worn away because it was applied lightly so in most cases it’s not a reliable identifier. Luckily, each different rod exhibits visual clues that reveal its origin.
The first-design rod was used primarily in standard passenger car bigblock applications. Two versions of this rod existed. One serviced passenger car applications and some high-performance 396, 402, and 427 engines. The other was used in 454s from 1970 to 1975. These were regular forged-steel rods with pressfit pins and knurled-shank 3/8-inchdiameter rod bolts. These rods are suitable for most restoration projects, even those that make 400 hp or more. For maximum durability use ARP (Automotive Racing Products) WaveLoc bolts when you prep the rods.
Second-design rods were used in all 1965–1969 high-performance bigblocks. This was still a pressed-pin design although some were modified for floating pins, specifically 435-hp Corvette engines. Stronger caps with stiffening ribs are characteristic of these rods and the 3/8-inch rod bolts were upgraded from 1038 steel to 4340 alloy for increased strength. You can identify these rods by the rod cap ribs and the shape of the machined surface on the big end. They are similar to truck rods, but truck rods are much bulkier near the big end.
The third design is the L88/L89 or ZL1 rod used in 430-hp Corvettes. It was the first rod configured for full-floating pins and larger bolts with the 7/16-inchdiameter boron-steel rod bolts that raised tensile strength to 220,000 pounds. This heavy-duty rod also featured thicker rod beams along with factory heat-treating, Magnafluxing, and shot peening.
GM engineers knew this rod would be raced and they responded with appropriate upgrades. The special rod bolts were also shot-peened and O.D. was ground for added insurance against defects; each end of the bolt featured bumps. Quality engine builders used stretch gauges to adjust rod bolt torque (even back then).
These rods are easily identified by their heavy-duty construction, dimpled rod bolts, and noticeable bumps on both sides of the rod beam near the small end.
A fourth-design pressed-pin rod was incorporated in all high-performance big-blocks from 1965 through 1972. This was the early LS6/LS7 rod that was similar to the L88 rod. It was Magnafluxed and shot-peened, outfitted with heavyduty 7/16-inch-diameter knurled-shank bolts. It has the same bumps on both sides of the rod beam.
The fifth design is a pressed-pin version also known as the late LS6/LS7 rod. It was also used in later Gen V and VI engines. It features the same characteristics as the fourth design but also has extra reinforcement below the pin boss. It bears the characteristic bumps on both sides of the rod beams.
The truck rod has a beefier radius merging to the big end and 3/8-inch bolts with knurled shanks. It is easy to identify because the side reliefs are Q-tip-shaped instead of splayed like other rods. Think of it as a heavy-duty first-design rod that is similar to the second-design rod. It is suitable for most street/strip applications up to 500 hp.
Gen V and VI Rods
As shown in the chart on pages 64–65, three Chevrolet rods are currently offered for Gen V and VI engines. They are also suitable for earlier Mark IV engines, although two of them are longer than stock and thus require pistons with the appropriate pin height. The 572 H-beam rods are bushed for floating pins.
Forged-steel rods offer almost unlimited cycles so used rods in good condition are almost always serviceable with bolt replacement and big-end resizing. The cost may or may not seem attractive because equal- or better-quality aftermarket rods are available for a surprisingly reasonable price. Those who choose to recondition a set of used rods should certainly have the rods Magnafluxed and checked for alignment before proceeding. Good pieces can then be resized and fitted with high-quality bolts that ensure reliable service.
Most restoration and street/strip applications are perfectly suited to reconditioned rods. Otherwise, you can simply pop for a good set of new aftermarket rods with all the most desirable features for about the same price and you have a much bigger choice of rod lengths if that matters to you.
Because all big-block rods, except for some recent Gen V and VI units, are the same length they interchange freely with the appropriate piston pin height for the crankshaft stroke you are using. If you use one of the longer late-model rods (6.535 inches) be sure to select pistons with the correct pin height for the longer rod. These rods require pistons with a shorter-than-stock pin height.
Pistons and Rings
Chevrolet Performance replacement pistons for big-blocks are limited in the factory catalog; consisting only of standard-bore replacements for 502/572 engines and a generic 427 piston that may or may not fit your needs according to chamber shape and size. The best course of action here is the aftermarket where ideal replacement pistons are plentiful and you can also order custom pistons if necessary.
A common trick is to use a reduced CR to accommodate today’s fuels. Many 11:1 and 12:1 engines are reduced to 9.5:1 for drivability. Custom pistons accommodate this easily and you can also configure the ring package to suit your needs. Speed Pro, KB Pistons, Icon, and others offer a broad variety of big-block pistons and you can usually find what you need in a Summit Racing or JEGS catalog or online. The important thing is to select the correct piston pin height for your application and then match the piston crown to the cylinder head.
Domed pistons are still available for both open- and closed-chamber heads and restoration applications can manipulate the CR to suit their specific concerns by adjusting dome height and configuration, gasket thickness, or even deck height in some cases. Your 11:1 engine may lose 10 to 15 hp when you lower the compression for pump gas, but drivability improves. Most high-horsepower engines already have enough power to tear your head off so what’s 10 to 15 hp when all you do is cruise Main Street on Saturday night and haul lawn chairs to a car show once or twice a month in the summer? The choice is yours.
Standard Mark IV big-blocks were generally equipped with cast pistons whereas high-performance versions often had forged pistons. Again the choice is yours and both types are widely available along with the now more common hypereutectic pistons from the aftermarket.
Cast pistons are inexpensive and easy to produce; they save you money on your rebuild if you don’t plan to lean on it too hard. They have a thermally stable crystalline grain structure and frequently incorporate cast-in steel explosion struts that allow a tight fi t in the bore for optimum stability and ring sealing.
Under normal use they stand up well to tens of thousands of miles of extreme use. However, they have speed limitations and limited thermal and detonation qualities. They were never intended to run much over 5,000 rpm. They do often survive 6,000 to 6,500 rpm without distress under the right conditions that may include infrequent short-duration bursts of speed and occasional drag strip use. In most cases they should be used in stock restoration rebuilds or moderate performance builds for which engine speed is limited and detonation is strictly avoided.
At the opposite end of the scale forged pistons are designed to take plenty of punishment. They are made with a forging die from a solid slug of heat-treated aluminum alloy. They have a dense grain structure and metallurgical properties designed to withstand severe use, including some degree of detonation resistance. However, forged pistons have less dimensional stability and require more piston-to-wall clearance due to greater thermal expansion. They remain the top choice where strength and durability are required for racing, turbocharging, supercharging, or when nitrous oxide injection is contemplated.
Normally aspirated street applications often experience a small degree of piston slap noise until the engine reaches operating temperature, but it’s generally not annoying except in low-performance applications for which smoothness and quiet operation are valued.
These are a recent casting variation that has about 21 ⁄2 times the silicon content of standard cast pistons for increased hardness and greater resistance to higher temperatures and cylinder pressures. They possess high dimensional stability and require minimal skirt clearance (as do cast pistons). In some cases they can operate with even less clearance than regular cast pistons. This keeps the piston and the ring pack well stabilized in the bore; ring sealing and blowby control are improved.
Although hypereutectic pistons are well suited to street performance applications, they do not possess the detonation and temperature resistance of forged pistons so it is easy to damage them. They should not be used with anything more than very light loads of nitrous oxide (if at all) with high-pressure turbo or supercharged applications.
Domed pistons are required on most performance big-blocks to gain compression. This is often the case because factory-style combustion chambers are quite large and it takes a fairly large dome to fill them enough to generate a reasonable CR for high-performance use. Street applications running pump gas require lower CRs to accommodate variations in fuel quality.
Different dome styles are required to accommodate either “open” or “closed” combustion chambers. Pistons with closed-chamber domes can be used with open chambers, but open-chamber pistons cannot be used with closed-chamber heads. Some aftermarket domed pistons fi t both closed- and open-chamber heads, but you have to follow the manufacturer’s recommendations. Even then you don’t know exactly what you have until you measure the components and do the math to accommodate the chamber size, dome volume, deck height, and gasket thickness. Of course fl attop pistons can be used with either chamber type, but it is diffi – cult to build adequate static compression, particularly with open-chamber heads, so you have to calculate it before purchasing pistons.
Hypereutectic pistons with press-fit pins are usually the best choice for basic high-performance street or occasional drag racing. They are stronger and more reliable than cast pistons in this application. They can be fitted with tighter clearances, making them quieter and more stable. They are only slightly more expensive. Higher-performance builds should use forged pistons. This includes turbocharging, supercharging, heavy nitrous use, or any normally aspirated combination with 10.5:1 or higher CR. These engines generate much higher temperatures and cylinder pressures and thus require tougher components.
Forged pistons expand more than the others and therefore require larger skirt clearances, but you can lean on them as hard as you want and they will take the punishment. If you’re performing a restoration you probably want to use the same type of piston as the original factory version, but for original cast pistons, it is certainly a good move to switch to a hypereutectic for added insurance. Cast and hypereutectic pistons generally call for .0007- to .002-inch skirt clearance while forged pistons typically call for .003- to .006-inch and even .008-inch or more on a supercharged or nitrous version.
The ring package should generally emulate the factory application unless you’re building a race engine. In most cases a standard pre-gapped moly ring set is best, but you can gain better sealing if you use .005-inch oversize rings that require individual file fitting to each cylinder bore. The general rule for gapping top rings in racing applications is to use .004-inch gap per inch of cylinder bore. For example, a 4.250-inch-bore 454 would require a minimum of .017-inch top ring gap. Street engine gaps should be larger, approximately .018 to .022 inch for the top ring, .012 to .014 inch for the second ring, and the same for the oil ring support rails.
Written by John Baechtel and Posted with Permission of CarTechBooks