The crankshaft, connecting rods, and pistons make up the reciprocating assembly of an engine. Although every engine component in an engine is important, the ones in the reciprocating assembly are critical because the reciprocating assembly is the biggest moving assembly in an engine. The components of the reciprocating assembly are under constant stress and must be properly selected and prepared for the performance target and application. In addition, for the 348 and 409 to perform at their best, the reciprocating assembly must be balanced and strong. Also, all the components must be compatible and suitable for the block and heads.
The crankshaft weighs almost as much as a W engine’s head, and it turns in excess of 6,000 rpm. Five main journals are found on a 348 or 409 crankshaft. When looking at a block from the bottom, the outer journals on the front and back of the block are larger as they can take advantage of the thicker outside walls of the block. The center three mains are not as wide, as they need to give clearance for the rod journals of the crankshaft. Make sure you use professional-grade micrometers when measuring the crankshaft journal diameter because these measurements must be accurate to obtain a reliable and well-running engine. For all W engines, the crankshaft journals stock measurement is 2.50 inches while the connecting rod journals meausrement is 2.20 inches.
This Tech Tip is From the Full Book “HOW TO REBUILD & MODIFY CHEVY 348/409 ENGINES“. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this post on Facebook / Twitter / Google+ or any automotive Forums or blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: http://www.chevydiy.com/348-409-cheat-sheet-crankshafts-rods-and-pistons/
The crankshaft has offset rod journals and large counterweights, and its main job is to convert linear motion into the motion of a rotating assembly. The W engine uses the same gallery oiling system as the small-block, and it has proven its worth in both engines.
With all that outward movement, the rotating assembly needs to be balanced and if it’s balanced within a few grams, it performs more efficiently, minimizes vibration, and produces better performance. You can internally or externally balance the crankshaft and the entire rotating assembly. An internally balanced engine uses a rotating assembly that is balanced without the use of any parts outside the oil pan. Externally balanced engines use the outside components of the harmonic balancer and even the torque converter of the transmission to balance the assembly. Race engine builders go to great lengths to balance every part within a few grams of one another, and then check the balance of the entire assembly when it is in place. It is generally accepted that a longer life and less fatigue come from an internal balance.
The flywheel is bolted directly to the crankshaft at the rear of the block. From there, the rotation of the crankshaft can be input to the trans- mission for vehicle movement. On the other end of the block, the front of the crankshaft is used to rotate a number of assemblies for use in other parts of the engine. Even before the crankshaft projects out of the block, its rotation is used to move the piston of the fuel pump and turn the timing chain that turns the camshaft. Once outside the block, the snout of the crankshaft has a pulley mounted to it to take advantage of the rotation to turn external engine assemblies such as the water pump, generator/ alternator, power steering pump, air conditioning compressor, or other accessories.
OEM crankshafts for production cars and trucks can be made in two ways: forged or cast. For many high- end racing applications, a billet-steel crankshaft is desirable for engines of more than 1,200 hp. All Chevy 348 and 409 crankshafts are forged steel and that includes the rare Z11 crank- shaft.
In the forging procedure, a foundry heats an alloy steel blank and a large die stamps the basic shape. From there, the pieces are machined to specifications and bearing surfaces are hardened. The stock forged crank- shafts are adequate for a stock rebuild or modified engine up to 500 hp. But if you’re building a max-performance engine with high-performance heads, high-lift cam, intake, and other high-performance parts, you need a strong foundation for the reciprocating assembly. An aftermarket forged crankshaft from Scat, Manley, Eagle, or another reputable aftermarket manufacturer is ideal.
Identification and Inspection
The biggest visual difference between 348 and 409 crankshafts is the design of the rear flange that attaches to the flywheel or flexplate. The 348 crankshafts have a round flange, and 409 crankshafts have a D-shaped flange. In addition, the 409 has bigger counterweights than the 348 because the 409 has a larger stroke so the counterweights extend farther out. And as a result, a 409 crankshaft cannot be used in a 348 block without machining for extra clearance.
The 409 crank with its bigger weights weighs 67 pounds, and this contributes to an 8-pound increase over the 348. All W engines have the same measurements for crankshaft mains and rod journals; the mains are 2.4977 inches and rod journals are 2.00 inches.
Inspecting a W crankshaft is critical when rebuilding an engine, and engine builders who don’t check the condition of the crank risk an engine failure. You should inspect the crank for cracks, chips, gouges, and heavy wear, but also for crankshaft runout. If the crank is out of alignment or bent, it must be trued. If you’re building your first engine, it’s a good idea to rely upon a qualified machinist to thoroughly inspect the crankshaft for you. However, if you’re going to do it yourself, you need to use a caliper micrometer to measure the crankshaft journal, which is 2.4980 to 2.4990 inches.
Crankshaft run-out is a critical dimension. It’s more than just a journal being out of round. Other critical factors play into this such as the size of the bearing and how much crush it has. If these factors are out as well, the measurement may be increased. Another factor is the level of the tools used in measuring the components. Professional engine builders have precision specialized tools for measuring engine parts. That’s because the dimensions of the block also affect the crankshaft’s movement. So while the crankshaft should not be out of round more than .004 inch, it’s equally important to know exactly how that measurement reacts with the surrounding components.
If the crankshaft run-out is greater than .004 inch, your machine shop must true the crankshaft. The machinist mounts the crankshaft to a hydraulic press, and the crank is literally bent back into spec. Once true and free from surface defects, look a little deeper with a Magnaflux test.
Shops use precision V blocks and dial indicators to check the straightness of the crankshaft. This is how they check for true on the main and rod journals. Machinists use magnifiers to reveal the surface condition of both main and rod journal faces. Likewise, they inspect and measure the width of those surfaces to ensure bearing spacing. When all the inspections have been done and the crankshaft deemed ready for use, a Magnaflux test should be done to determine if any cracks or defects not seen by the naked eye are present.
After checking straightness, inspect the surfaces of both the main and rod journals. They should be straight acrosss, without any deep scratches or gouges and smooth to the touch. Look at the wear patterns on these journals, especially at the edges where the color of the surface goes from shiny to dark. The stock rod journals measure 2.20 inches, so use either a micrometer to take that measurement.
The corresponding size of the material is laid on the centerline of the journal. Then, the cap is attached and fasteners torqued to the desired level. When the cap is removed, the material has been compressed by the torque and the width of the product after torquing corresponds with a gauge printed on the packaging of the material to show just what the clearance is. This determines the clearance so the engine builder can verify that these are within accept- able limits.
To best measure the clearance, a strip of the material should be long enough to cross the entire width of the journal, not just part of it. You measure a journal in steps all the way across the journal, looking for variances. Pay attention to the journal’s small area where the rods would attach.
Inspect the all-important key- way on the snout of the crankshaft. It should be free of gouges, not enlarged in any way, and have firm but not sharp edges on its opening. Inspect the rest of the snout for wear, scratches, and anything that could damage the harmonic balancer and the way it mounts to the snout. Finally, the threads inside the snout that are used for the balancer bolt should be clean and not damaged or the bolt is not able to be used.
The W engines share the same snout measurements as the small- block Chevy, and therefore the correct measurement for the 348 is 1.249 inches and the 409 measurement is 1.250 inches.
Once the crankshaft has been determined to be useable, it should be internally and externally balanced. For internal balancing, weight is either added or taken away from the crankshaft’s counterweights by a professional machinist. The machinist finds the “light spot,” drills a hole, and inserts a heavier material, such as Mallory, to add weight to that location.
If you choose to have the crank internally balanced, a professional machine shop must set it up on a balancing machine and perform the procedure.
External balancing is done with the harmonic balancer and/or fly- wheel. When those are used, the weights are positioned as they would be on the crankshaft and mimics weight added to the crankshaft.
An external balancer is one of best options for the enthusiast rebuilder. Mount the harmonic balancer and flywheel on the crankshaft before balancing.
Harmonic Dampeners Harmonic dampeners (or balancers) counteract the torque on the crankshaft that comes from cylinder firing. These also dampen the vibrations of the crankshaft’s constant rotation with rubber or even a liquid stored internally. After 50 years, an original OEM W engine dampener must be rebuilt, but if the housing is damaged, it should be replaced. For high-performance builds, a Fluidampr, ATI, or similar dampener is recommended.
Fluidampr makes a harmonic dampener for the 348 and 409 W engines. It features a laser-welded outer housing that holds silicone fluid surrounded by a nylon-coated inertia ring. These dampeners are a definite step up from the stock dampener and prevent dangerous frequencies from traveling through the engine and cracking vital components. These SFrated dampeners have timing marks every 2 degrees, making it easier to set timing.
ATI has a superior performance reputation on the street and track, and its durable dampeners are designed specifically for Chevrolet engines, including the W engine. The tunable and rebuildable Super Dampener is 6.235 inches OD, avail- able in aluminum or steel, and features a steel inertia weight, which has computer-machined grooves to maintain the proper durometer O-rings. This top-quality dampener has engraved 360 degree timing marks for easy reference, and it exceeds SFI 18.1 specifications.
When inspecting a balancer, the rubber between the two pieces is the first thing to check. The holes for pulleys need to be checked to ensure they are still properly threaded. The keyway should be free of cracks, gouges, and wear indicators. The hub itself should be checked for the same. Rebuilding a balancer is typically not a home garage job. The critical timing marks need to be in the original factory position or timing is affected. Making sure the balancer is correct is another cheap insurance aspect of engine building.
Today more than ever, the after- market is full of options for W engine rotating assemblies. From stock sizes for replacing OEM parts to new configurations of crankshafts, connecting rods, and pistons, if a builder can’t find what is needed for a project, it likely does not exist.
Aftermarket crankshafts and harmonic balancers are readily available for both restoration and performance builds. Typically, both the crankshaft and balancer should be selected for a specific type of build. These cranks use lighter, yet stronger materials, counterweights with knife edges to throw off oil, reduced windage, reduced rotating mass that rotates with better aerodynamics, and hardness finishes tough enough to stand up to racing race after race, and more.
The aftermarket offers a variety of standard-stroke and Chevy big- block crankshafts that can be used for the 348 and, particularly, the 409. Eagle, Scat, Callies, and others offer forged steel 348 and 409 crankshafts, so you don’t have to rely on an OEM crankshaft.
Eagle produces a 4.00-inch 4340 forged steel crankshaft that has a .125-inch radius on rod and main journals for increased strength, but it’s also shot peened and nitrided for exceptional durability.
Scat offers a number of big-block Chevy cranks in different strokes that are suitable for the W engines. The forged 4340 standard-weight crankshaft is suitable for street or race engines, so in certain all-out builds (and with the right parts), horsepower ratings of 1,000 can be achieved. Features include straight- shot oiling holes, lightening holes for all rod throws, pendulum undercut counterweights, large fillet radii, and chamfered oil holes.
Callies Compstar offers other crankshaft options for the 409, and these are Chevy big-block crankshafts that fit the 409 block. The Compstar crank is 4340 forged steel similar to the Eagle crank. And it shares many features with the Eagle unit. It’s nitrided, has drilled rod journals, and the counterweights are profiled for piston clearance. Since these were developed for the big-block Chevy, you can select this crank with a 4.250-inch stroke and a pin size of 1.771 inches for the 409.
One of the best examples of after- market advancements is the proliferation of crankshafts available for building a W stroker engine. In the past, machining work had to be done to the blocks to accept a stroker crank- shaft, but many of today’s products are virtual dropins. This is an example of the rapid technology advances in 348 and 409 parts (see Chapter 9 for more information on strokers).
The forged connecting rods for the W were adequate for a stock or slightly modified engine, but when the 409 was released and engine builders increased horsepower, the connecting rods often failed. For that reason, high-performance 348 and 409 W engines demand a stronger- than-stock connecting rod. Thank- fully, there’s an abundance of billet steel and forged connecting rods that are suitable for almost every kind of high-performance engine build.
For 348 engines, the con rods measure 6.135 inches long, and 409 rods are 6.00 inches long. They both share the same basic construction in their makeup, and the 409 rods are shorter yet weigh more. The bulk of that weight difference is centered along the most important part of a rod, the main beam between the two holes, and the 2½-ounce-heavier 409 rods definitely are stronger.
For W engines as well as most production engines, the pistons are press fit onto the wrist pins and connecting rods. Racing and high-performance engines use floating wrist pins held in place by keepers or clips to keep the movement of the piston free of drag and minimize resistance that would rob horsepower.
Like the crankshaft, the rod journals utilize bearings to take up the wear. The connecting rod uses two- piece bearings placed into the opening when the rod cap is removed. These bearings are oiled via a small hole or groove in the bearing and connecting rod that receives oil from the holes in the crankshaft. Because of the two directions of movement, rods wear out by developing an egg shape to both of the holes in the rods. Another area of wear is more of a stress nature and that is the rod bolts. The bolts may not wear like the rods but they do stretch and that can prevent them from holding to the journals effectively.
The wrist-pin end of the connecting rod gets its oil from the cylinder walls. As oil is sprayed or splashed onto the cylinder wall, the piston rings scrape it off and return it to the bottom of the engine. Some of that oil is worked into the piston rings and through small holes in the ring grooves or lands, so it works its way into the area of the wrist pin.
Because of the extreme load connecting rods work with, there are a number of operations to reduce their stress or strengthen them for use. These procedures are explained in the machining part of this chapter. Those building 348/409 engines should opt for a new set of rods. The strength of an original set of rods has been compromised because over 50 years, they have been heat cycled countless times that they have lost their integrity. Also, unless the owner can be absolutely sure of the miles run on the engine, there is no telling what the true condition of OEM rods can be when reused. If there is an Achilles Heel of an OEM engine, it might very well be the connecting rods. If you want to reuse the existing rods, you should Magnaflux them because a tiny crack can develop into a bigger one and break a rod, but at engine speeds of 6,000 to 7,000 rpm, the crank, heads, and other vital components can be turned into junk.
Chevrolet used forged steel to make the connecting rods for the 348 and 409s. The 348 rods are 1/2 inch longer than 265 small-block rods yet only weigh about an ounce more. The 348 rods are longer than 409 rods and may look the same but are not. The 409 rods are beefier to with- stand the forces of the longer stroke. Extra metal was used on the area of the crankshaft end of the rod for more strength. These rods had no problems supporting both the 348 and 409 pistons in stock configurations. But stock rods, in most cases, are not suit- able for a high-performance or race build. Therefore, if you’re building a high-performance W engine with a high-lift cam and aftermarket heads, you need to step up to aftermarket forged or billet steel connecting rods to support more than 500 hp. In most cases, the stock rods can sup- port up to 500 hp.
Connecting Rod Bolts
Rod bolts are subjected to more stress than any other fastener in the engine, and therefore, the proper selection and the torqueing of the bolts is essential. A stock rod bolt is adequate for a strictly stock rebuild, but a top-quality rod bolt is an extra insurance policy against a rod failure. ARP makes a top-quality bolt kit for stock and aftermarket 348 and 409 connecting rods, and these bolts are adequate for stock and highly modified builds. ARP is widely regarded as building the best rod bolts for both street and track usage. A set of ARP Hi-Performance Connecting Rod Bolts for a Chevy 409 are typically 190,000 psi and made from a premium grade 8740 alloy chromemoly steel. The heat-treated bolts yield a tensile strength in the 200,000 psi range. These specs roughly calculate to be five times more reliable than stock bolts. A common upgrade is to go from the OEM size of 3/8 to 7/16 inch with aftermarket rods.
The bolts stretch like any other bolt when tightened. This is easily confirmed by a tool especially made to measure rod bolts for stretching. As this is a common known fact, many different brands are available for use on 348/409 engines. Those using stock or stock size rods are limited to the size that fits those rods. Usually, they are the same 3/8-inch rod bolts used by Chevrolet.
Aftermarket Connecting Rods
The stock Chevrolet connecting rods were a weakness of the original engine, and if you’re building a mildly modified up to a race engine, you need to upgrade the connect- ing rods to handle with additional stresses encountered. If you do not opt for stronger rods, you risk a catastrophic engine failure, and you don’t want to put your investment in your engine at risk.
Aftermarket forged connecting rods are made of 4340 steel and often have a higher amount of nickel and chrome. Eagle offers forged I- and H-beam connecting rods for the 348 that provide the necessary rod strength for modified W engines. These H-beam rods come with ARP fasteners and weigh 780 grams a piece. The I-beam rods are suited for both stock and high-performance rebuilds. These are made of 5140 steel and use 3/8-inch rod bolts. In addition, you can fit big-block Chevy forged I-beam or H-beam rods to the 348 or 409 crankshaft, and these are available from almost all major connecting rod manufacturers. So, select the design, material, and service life of the rod you need for a particular horsepower target and application. When sizing, the big-block Chevy rods, make sure the center-to-center distance is correct, and that small and big ends are the correct size. Aluminum as a connecting rod metal is widely accepted in racing and for high-performance applications. A serious race engine is often fitted with billet aluminum rods to shave weight and provide the necessary strength on any movement of the piston, but aluminum rods do not have a long service life and are not recommended for a high- performance street engine. For street engines, forged steel rod is the way to go. Although most companies don’t reveal the type of aluminum used for rods, it is generally in the 7075 T-6 family. All types of racing connecting rods are machined on CNC machines for accuracy and production of horsepower.
Buying aftermarket rods depends on the type of build desired. Forged rods cost less than billet rods, and generally billet aluminum rods cost the most as they have to have sleeves for the bearing surfaces and threads. And while it is believed aluminum rods have a shorter service life, their manufacturers don’t always agree with that.
There are many brands of connecting rods for the W engine and, depending on the use intended, tailoring the type of rod to the build is easily accomplished. The Eagle H-beam forged connecting rods mentioned earlier retail for $345 for a set.
If nothing other than the mentioned disastrous scenario, all the connecting rods should be balanced, not only against each other, but with the other components of the rotating assembly. With balanced rods, they can transmit the power they are helping to make to the driveline where it belongs.
The piston’s design of course complements the needs of the engine and works with the design of the engine. W pistons, as described ear- lier, have a number of unusual design characteristics and are not symmetrically balanced. With the combustion chamber located in the block, the upper part of the pistons is made in a “gable” or reverse V-shape for a number of reasons. One 16-degree side of the gable provides the squish area of the piston flatly against the head while the other, combined with the cylinder bore, creates the bulk of the combustion chamber. Domed pistons are prevalent in high-performance engines, and as most know, the Hemi is equipped with domed pistons. But those same engines don’t usually have the other design features seen on a W engine. As such, W engine pistons are their own creation and that goes all the way to high-performance Ws, too.
The 348, the first generation of the W engine, used pistons that all had the same design on both sides of the gable shape. As the engine was further developed, and increased compression and power became more of a goal, those pistons underwent design changes that first made them usable in only certain sequences of cylinders and then later, directional in nature as to not impact the valves as compression ratios were increased and valve-to-piston clearances decreased. These evolutionary guidelines still hold true today when outfitting a W engine. Cylinders number-1, -4, -5, and -8 have a completely diffeent valve orientation than cylinders number-2, -3, -6, and -7.
Because of the unusual design of the combustion chamber, the resulting shape of the piston and the size of the bigger bore, W pis- tons were made bigger with more mass and that equals more weight. The inverted V-shape of the pistons required more metal to withstand the pressures exerted both up and down in the bore. The design puts more metal into that V to give it the strength it needs but also creates more mass. And even though it is an aluminum piston and lighter than a steel one would be, W engine pistons are heavier by design. The stock 348 cast pistons at 9.5:1 tip the scales at 787 grams, but modern, forged JE pistons weigh 814 grams. Although this is a nominal increase in weight, the JE pistons deliver the strength to support 500 hp or more.
It’s not enough to aim for using the lightest pistons. The shape of a pis-ton is just as important as the weight. As temperatures rise in the combustion chamber, the piston expands and that means it can cause more friction or drag on the cylinder walls. In the past, pistons were configured in a straight profile. Today, there is a slight taper incorporated into their shape, with a reduction in the diameter of the piston just under the rings.
About half of the piston crown is flat while the other half is domed. As a result, the raised dome half of the piston is heavier than the flat- top half. Therefore, in order to balance the heavier domed half, many engine builders mill down the inside of the piston dome from under- neath to remove weight and achieve the optimal balance. However, the pistons don’t have to be milled for balancing unless you’re building an extreme high-performance or race engine. Some are concerned about the uneven distribution of mass in the piston, and that this creates a durability concern, but it’s only a concern if engine speed (RPM) exceeds 8,000.
The stock engines ran 9.5:1 to 11:1 compression ratios, but today a variety of aftermarket pistons are available with compression ratios from 10:1 to 13:1. Beyond the unconventional piston design, the stock cast piston needed a fair amount of material to withstand the cylinder pressure. To accommodate this, Chevrolet had to use heavy pistons and the heavy pistons strained the valvetrain on the W, so if the 409 or 348 revved more than 6,100 rpm, it often didn’t hold together.
If you’re building a naturally aspirated high-performance or race engine, you need to select a piston that’s compatible with the high-performance head, achieves the correct squish clearance, and is the compression ratio for the performance target. For race and high performance, a 11:1 to 13:1 compression ratio is preferable. On the other hand, if you install a supercharger, turbo, or nitrous, you must lower compression to reduce cylinder pressure, and therefore you need a 9:1 or less compression ratio.
Compression height is the measurement between the top of the piston and centerline of the wrist pin bore in the piston. You need to determine this spec before selecting a compression ratio. To determine the compression ratio desired, three other measurements are needed– block height, the length of the connecting rod, and the stroke of the crankshaft. Deck height is measured from the centerline of the crank- shaft to the block’s deck. Connecting rod length is measured between the centers of the two ends of the rod. Thus, the center-to-center distance between the crankshaft’s main journals and the connecting rod journals determines the stroke of a crankshaft. Taking that measurement and doubling it gives the stroke or the distance the piston moves in the cylinder.
But it’s not all about raising the dome. You need to ensure clearance for the valves and that they do not impact the higher dome. That’s where valve reliefs are cut out of the dome and the piston top ends up looking like a mirror mold, matching the face of the head.
Selecting a compression ratio comes down to the application of the engine and how it will be used. If it’s a straight stock rebuild, the 348 had 9.5:1, and the 1961 model 409 had 11.25:1, a particular ratio that was one of the highest used on a street vehicle. Most owners don’t want to run a compression ratio above 11.5:1 because you need a high-octane race gas. In addition, the chosen compression ratio must be compatible with the induction system.
Building one with a turbocharger or blower dictates a lower compression even if it is a race engine. The compression ratio needs to be part of your engine build plan. Other factors are the octane of the fuel as well as the operating conditions. A hardcore race car faces more harsh conditions than a weekend cruiser and adjustments need to be made. Calculating compression ratio is actually easy. It’s the ratio of the difference between the two measurements at both top dead center (TDC) and bottom dead center (BDC) of a piston in a cylinder bore.
Calculating Effective Dome Volume
The dome volume is the part of the piston that resides above the deck, and this includes the dome. Any valve notches have to be factored in for an accurate measurement. Pis- tons with domes reduce the volume of the clearance, and flat-top pistons add to that clearance. Here’s the formula for calculating dome volume:
Dome Volume (ci) = (A + B + C) – D ÷ E-1
Where: A = head chamber volume B = gasket volume C = deck to piston volume D = cylinder displacement E = desired compression ratio
Here’s the formula for calculating head gasket volume:
Head Gasket Volume (ci) = A2 x B x C
Where: A = bore size B = .7854 (standard) C = gasket thickness
Here’s the formula for calculating deck clearance volume:
Deck Clearance Volume (ci) = A2 x B x C
Where: A = bore size B = .7854 (standard) C = gasket thickness
Place the cover plate so that it completely covers the combustion chamber. The only criteria is that it be flat, clear, have one hole for the burette, and several smaller holes to allow the air to escape when the chamber is filled with liquid.
Calculating Swept Volume
The swept volume of a cylinder is the amount of air displaced by the piston as it moves from BDC to TDC, evacuating the cylinder. Any change in this measurement affects compression.
Here’s the formula for calculating swept volume:
Swept Volume (ci) = A x B2 x C
Where: A = stroke B = bore C = .7854 (standard)
Increasing the compression ratio increases power throughout the entire rev range and can increase fuel mileage. But care must be taken to ensure any and all components are capable of working at the higher rate. Often in most engines, a compression ratio about 11:1 requires high- octane gas and that’s gas with 91 to 94 octane, depending on location. At some point, raising the compression requires a higher octane fuel be used. This is seen in hardcore racing engines that cannot function properly without the use of higher octane racing fuels. A compression ratio of 14:1 requires such fuels.
Wrist pins are retained by two methods—pressed in pins or floating pins with locks. The most common method is to press the pin into the piston such as the OEMs do.
Pressed-in pins require a press to install the pin after heating the small end of the connecting rod. Full floating requires some kind of retainers on the ends of the pin to prevent travel. In both methods, the pin is inserted into the smaller opening of the connecting rod.
Pins that float are usually used in high-performance engines and are retained by one of three different devices. Spiral locks, round wire locks, or snap rings are used and fit into a groove machined into the pin. Sometimes, two retainers are used and a good rule of thumb is to never use a retainer twice. The tension of what is essentially a spring may be affected, as well as the shape of the retainer.
Piston rings for both street and performance engines do more than seal the combustion chamber. Rings control the movement of oil on the cylinder walls and, if matched perfectly to the build, can add strength and maintain horsepower. Selecting the proper rings includes factors such as compression ratio, fuel to be used, horsepower, and even the rev range the engine operates at. Most stock engines use moly faced rings and can take advantage of oversized rings for worn cylinders. Racing engines lean toward coated rings. The metal used can be steel or iron and can be coated with molybdenum, nitrate, or hard chrome. The top ring experiences the most heat and temperatures come close to 600 degrees F. The second ring sees much less heat, often half as much. The second compression ring helps in sealing the combustion chamber and helps the third ring which is the primary ring for keeping oil out of the burn area.
Many of the aftermarket forged pistons use a 1/16-, 1/16-, and 3/16- inch ring configuration. The JE Pro seal rings are a popular choice and deliver far better performance than stock iron rings. The top ring is ductile iron. It has a barrel-faced design and features plasma-moly inlay on the face to reduce friction and provides excellent sealing. The reverse torsional second rings have a phosphate coating and a tapered face. You have the option of selecting low or standard-tension oil control ring.
Total Seal piston rings are ideal for racing-type W engines. The diamond finish minimizes friction and delivers exceptional cylinder pressures through ring sealing. These rings are used in about every major automotive racing series.
It’s essential to get the correct piston ring gap to maintain sealing pressure. A good way to check gap is to use a torque plate much like when boring or honing the cylinders. That way, the bore is correct as it would be under a torque load of the head. Insert the ring about 1 inch into the cylinder, using a pis- ton to align the ring square to the bore. Use a feeler gauge to check the gap to the manufacturer’s specs. If required, the ring gap can be increased by filing the ends of the ring. For most carbureted, stock, or light-performance builds, the top ring end gap should calculate to .004 inch per inch of bore diameter. This may vary with the horsepower of the engine. The build uses the same end gap for the second ring as the top ring. For most oil rings, the recommended ring end gap is usu- ally .015 inch.
Generally, the more built up an engine is, the more gap it needs. One example is the difference between a street/strip engine and one equipped with a blower for racing. That difference would be .0020 inch. Installing rings is important enough to have its own tool. A ring expander should be used to fit the rings onto a piston to reduce the chance of bending or twisting the ring.
Aftermarket Piston Options
Chevrolet built the 348 and 409 W engines using cast-aluminum pistons. As mentioned, these were relatively heavy pistons to accommodate the unconventional combustion chamber design, and today’s forged pistons are far stronger than the OEM cast pistons. The heavy OEM pistons carried more mass, so if RPM exceed 6,200, the valvetrain couldn’t cope with it and the valves would float. Often, the valvetrain couldn’t tolerate the additional revs and failure was common. There- fore, stock cast pistons are not the best option even when rebuilding a stock engine.
Forged pistons are impact extrusions and these are the strongest pistons available other than billet aluminum pistons. Forged pistons are heavier than cast as the metal is compacted, yet they are manufactured more easily. With the cost difference between cast and forged pistons being nominal, it is best to opt for the higher strength forged pistons. If you’re building a high- performance or race engine, forged pistons from Ross, JE, or Keith Black are the most suitable option.
A number of manufacturers offer forged pistons for the 348 and 409 for both street and race applications. Keith Black forged pistons use a V-rib design because the W needs increased support for high-RPM operation. These pis- tons feature a high-strength T-6 heat-treated 2618 primary aluminum alloy. In addition, the pistons incorporate a machined skirt, head, gas accumulator, and ring grooves as well as a pressure-drilled pin- hole oiling.
Ross also offers forged pistons, and these are said to have a design that improves the intake signal during valve overlap. Unique to the pis- ton, the intake valve relief is cut at an angle to the exhaust valve. The distinct crown design features a modified dome and rounded outer bevel. One piston model is suitable for 10 or 10.5:1 compression ratio, and that can be used with cams up to .600 inch of cam duration. Another model is suitable for 11 or 11.5:1 ratio and can withstand up to .700-inch lift. These pistons come with full floating wrist pins. Even with aluminum as the piston base material, there are different pistons by how they are manufactured.
The three types are cast, hyper- eutectic and forged. Cast pistons are often used for stock rebuild. Although they can sometimes be a little more brittle, they keep their shape over long miles. They are also known for larger quantities and are not seen in performance builds. Cast pistons are lighter than other types such as forged, and that factors into better mileage. The W-engine uses heavy gabled-roof pis- tons, and because these pistons are under more stress in the combustion process than a conventional head- based combustion chamber, engine builders do not opt for hypereutectic piston when building a high- performance engine. Instead, builders select the appropriate profile forged pistons that withstand much greater stress and horsepower levels than a hypereutectic piston.
The three components of a W engine’s rotating assembly, the crankshaft, the connecting rods and the pistons all typically require machining during a rebuild of a high-mileage or worn engine. The goals of these operations are the same: ensure the most strength and longevity of the piece for the best possible performance and durability. Using the following machining methods has proven to accomplish such goals. A common procedure early in the engine-building process for crankshafts and connecting rods is Magnafluxing where cracks and imperfections not visible to the naked eye are found.
On crankshafts and rods, stress relieving is an important procedure that helps increase component longevity. Using a rotary tool and a sanding pad, lightly grind or sand of all the straight edges of crankshafts and connecting rods into a radius. This makes them stronger and less susceptible to cracking. Use a grinder/sander to get rid of any casting flash or extra metal. Grinding down seams and creating a smooth radius strengthens any part. After stress relieving has been done, shot peening finishes the rough metal or non-machined surfaces. This is where the part is masked off to pre- vent peening of machined areas and then shot peened with metal media much like sand blasting. The peening reduces the surface tension and stress of the part, making it stronger.
After this, the crankshaft and connecting rods go their separate ways for further machining operations. The crankshaft can have the oil holes in the journals opened up by countersinking the edges of the holes. As the hole is on a round surface, only about half the hole is countersunk or opened up to improve oil flow. Much like stress relieving, taking the edges off the hole also makes the journal stronger.
Sometimes a crankshaft is dam- aged or worn on the journals. A bearing can turn or spin damaging the face of the journal and often disabling the engine. The alternative to buying a new crankshaft is to resurface or “turn down” the journals and use compensating bearings. As little as .100 inch is all that is needed to reuse the crankshaft. After turning down, the part is sometimes polished, or replated with a hard finish. Specifications would then describe the parts as .010 inch under.
At the machine shop, connecting rods are reconditioned and returned to their true dimensions and roundness, but W-engine connecting rods are placed under enormous stress. Sometimes stock connecting rods failed if the motor was revved up to 5,500 rpm or if the engine was modified. If a W-engine connecting rod has had a long service life, it’s a wise idea to replace it with a connecting rod compatible for a particular build. When rods are moved up and down hundreds of times a minute, they tend to concentrate that force on the centers of the holes. After years of service, those holes end up out of round or egg shaped. Reconditioning rods starts with a very good cleaning to remove all oil, scale, and other impurities from the surface of the rods and bearing areas. The next step is to Magnaflux the rods for cracks or other damage. If the rods pass that
test, they are checked for straightness. A visual inspection follows look- ing mostly for discoloration that can indicate overheating. If the rod has a blue cast to it, there is a very good chance it has been overheated. The result would be the metal hardness of the rod may have been affected, thus losing some of its much needed structural strength.
The rod bolts are also checked for excessive stretching. To do this, the rods are assembled with the bolts torqued to their proper specifications. Then the bolts are checked for length. If the bolts are out of specification, replacement is the answer. With the new or passing rod bolts torqued correctly, inspect and measure the bigger of the two rod bores. If they are out of round, the correcting procedure is to mill down the flats on the caps and resize the bore by honing it. Much like align- honing in a block, this procedure takes off less material and maintains more control of the operation. If the smaller-sized bore of the rod needs to be resized, a bushing is used. Clearances of the wrist pin are critical in this operation. Also critical is the distance from center to center of the rod. It is also understood that rod end caps should always be used with the original rod and not swapped with other rods. One trick is to use a number stamp on both sides of the connecting rod and its cap so the numbers face one another. This indicates which rod they are, as well as the correct way to mount the rod cap. The number chosen should reflect to the number of the cylinder it came from. If number stamps are not available, using a center punch and stamping in the number in dots also works.
The rotating assembly of any engine is very critical to its overall operation. Time spent on ensuring the right parts and, just as importantly, how they work together is an investment in a long and strong running W engine.
Written by John Carollo and Posted with Permission of CarTechBooks