An engine’s rotating assembly obviously includes the crankshaft, connecting rods, pistons and pins, piston rings, rod bearings and main bearings (as well as damper and flywheel/flexplate in the case of externally-balanced setups). While many engine builders prefer to design their engines using specific components (brands and versions of components with which they’ve had previous success, etc.), the option of purchasing a rotating assembly “kit” offers a time savings, as well as potential cost savings.
A pre-packaged rotating assembly (available from a variety of aftermarket crank and rod manufacturers) provides a level of convenience by offering a system that has been pre-determined to suit specific goals, in terms of engine displacement and compression ratio. Want to build a 347 stroker Ford starting with a 302 block? Naturally, you’ll need a 3.400” stroker crank and 5.400” rods, along with the appropriate piston CD to accommodate your block deck height requirement. A pre-designed system will include the necessary crank stroke, bore-diameter-appropriate pistons, required rod length, as well as suitable rings for the included pistons, and main & rod bearings for the application (simply specify stock or suitable undersize). As far as compression is concerned, the rotating assembly supplier will provide a reference chart which easily allows you to match-up the pistons to the head that you plan to use in order to obtain your desired squeeze ratio. If you’re not picky about brand of pistons, rings and bearings, you’re able to obtain the assembly that you want by ordering a single part number, as opposed to flipping through catalog pages or perusing numerous websites.
PROS AND CONS
Pros: A rotating assembly “kit” provides a matched system without the need to purchase individual parts. This saves both time and money.
Cons: If a builder prefers to create his own system by cherry-picking specific brands/models of components, and/or prefers to use his own design ideas (re: rod length, piston CD, ring spacing, bearing clearances, piston dome design, etc.), buying a complete pre-packaged system simply doesn’t make sense.
POPULAR STROKER COMBINATIONS
While a pro engine builder (especially one who builds competition engines) may likely opt to piece his own system together, the ready-availability of pre-packaged rotating assembly combinations make it easy to obtain the customer’s desired displacement and compression package by simply ordering a single part number. Especially in the area of stroker builds, the aftermarket performance crankshaft makers make this easy.
While a wide variety of combos can be had (either by purchasing individual components or by ordering a pre-coordinated rotating assembly), following is a sample list of available stroker packages, which will include the crankshaft, connecting rods, pistons and pins, rings, rod bearings and main bearings. This list is courtesy of Ohio Crankshaft, but similar packages are available from most crankshaft manufacturers. Note that additional stroker/displacement combos not listed here are also available from other suppliers)
- SMALL-BLOCK CHEVY: 434, 421, 377, 383, 408, 434, 428, 421, 447
- BIG-BLOCK CHEVY: 509-540, 555-565, 496, 605, 620, 572, 582, 588, 598, 632, 698, 706
- GM LS: 403 – 416
- SMALL-BLOCK FORD: 302 – 347, 351w – 408, 302 – 331
- BIG-BLOCK FORD: 514, 533, 545, 557
- PONTIAC (400/455): 463, 468, 501
- MOPAR: 360 – 428, 360 – 408, 440 – 500, 440 – 511, 440 – 493, 440 – 541, 440 – 572, 426 – 472
Crankshaft makers who offer complete rotating assemblies naturally feature their crankshaft in a rotating assembly kit. If that maker also manufactures connecting rods, naturally their rods will be included (choices in cranks and rods are common, between cast and forged cranks, forged or billet I-beam or H-beam rods, etc.). As part of the “kit,” the crank maker will include pistons, rings and bearings of various brands (they may offer choices of brands). For example, pistons of choice might include Mahle, JE, SRP, KB, Wiseco, Arias, Carrillo, Diamond, CP, Ross, Probe, etc. Bearings might be from Mahle Clevite, Federal Mogul, King, etc., while rings may be sourced from Mahle Clevite, Total Seal, PBM, Federal Mogul, Hastings, etc. many assembly suppliers offer a choice of specific component brands. The use of specific pistons, rings and bearings might be based on either performance or pricing factors, depending on the level or intended application. In any case, the quality aftermarket crank makers take advantage of the highest quality in terms of completing their rotating assemblies. In other words, if you stick with the leading suppliers of rotating assemblies, you’re not gonna get stuck with junk pieces.
PRICE COMPARISON EXAMPLES
What’s the approximate cost difference, if any, between buying a rotating assembly kit as opposed to piecing-together a system using the same/similar components? We wondered that as well, so I took a look at one of the big box parts discounters websites (using their pricing merely as a representative example). Merely for the sake of illustration, I chose three popular systems — one for the 383 stroker Chevy small-block, one for the 347 stroker Ford small-block and one for the 408 SBC. Again, just for the sake of providing examples, I referred to one of Eagle’s rotating assembly for the Chevy, one of Scat’s rotating assembly for the Ford and one of Callies assemblies for the 408. Note that the first two examples include packages that feature cast cranks. Both Scat and Eagle, for example, offer cast and forged cranks, as well as hyper and forged pistons, with prices varying depending on selected components.
SMALL-BLOCK CHEVY 383
An Eagle rotating assembly for a small-block Chevy 383 was offered at $838.95. This kit included a cast 3.75” stroke crank, 4.030” hypereutectic pistons, Eagle I-beam 5.700” rods, plasma moly rings, main and rod bearings, and a damper and flexplate. The brand of pistons, rings, bearings, damper and flexplate wasn’t listed, so I made my best guesses with regards to those components. Piecing the kit together, purchasing all items individually, I came up with a total of $996.95, a savings of about $158.00.
Big box parts discounter prices per item:
- Eagle cast crankshaft $165.95
- Eagle I-beam forged rods 228.95
- Speed Pro hyper 4.030” pistons 127.60
- Speed Pro rings 38.95
- Clevite H-series main bearings 89.95
- Clevite H-series rod bearings 71.60
- TCI flexplate 93.95
- Damper 180.00
Rotating assembly price $838.95
Savings by buying kit $158.00*
SMALL-BLOCK FORD (302 STROKED TO 347)
One big box parts discounter offers a Scat rotating assembly for a 347 Ford that includes a Scat cast 3.400” stroke, Scat forged 5.400” rods, 4.030” flat-top forged pistons, plasmamoly rings, main bearings and rod bearings. The package sells for $872.95. Without knowing the brands of pistons, rings and bearings, I approximated using Speed Pro forged pistons, Speed Pro rings and Clevite H-Series bearings. When I added individual pieces together (again, citing one of the big box parts disounter prices as reference), I obtained a total of $1,128.00. Using these examples, that represents a savings of about $255.05 when buying the kit as opposed to buying parts separately.
The breakdown is as follows:
- Scat cast crankshaft (3.400” stroke) $242.95
- Scat forged rods (5.400”) 257.95
- Speed Pro forged flat-top pistons (4.030”) 319.60
- Speed Pro plasmamoly rings 111.95
- Clevite H-Series main bearings 99.95
- Clevite H-Series rod bearings 95.60
Rotating assembly price $872.95
Savings by buying kit $255.05*
One big box parts discounter lists a Callies rotating assembly for a 408 Chevy at $2,460.00, which includes a Callies forged crank, Callies forged Compstar H-beam rods, forged pistons, plasmamoly rings and main & rod bearings. Note that, according to Callies’ website, each rotating assembly also includes precision balancing, so we can also factor in that cost as well.
- Callies forged crank (4.000” stroke) $931.00
- Callies Compstar rods 662.00
- JE forged pistons (4.030”) 819.00
- Total Seal rings 115.95
- Clevite main bearings 178.00
- Clevite rod bearings 150.00
- Estimated balancing cost value 200.00
Rotating assembly price $2,460.00
Savings by buying kit $595.95
* Disclaimer: Granted, without knowing exactly which brands and part numbers of rings, pistons, bearings, damper and flexplate that the rotating assemblies include in the above examples, the prices of individual parts and the resulting savings differentials may not be accurate. However, the sample comparisons still make it clear that purchasing a pre-packaged rotating assembly should represent some degree of savings as compared to piecing the system together.
COMPRESSION RATIO CALCULATOR
Rather than take space here to list a host of formulas, a very handy and easy to use calculator is available on Diamond Racing’s website. Simply plug in your numbers (deck height, stroke, bore diameter, gasket thickness, rod length, piston volume, chamber volume, etc.), and this calculator does the work for you in a heartbeat. This is a very handy reference to help you to make decisions relative to piston volume, gasket thickness, piston CD, etc., to help you to tune-in your desired compression ratio. Yes, some may view this as a form of cheating (it does the math for you), but it’s darned handy. Just go to www.diamondracing.net/tools. Once you start to play with this calculator, you’ll be hooked.
ROD RATIO… SOMETHING TO CONSIDER
Connecting rod ratio refers to the length of the rod in relation to crankshaft stroke. Rod ratio is determined by simply dividing rod length by crankshaft stroke. For example, a rod length of 6.700” mated to a crankshaft stroke of 4.500” will result in a rod ratio of 1.488:1 (6.700 divided by 4.500).
Rod ratio directly affects piston side-loading due to the maximum operating angle of the rod. As rod ratio is lowered (moving to a shorter rod with a given stroke, for example), the angle of the rod relative to the cylinder bore increases, which results in potentially increased piston skirt wear, increased friction (skirt and rings to cylinder wall) and resulting increase in heat. Basically, as rod ratio decreases, the rod (at its maximum angle) is trying to push the piston through the cylinder wall (“side loading”) in addition to moving the piston up/down. Obviously, if side-loading is excessive, this not only poses a threat to component longevity but also results in a horsepower loss due to the increase in frictional drag (as well as a potential change in bore geometry due to both frictional drag and elevated heat, which can affect ring sealing). Due to modern boring/honing technology (where cylinder bores are machined with greater concentricity) and modern piston design (stronger designs along with friction-reducing skirt coatings) reduces the potential for increased wear and drag, but we still need to consider rod ratio in order to maximize durability and power for a given engine application.
Side-loading increases as the rod ratio decreases, and the effect of side-loading decreases as rod ratio increases. Rod ratio should be considered when choosing rod length and stroke combinations.
According to Eagle, a rule of thumb is to stay above 1.45 or so for a street engine with modern pistons and boring technology. It is generally accepted that past 1.72:1 you won’t realize any significant gains (diminishing returns). The improvement from 1.40 to 1.50 is significant, but going from 1.75 to 1.85 won’t have much affect.
Following are a few examples of rod ratio in specific setups (courtesy Eagle):
- Ford 302 (5.090” rod / 3.000” stroke) = 1.70 rod ratio
- Ford 351W (5.956” rod / 3.500” stroke) = 1.70 rod ratio
- Ford 460 (6.605” rod / 3.850” stroke) = 1.72 rod ratio
- Chevy 350 (5.700” rod / 3.480” stroke) = 1.64 rod ratio
- Chevy 400 (5.565” rod / 3.750” stroke) = 1.48 rod ratio
- Chevy 454 (6.135” rod / 4.000” stroke) = 1.53 rod ratio
- Chrysler 440 (6.760” rod / 3.750” stroke) = 1.81 rod ratio
DIMENSIONAL FACTORS TO CONSIDER
When choosing a complete rotating assembly, naturally one of the critical elements deals with dimensions in terms of making the reciprocating movement of the assembly fit the block in question. A nice feature of a pre-designed rotating assembly is that the supplier has already determined which stroke variations will fit a particular OE-design block (yes, some clearancing may be needed, but it’s do-able).
• Block deck height: Distance from the main bore centerline to the cylinder head deck surface.
Note: We need to consider the finished block deck height, not a theoretical OEM or otherwise assumed deck height.
• Crankshaft stroke: The published stroke distance represents the crank’s total stroke (distance from the rod journal centerline at bottom-dead-center (BDC) to the rod journal centerline at top-dead-center (TDC)). For purposes of configuring connecting rod length and piston compression distance, you’ll only factor-in one-half of the published crank stroke….the distance from crank centerline to rod journal centerline with the rod pin at the top of its stroke (TDC).
• Connecting rod length: Distance from the rod’s big-end bore centerline to the small-end bore centerline.
• Piston compression distance (CD): This refers to the distance from the wrist pin bore centerline to the “flat” of the piston dome/deck.
Example: In order to achieve a “zero deck” (where the piston deck meets and is flush with the block’s deck), we need to determine crank stroke, rod length and piston CD dimensions that will add-up to equal the block’s deck height with the piston at TDC.
½-crank stroke + rod length + piston CD = block deck height.
For example, if we’re dealing with a block deck height of 10.210”, and assuming we want zero-deck, the distance from the centerline of the crank’s rod journal (when at TDC), the rod length and piston CD must equal the block deck height. In this example, using a crank stroke of 4.500”, a rod length of 6.700” and a piston CD of 1.260” will result in a zero deck. Note that one-half of the crank stroke is 2.250”.
½-Stroke 2.250 + rod length 6.700 + piston CD 1.260 = 10.210”
Let’s say that we prefer that the piston at TDC is located 0.015” below deck. Using the above example, we could reduce piston CD from 1.260” to 1.245” (by ordering a custom piston). By the same token, if we wanted the piston to protrude, say, 0.010” above deck, we would increase piston CD to 1.270”.
The need for crankshaft balancing should be obvious. After all, you wouldn’t want to run a fresh set of tires on a vehicle without first having the tire/wheel assemblies balanced. An out-of-balance condition would result in radial imbalance due to uneven centrifugal forces (vibrations/harmonic changes, etc.). Why would we risk an imbalance condition in an expensive engine assembly? A proper balancing correction eliminates unwanted stresses and harmonics, which helps to optimize engine longevity and horsepower. Eliminating this easily-correctable parasitic influence simply makes sense, even in a street build.
One annoying aspect is worthy of mention here. All too often, uninformed consumers confuse the terms “balancing” with “blueprinting.” It’s very common for a novice to boast that his engine has been “balanced and blueprinted,” when in fact a blueprinting has not been performed. The work involved in blueprinting an engine (optimizing every aspect of the build, in terms of clearances, dimensions, airflow, chamber volumes, etc.) adds mega-dollars to the build and realistically is only justifiable in all-out competition engines where every fraction of horsepower and optimized engine durability is absolutely critical. In other words, if a crank/rotating assembly has been balanced, that does not mean that the engine has been blueprinted.
Whether we decide to zero-balance, under-balance or over-balance (we’ll address these issues later in this article), the reciprocating components must be weight-matched. All piston and pin combinations must be of equal weight. All connecting rod big ends must match, all rod small ends must match, all total-rod weights must match, all rod bearings must match and all ring packages must match. In the “old days,” a set of pistons may vary in weight by as much as 5-8 grams or so, requiring the balance technician to weigh all pistons, determining which piston is the lightest. All remaining pistons must then be weight-relieved (by removing material from the pin boss areas) in order to achieve a set of weight-matched slugs. With today’s casting and forging technology, coupled with CNC machining, it is very rare to find the need to weight-correct any pistons made by any of the high-quality performance aftermarket piston manufacturers. The same holds true for connecting rods, where all big ends must weigh the same, all small ends must weigh the same and total weights must be equal.
When checking for equal/matched component weights, staying within about 0.5g (piston to piston, or rod-to-rod) is acceptable.
Again, due to close-tolerance manufacturing techniques used by today’s performance aftermarket rod makers, it is very unlikely that you’ll need to perform any weight corrections to a set of rods.
Nonetheless, you should never assume anything, even when using the highest quality components. It’s good basic practice to weigh each piston and each rod, if for no other reason than to obtain peace of mind and to create a detailed record of the build.
INTERNAL vs EXTERNAL BALANCE
Internal balancing relies on the crankshaft counterweights alone to handle the reciprocating mass of the rods, rod bearings, pistons/pins and rings (as well as anticipated clinging oil weight). External balance feature additional counterweights on the damper and flywheel to provide assistance to the crankshaft counterweights. Certain engines, largely due to limited room inside the engine block that may not be able to accommodate large-enough crank counterweights, must be assisted by the damper and flywheel (i.e. external weight) in order to achieve crank balance. Based on limitations of the room inside the block, this is true for some builds that feature longer strokes and larger bore diameters.
With an internal balancing job, the damper and flywheel are not considered (here we simply use zero-balanced damper and flywheel). With an external balance, the damper and flywheel must be attached to the crankshaft during the crankshaft spin balance check.
Internal balancing is always preferred, if you can get away with it. If the crank counterweights are too light, you can always drill and add tungsten (heavy metal, which is about 1.5 times heavier than lead), although this adds to the expense of the balance job, due to the high cost of heavy metal slugs, which can easily add $100 to $200 or more to the balance job). By maintaining the necessary counterweight closer to the crank centerline and within the confines of the block, you place less strain on the crankshaft. External counterweights apply more dynamic force at the ends of the crank, potentially inducing more crank deflection. For mild street applications, it really doesn’t matter, but for high-stress applications, internal is always a better way to go. This is especially true if you’re running a blower, where more stress will be applied to the crank snout. The less weight you have hanging out on the snout (weighted balancer as compared to a harmonic damper or pulley hub), the better.
By the way, when adding heavy metal slugs, while these may be placed on the outer face of the crank counterweights (90-degrees to the crank centerline), a preferred placement is through the counterweights (fore/aft), placing the drilling and weight parallel to the centerline. This eliminates the possibility of slinging the weights out during crank rotation.
While achieving a “zero” balance is the common goal, some builders prefer to intentionally over-balance. This means that you’re allowing a bit more weight on the crank counterweights than needed for zero balance. This theoretically moves the ideal balance point further out on the rpm scale (smoothes out more at higher rpm). For instance, instead of balancing a V8 crank at 50%, the crank is overbalanced by a few (for example 1 to 3 percentage points), 51 to 53%. This is done by making the balancing bobweights 1 to 3 percentage heavier when spin-balancing the crank. The theory is that you’re trying to compensate for higher dynamic forces (in addition to static weight) that occur at higher rpms. The engine may idle a bit rough, but will smooth out at a target high engine speed. Several builders I spoke with told me that “this is something that you can try…if it works out for you, great. If it doesn’t, at least you learned something.”
Mike Mavrigian has written thousands of technical articles over the past 30 years for a variety of automotive publications. In addition, Mike has written many books for HP Books. Contact him at Birchwood Automotive Group, Creston, OH. Call (330) 435-6347 or e-mail: email@example.com.
For a PDF of this article (complete with photos), go to: