Rotating Assembly Design
By definition the engine’s rotating assembly consists of the crankshaft, the connecting rods, the connecting rod bearings, the pistons, the piston rings, and the piston pins.
Words: Larry Shepard
By definition the engine’s rotating assembly consists of the crankshaft, the connecting rods, the connecting rod bearings, the pistons, the piston rings, and the piston pins. In the past few columns I have discussed the design aspects of the cylinder blocks, cylinder heads and intake manifolds. These would all be considered high-profile topics. The rotating assembly topic would be on the other end of the popularity scale. Typically this group of parts is not looked at as a team that must work together and generally are not put into the design equation. However, each major part of the rotating assembly has an important aspect or specification that is a big concern—the crank has stroke, the piston has bore-size, and the rod has engine-type. While these basic design theories apply to all engines, it is not very popular to change the crank-rod-piston package in 4 and 6-cylinder engines so I’ll focus on the popular V-8s.
The largest piece of the rotating assembly is the crankshaft. Many of the design aspects of the crank are defined by the block. Main bearing spacing and main bearing size, both diameter and width, come from the cylinder block. There are many clearances such as to the pan-rail and the cam lobes and the bottom of the cylinder-bore that have to be considered. With any given engine or block, these concerns come down to stroke—more stroke equals more cubic inches and more inches means more performance. The standard stroke 426 HEMI® Gen II has a 3.75" stroke, P5007251. The popular stroker crank for the 426 HEMI has a 4.15" stroke, P5007250. Both are made of forged steel and upgraded to 4340 steel for added strength. Because these two engines are closely related, the cranks also work in the big block wedge engines such as the 440. They do not work in the low-deck versions of the big block wedge, commonly called the 361/383/400, because the low-deck engines use a smaller diameter main size. There is a similar situation on the small blocks between the small main 273/318/340 group which includes the 5.2L Magnum® engines and the large main 360 group which includes the 5.9L Magnum engines. For example, the 318/340 small main forged crank with the 3.58" stroke is P5007253 while the large main version is P5007255. Getting back to the big blocks, one of the most popular conversions is to install the basic 440 large main crank into the 383/400 blocks. Obviously you can have a custom, billet crank made (www.scatcrankshafts.com) but this tends to be expensive. However, the popular choice is made by cutting down the mains of the 3.75" stroke, 440 crank to the 383/400 size. This cutting down operation can be done by your engine/machine shop. Caution: if the forged crank is cut down, Mopar Performance recommends a surface hardening (such as nitriding) after machining for severe-duty high-horsepower applications.
On all big block engines—383, 440 and 426 HEMI—the block and crank use a rear seal retainer to hold the rear main seal which keeps the engine from leaking oil during operation. The rear main seal itself, P4271961, is made of neoprene/rubber and is held by the aluminum seal retainer and two side seals. The crank’s rear flange on the 426 HEMI has an 8-bolt design as do the HP/Race RB cranks listed above. Most production 383s and 440s used a 6-bolt design. The flexplate (automatic transmission) or flywheel (manual transmission) attach to the crank flange. There are 6- and 8-bolt flexplates and flywheels, so you just have to match the parts with your application. Rod and main bearings are available from Clevite – www.engineparts.com.
Although not technically a rotating part, the RB windage tray is directly related to the crank because it must be changed if the long-stroke cranks are used. The standard 3.75" windage tray, P4120998, fits both the 426 HEMI and the 440 but you must switch to the 4.15" tray, P5007345, if you install the 4.15" long-stroke crank.
While I have not mentioned it before, the rotating assembly includes the vibration damper which attaches to the nose of the crank. To cover all the possibilities, it needs its own column so I’ll only hit some highlights. There are two general categories: production and Race or SFI. The SFI dampers are safety tested and tend to be required for the faster classes in drag racing. The forged-crank 383 and 440 engines use damper P3830183 as do the ’65 426 Race HEMIs. This is considered a thin damper. The ’66–’71 Street HEMI uses a thick damper P5249699 (SFI version). The 383-440 SFI damper is P5249694. The damper is held onto the nose of the crank by a large bolt and washer—P5249562 (1.25" long for thin-damper use) or P5249557AB (2.25" long for thick-damper use).
While most crankshafts are made of iron there are two main types – cast and forged. All 426 HEMI cranks are forged and most of the 383 and 440 cranks are also forged but there are cast iron versions of these two cranks, typically used in the newer versions. These cast-crank 383/400 and 440 engines use a thicker damper which is also offset weighted or unbalanced by itself. The LA engine (273/318/340) used forged cranks but all the 360s used cast cranks. The ’72–’74 318s and 340s switched to cast cranks. All of the 5.2 and 5.9 Magnum engines used cast cranks. For example the small main 318/340 forged crank, P5007253, has a 3.58" stroke while the cast crank version is P5007257. Note: the stock stroke (3.31") 318 and 340 crank is not very popular, so the longer stroke (3.58") is used for added cubic inches. The most popular performance crank for the LA and Magnum engines is the 4.00" stroke—P5007252 forged (318/340 main) or P5007256 cast (360 main). Other versions also available.
Although similar the small blocks use a unique damper from the big blocks. The 318/340 forged crank damper is P5249690 and is SFI approved. The production-style, thin damper for the 318/340 forged cranks is P4452816. The 360 cast cranks use the thick damper P5007301 (offset balanced).
Caution No. 1: If you change any rotating assembly part, be sure to have the engine re-balanced. All the cast-crank engines use some form of external-balance which means that part of the engine’s overall balance is in the damper and/or the flywheel/flexplate/converter. For example, the 360 cast-crank production engines have one very large or two smaller weights added to the engine side of the torque converter. The ’70–’71 440-6 bbl also used weights added to the torque converter – kit P4120241. The 5.9L Magnum engine uses different weights than the LA 360 engine.
Caution No. 2: If the engine’s front cover or damper has been changed, assume that you must check the timing marks against the actual TDC (Top Dead Center) on the No. 1 piston. While not related directly to the rotating assembly design aspects, one of the dampers functions is the timing mark cut into the outside diameter of the damper surface. To set your timing, you line this mark on the damper up with one mark on a timing tab located on the front cover. These two marks must line up when the No. 1 piston is at TDC. There are now too many possibilities of dampers and front cover combinations to go into detail just double-check the marks against TDC. Re-mark as required.
The new HEMI or Gen III uses a unique crank from the other Mopar® small blocks (or big blocks). There are currently three—5.7, 6.1 and 392. The 6.1 forged steel crank is serviced separately with OEM part no. 05037458BD. It can be used as a replacement for a 6.1, or to upgrade a 5.7 from cast to forged. The SFI damper for the 5.7 truck engine is P5153631 and the SFI damper for the 5.7, 6.1 and 392 car engines is P5153630. These SFI dampers are unique because the serpentine belt runs on the outside diameter of the damper.
The connecting rod connects the crank’s rod journal to the piston’s pin. In this configuration the rod rotates with the crank and slides with the piston. Longer stroke cranks require that the rods be clearance-checked to the block’s pan rail, the bottom of the cylinder bores, the cam lobes etc. The rod’s center-to-center distance is the most popular of the rod’s specifications. The rod’s big-end diameter and width are defined by the crank. The small-end of the rod is defined by the piston’s pin—floating or pressed, size or diameter, length. A floating pin in the piston requires a bushing in the small-end of the rod. For example, the 440 forged-steel rod has a center-to-center length of 6.76" and uses a pressed pin—no bushing. The 426 HEMI forged steel rod, P5249898, has a length of 6.86" and uses a floating pin with bushing. There is an equation that relates the crank’s stroke, the rod length and the piston’s height (pin centerline to top of piston) to the block’s deck height. To allow longer stroke cranks, the rod length and piston height must change to keep the final piston position at or below the deck surface so you don’t have clearance issues with the cylinder head. Typically the rod length is left the same and the change is made to the piston, if possible, because new pistons change only one set of parts, not two, which is much less expensive.
The piston tends to be the part that is used to fix problems in all the other areas – heads, blocks, cranks and rods. The basic size (bore) of the piston is dictated by the block or more specifically the cylinder bore size in the block. For example, the aluminum forged 426-HEMI piston, P4876026, is a 4.500" bore piston and these big-bore pistons require the siamesed-bore HEMI block. This piston cannot be used in the standard HEMI blocks because these blocks can’t be over-bored to these large bore sizes. The next important piston specification is piston height – the distance from the pin centerline to the top (flat) of the piston. This height is adjusted to keep the piston at the desired relationship to the top of the block. This height is decreased when the 3.75" stroke 440 crank is installed in the 383/400 block or the 3.58" or 4.00" LA-engine stroker cranks are installed in the 318 and 340 blocks—pistons available from KB Pistons (www.kb-silvolite.com), Wiseco (www.wiseco.com), Diamond Pistons (www.diamondracing.net), plus many others. While pistons are typically made of aluminum there are several types: cast aluminum, forged aluminum and hypereutectic. The cast pistons tend to be the least expensive. Forged pistons tend to be more expensive and the hypereutectic fits in the middle. Hypereutectic is a hi-silicon aluminum which adds strength. The piston weight is important to the engine’s performance but is also related to the engine balance so you should not balance your rotating assembly until after you have purchased your pistons. Less weight tends to cost more money. The top of the piston can be flat or domed (high compression ratio pistons) or dished (low compression ratio). Domed pistons must be made based on the cylinder head (combustion chamber shape) that is going to be used. The application that the engine is going to be used for (street or racing etc.) and the fuel that is going to be used (pump gas, racing gas) will dictate the engine’s compression ratio. The head’s combustion chamber can only be adjusted by a few cubic centimeters and there are only a couple head gasket thicknesses, so most compression ratio adjustments are done with the piston. Therefore, the piston tends to be the last part selected because you may want to use it to get the compression ratio where you want it.
More Mopar rotating assembly parts—crankshafts, connecting rods, connecting rod bearings, pistons, piston rings and pins—are being added to Mopar’s lineup. For additional information, refer to the latest Mopar Performance catalog or visit www.mopar.com. Space limits my discussion; if you have specific questions, please call the Mopar Direct Connection Tech Line at 1-888-528-HEMI (4364).
