Small Block Chevy Rods and more.
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This six page article is about small-block Chevy "mouse motor" connecting-rods. It has good overall detail with an emphasis on connecting-rod bolts. The four states of material are covered.
One, The tensile state (bolt stretch).
Two, The yield state (stretched (loaded) but not deformed).
Three, Plasticity, the yield state (loaded) with permanent deformation.
Four, The ultimate tensile load (point of failure).
A bolt is tighten to a point of yield that does not permanently deform the bolt. When a state of yield is reached that permanently deforms the bolt, this is commonly called the plasticity point. A point where the bolt has reach maximum yield and become permanently deformed and must be replaced.
Fasteners work within a range between yield and plasticity, avoiding permanent deformation. Clamp load between two surfaces with additional pull (lift) loads determine the overall loading a fastener must be subject to.
NOTE: The Chevy Hi-Performance guides from Chevrolet Division recommend that stock connecting-rods be Magged and shot-peanned, resized with high quality bolts and a 0.001" inch piece of steel shim be put between the big end parting surfaces before honing the big end to a minimum size. Remove the steel shims after the honing process.
This gives more relief between the bearing crush areas to allow for deformation of the big end bores under load. A major contributor to bearing spin. Also, a pin can be placed into the bearing cap area to accept a connecting-rod bearing that has been predrilled to assist with bearing stability.
Any comments and/or questions are always welcomed.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS
TECH TIPS SMALL BLOCK ENGINE PERFORMANCE LS1 LS2
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS
The connecting rod is a simple enough part. While it accelerates and decelerates at incredible speeds, it has no moving parts. As unassuming as it appears, the connecting rod is one of the most highly stressed and abused components in any internal combustion engine. Step into the world of high performance, high horsepower, and most importantly— high RPM, and it pays to spend some time to make the most of your connecting rods.
front_cover_image_3537small This Tech Tip is From the Full Book " HOW TO BUILD BIG-INCH CHEVY SMALL-BLOCKS ". For a comprehensive guide on this entire subject you can visit this link:
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BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 1
Stock small-block Chevy connecting rods for the 400 small block came in two lengths, 5.7 and 5.565-inches. Don’t be lured in by the idea that the so-called "pink" rod is a stronger rod. The only thing the pink rods offered was that they were shot peened and Magnafluxed. Otherwise, they were forged of the same mild steel as other production rods.
In stock street applications where engine speed rarely exceeds 6000 rpm, stock connecting rods do an excellent job of transferring the power from the piston to the crank. But add cylinder pressure, big cylinder heads, and long duration camshafts that require higher RPM to produce horsepower, and that’s when we have to start paying closer attention to the lowly connecting rod.
When you talk about stock small block Chevy connecting rods, the two most popular overall lengths are 5.7 and 5.565-inches. Go all the way back to the original 265ci mouse motor and you’ll find that Chevy elected to use a 5.7-inch long connecting rod. As stroke increased from 3.00 to 3.25 to 3.48-inches, rod length remained the same.
With the arrival of the 400 small block in 1970, there wasn’t room to maintain the piston compression (dome) height that GM desired with a 5.7-inch rod, so the engineers shortened the rod by 0.135-inch to 5.565 inches (one half of the .270" inch increase in stroke of the 400ci). At the same time, the engineers also shortened the rod bolt slightly to create more clearance between the bolts and the cam.
There is a third small block rod used in the 350 H.O. and ZZ4 engines that is a 5.940-inch powdered metal (PM) rod that is every bit as strong as the original "pink" small block rods. This rod was also used in the baby 265ci LT1-based engines from 1992 to 1994.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 2
There are two basic connecting rod designs. The production-based I-beam rod (top) is the most popular, but the H beam rod is now a popular performance alternative.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 3
When you add stroke, it’s best to increase the rod length at the same time in order to reduce the acute angle created by the longer arm on the crank. Popular long rod lengths for the small block are 5.850, 6.000, and 6.125, but some companies make rods as long as 6.25 inches.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 4
The most highly stressed fastener in any engine is the connecting-rod bolt. ARP offers rod bolts in three quality steps from the standard 190,000 psi 8740 bolt (left), to the Wave-Loc bolt (center), to the Pro Series Wave-Loc (right) made from a proprietary ARP 2000 material offering about 220,000 psi tensile strength.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 6
Longer strokes create problems with cam-to-rod clearance. This often requires the use of stroker rods that offer additional room in the area just above the rod bolt. As stroke grows, this clearance becomes increasingly tight.
Production small-block rods are all constructed using a basic forged I-beam design with a stud-and-nut fastener.
Gen III small blocks also employ the powdered metal forging that is actually a little stronger. The other interesting technique employed in conjunction with these rods is the one-piece forging uses a cap that is broken off the main part of the beam. This jagged edge looks nasty but does a superior job of locating the cap to the rod, preventing what is generally referred to as cap walk. Unfortunately, it is not rebuildable since the jagged mating surface cannot be machined to make the big end smaller to resize it.
Stock rods work well on street engines that will not see more than 6,500 rpm and see that speed only for very short periods of time. But since high performance engines are all pushing the envelope, there are better quality rods out there for such an attractive price that the aftermarket rods deserve significant attention.
If you’re on a tight budget and plan on using a 5.7 rod in an affordable 383 or 406ci small block, the smart move is to Magnaflux the rods for cracks, have your local machine shop install new ARP bolts, and then resize the big end of the rod. The big end must be resized anytime a new fastener is used since the new bolt will drastically alter the shape of the big end when it is torqued in place. You can also polish the side beams to prevent the formation of stress cracks.
Stock Chevy rods are constructed of 1038 mild steel that offers decent strength combined with great ductility.
Performance applications apply much more stress to the connecting rod than stock applications, so long ago companies like Carrillo, Oliver, and Crower started building extremely strong 4340 steel rods for racing applications.
In the last few years, several new companies have jumped into the high-performance connecting rod market with inexpensive, yet high-quality 4340 and 4130 forged steel rods. For mild street engines, the next step up from stock would be the 4130 steel rods, and then on to the higher-quality 4340 forging.
There are several companies building 4340 steel rods. The difference in price can be substantial when you compare a Scat or Eagle 4340 steel rod to a rod like a Carrillo or Oliver. There’s no disputing the quality of the Carrillo or Oliver rods, but the additional quality may or may not be worth the additional cost.
Crower perhaps has the widest range of rods, all forged with 4340 steel, but varying widely in price and application.
Selection then is more a question of application than price. For example, the Crower Sportsman is the entry-level 4340 chromoly steel rod that includes a 3/8-inch, 8740, 180,000 psi steel alloy bolt and nut. Increasing in price are the Sportsman Stroker, Billet, Billet Stroker, and Lightweight rods using through style bolts and nuts.
There are also Billet, Billet Stroker, and Mid weight rods using cap screws. There’s even an Ultra-Light cap screw rod. All told, Crower offers 14 different connecting rods for the small block Chevy. All of these rods are built from 4340 steel, but vary dramatically in quality, application, and price. As you can see, there are plenty to choose from.
But don’t be misled into thinking that you need the trickest rod. The reality is probably the basic 5.7 or 6.0-inch Sportsman or Stroker rod would probably work best in most applications.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 5
Any time you replace rod bolts, you must also resize the rod in order to create the proper bearing housing bore diameter. This housing bore diameter determines the proper inside diameter for the rod bearings in order to create the proper bearing clearance.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 7
High performance rods are also available in either pressed pin (left) or full floating styles (right) identified by its bronze bushing. Full floating rods require using some type of piston locking rings to keep the pin inside the piston.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 8
In order to determine bob weight, you must know the weight of both the big and small end of the rod. Each end is weighed using a fixture like this one. Those weights, when added together, must match the overall total weight of the rod.
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Another reason to not use stock rods is that in the mid 1980s, Chevy went to a thinner I-beam rod. The weights are the same, but the center portion of the rod is roughly 0.070-inch thinner. It’s best to avoid these rods.
We mentioned billet rods as part of the Crower lineup. Oliver, Carrillo and others also offer billet rods, as do several other companies. A 4340-chromoly steel billet rod is generally considered to be the strongest, most durable rod made.
It comes in either through-bolt or capscrew configurations. Billet rods are cut from pure parent material, as opposed to being pressed in a forge. This offers excellent flexibility in design, but dramatically increases the price. For street engines, billet rods would be considered a rather expensive luxury.
Pure drag race engines often employ aluminum rods in an effort to further reduce reciprocating weight. The only reason we mention them here is because some enthusiasts have the mistaken impression that there might be some advantage by employing aluminum connecting rods. Bill Miller Engineering (BME) contends that they have successfully used aluminum connecting rods in many street engines without failure, and we certainly are not in a position to refute those claims. However, any metallurgist will agree that even the best aluminum alloy is no match for 4340 chromoly steel in the strength department and that aluminum is far more prone to work hardening with use.
These factors alone make an aluminum rod choice difficult to defend. The weight reduction may offer few, if any, advantages when it comes to creating a given bob weight. With all these considerations working against an aluminum rod, there are few credible reasons to support risking a rod failure.
The whole point of this book is to show you how to build a big-inch small block. One of the stumbling blocks to building a long-stroke mouse motor is camshaft-to-rod clearance. With the original 400, Chevy resorted to a shorter 5.565-inch rod with a shorter bolt. This produced plenty of cam clearance, but the short rod creates several problems when venturing beyond the factory 3.75-inch stroke. Because additional stroke makes the crankshaft counterweights larger, this also requires a longer rod in order to allow enough room for the piston to clear the counterweights at bottom dead center (BDC). Generally, a 5.7 or 6.0-inch rod is preferable, but you can find small-block rod lengths all the way up to 6.250-inches in length.
In addition to the length question, you will also need a rod with sufficient clearance around the upper portion of the big end. This is where the rod comes closest to the camshaft, just past top dead center (TDC). Stroker rods offer additional clearance around this area, making it easier to stuff a longer stroke crank into a standard deck-height block. It is possible to cram a 4.00-inch stroke crank into a standard cam-height block. The tightest clearance between the rods and the cam occur with rods 1,2, 5, and 6; but because of balance considerations, all the rods must be clearanced the same.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 9
Pro Form makes a tool to remove the rod cap from a rod, but you can also use a rod vise. Loosen the nuts and then gently rock the rod until the cap loosens up.
Many rod companies do not differentiate between their standard 4340 rods and stroker rods, so you need to contact the manufacturer to determine if the rods you want to use will clear the stroke you intend to employ. Crower’s Stroker Sportsman rods are only available in 5.7 and 6.0-inch lengths but the stroker billet rod has more options ranging from 5.7 to 6.250-inches. Not only is the rod stronger, but also substantially heavier at 690 grams for a 6.0-inch rod compared to the Sportsman Stroker 6.0- inch rod that weighs only 615 grams. Clearly, the heavier billet rod is stronger and much of its strength comes from its additional material.
You must also consider rod clearance around the pan rail as well when shopping for a stroker connecting rod. Not all rods are shaped the same. Certainly, the stroker rods will offer some additional clearance is this area as well.
The most highly stressed fastener in the entire engine is the connecting-rod bolt. The sidebar on Strength vs. RPM will detail the loads imposed on a connecting rod and its fasteners, but the bottom line is that as the weight of the reciprocating system and RPM both increase, the forces that are trying to stretch the two bolts holding the cap on the connecting rod increase radically. Both weight and RPM are critical to this formula but certainly the most damaging is RPM. For example, a 4.0-inch bore, 3.875-inch stroke engine can impose as much as 12,380 pounds of tensile load on the big end of the connecting rod as the rod accelerates away from TDC at 6,500 rpm. This happens every time the piston comes across TDC on the overlap stroke. When you consider the number of cycles an engine goes through (even though a majority are not at high RPM), this still requires tremendous rod-bolt strength.
Fasteners are a study unto themselves, but we’ll try to condense the essentials down to a few simple paragraphs to give you the information you need to choose the right rod bolts.
Most rod bolts are rated in terms of psi, which is the rating system used to define the stress or the load applied to the bolt divided by its cross-sectional area. This is why a larger diameter bolt made of the same material is stronger and offers more potential clamp load than a smaller fastener. Unfortunately, larger bolts weigh more so the ideal compromise is a strong material in a smaller bolt.
The ultimate tensile strength is the maximum stress that a fastener can withstand without breaking.
Each fastener also has a maximum amount of strength that it can withstand and return to its normal length. This is called the fastener’s yield strength. If you exceed a fastener’s yield strength, the fastener is ruined and should be replaced.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 11
Rod side clearance is one of those clearances that can be easily overlooked in the rush to assemble an engine. But if you are using all new parts, you absolutely must ensure that the clearances are within spec. Minimum rod side clearance for steel rods is between 0.010-0.012-inch. Greater clearance allows more oil to splash around the crankcase.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 13
Always check that the chamfer on the rod bearing is adjacent to the fillet radius on the crank (arrow). More than one engine builder has inadvertently installed the rod backwards on the piston.
Extremely high tensile strength fasteners also tend to be somewhat brittle. That means that the difference between the fastener’s yield strength and its ultimate tensile strength is a fairly small amount.
Clamp load is force exerted by a tightened bolt and is the same as its preload. What makes life difficult for rod bolts is that they must have high yield and tensile strength but they must also be able to withstand well over a million cycles of tensile, or stretch, loads.
A high-quality connecting-rod fastener should therefore have a high enough tensile strength, but also have a fairly wide load range between its yield strength and its ultimate tensile strength. It is in this band where it may be asked to operate. This is why creating the proper preload or clamp load with a rod bolt is so important.
This is most accurately measured in bolt stretch. Typically, ARP specifies a bolt stretch dimension that is 75 percent of the bolt’s yield strength. So if a typical 3/8-inch ARP rod-bolt stretch figure is 0.006-inch, then its yield length is probably very close to 0.008-inch. The point here is to establish a clamping load that is far greater than the load imposed on the bolt during engine operation.
During the point where the piston changes direction downward after top dead center (TDC)
during the overlap cycle, the crankshaft pulls down on the piston. This places the rod bolts in tension. At high RPM, this can actually deform the big end of the rod, making it oval and placing extreme loads on the rod bolts. This is why the clamping load is so important.
The only way to achieve this optimal stretch figure is to measure the bolt. This is accomplished with a rod-bolt stretch tool. Several companies, including ARP, sell this tool, which is really nothing more than a steel fixture mounted to a dial indicator. The dial indicator uses a heavier return spring than common dial indicators to help the tool remain in place on the rod bolt. This is helpful in the engine since this tool is most often used in the tight confines of a lower rotating assembly. The checking procedure is also simple.
Place a box end wrench over the rod bolt with the fastener hand tight. Then position the stretch gauge so that you can read the dial indicator with the two points of the indicator located securely on the small divots on both ends of the fastener. These divots help hold the indicator points in position. Zero the indicator and then slowly tighten the rod-bolt nut until the desired stretch is achieved. This must be accomplished on all 16 rod bolts for a V8 engine.
If this sounds like way too much effort — it is. But if you are investing a couple thousand dollars in a quality rotating assembly, this is the only acceptable way to ensure that the rod bolts are adequately loaded. Some may contend that using a torque wrench is sufficient, but the reality is that there are entirely too many variables that affect bolt torque that have little to do with stretching the bolt.
For example, ARP specifies a torque setting intended to be used only in conjunction with ARP’s moly lubricant. This lubricant offers a reduced coefficient of friction than motor oil, which is why the lubricant torque is always less than the torque specified when using oil. We have tested these torque figures against actual bolt stretch and because ARP cannot account for all the variables, we’ve found the ARP torque figure usually creates a bolt stretch that is less than the optimal spec. We have tested this on several occasions and we consistently come up at least 0.0008 to 0.0010-inch shy of ARP’s specified bolt-stretch figure. This is why measuring the actual bolt stretch is the only way to go.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 12
Checking bearing clearance requires torquing the rod bolts outside the engine. Always use a rod vise when torquing the rods to prevent tweaking the rod. Make sure the vise supports both the cap and the beam of the rod.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 14
Always measure bearing clearance in the vertical. All engine bearings employ a certain amount of taper towards the horizontal component. Measuring the inner diameter of the rod at the parting line will produce an erroneously wide clearance.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 15
Another advantage to using a higher quality aftermarket rod is these locating dowels that prevent rod cap walk under high-RPM applications.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 16
Cycling the rod bolt several times also helps seat the rod bolt in the rod. Sometimes the machine shop does not fully seat the bolt in the rod due to a tight fit between the bolt and rod. Torquing will usually fully seat the bolt in place.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 17
Many higher-quality rods employ cap screws rather than bolts and nuts. The cap screws offer certain height advantages for stroker applications and can be measured for bolt stretch just like any other rod bolt.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 18
This cutaway shows just how close all the rotating parts are when you stuff a long stroke crank and long rods in a small block. The minimum clearance for the rod to the camshaft is 0.050-inch.
BIG-INCH CHEVY SMALL-BLOCK CHEAT SHEET: CONNECTING RODS 19
If you must torque rod bolts in place rather than using a stretch gauge, ARP recommends you torque the rod bolts at least three to five times. This burnishes and smooth's the threads and the rod-nut to rod interface, which creates less friction. Also be sure to use only the rod-bolt moly lube that comes with the ARP bolts.
There are actually no excuses for not using a rod-bolt stretch tool. These tools are not that expensive (usually around $100) and are easy to use. However, in an emergency, you certainly can merely torque the rod bolts in place.
If you are using brand new fasteners, ARP recommends torquing each fastener at least five times before the final torque is applied during assembly. This is because new fasteners need to establish a wear pattern between the nut and bolt and within the threads. This may also take into consideration the rare time when a rod bolt is not fully seated in the rod. Torquing the fastener at least five times should seat the bolt in the rod and establish a reasonable wear pattern that will allow more of the actual torque to create bolt stretch as opposed to merely overcoming friction.
Torque wrench accuracy is another variable that affects the fastener torqueand stretch. It is not uncommon to find a torque wrench that is off by as much as 10 ft-lbs when checked for accuracy. If you have not had your wrench calibrated, have it checked at the specified torque setting for the rod bolts. That way you can at least account for this variable.
If you really want to know if a rod bolt has been stretched beyond its yield point, then you need to also keep track of the relaxed length of each fastener when it was new. During subsequent teardown, the bolts can be re-checked for length. If a used bolt is even as little as 0.001-inch longer than its original relaxed length, it has been stretched beyond its yield point and must be replaced.
We’ve expended a significant amount of this chapter on rod bolts and their use because of the catastrophic damage that can occur if these specifications are not followed. Spend time making sure this area is assembled properly and you can avoid most of the expensive problems.
In addition to all the other variables involved with connecting rods, the aftermarket has also begun offering a different style of rod called the H-beam rod. H-beam rods were developed for racing engine use and were designed to be stronger in high-RPM use where the rod’s ability to withstand increased load in tension are an advantage.
The traditional connecting rod design is the I-beam style, which, according to the manufacturers that we’ve interviewed, is stronger in compression compared to the H-beam rod, mainly due to reduced stress concentrations.
Since most large displacement small blocks will be built more as torque engines rather than stratospheric- RPM power plants, the I-beam rod appears to be the more sensible choice. However, there is certainly nothing wrong with using H-beam rods in almost any application and the differences in strength are probably not going to make a difference in terms of a potential failure.
After wading through all this material on connecting rods, hopefully you’ve gained a greater appreciation for both selection and application of this often-overlooked component. Connecting rods are not generally associated with massive power improvements, but they do play an essential role in making sure that all the power you do make is created durably and reliably.
Written by Graham Hansen and Posted with Permission of CarTechBooks