Steel Connecting Rods

Written by on October 1, 2000 What You Really Need to Know
Steel Connecting Rods View All 7 Photos

Connecting rods are a vital link in the Chevy engine chain. And if you've ever heard terminal death knock in an engine, you'll know exactly what we're talking about. But when it comes to steel connecting rods, there's more to know than you might have first imagined. For example, the actual assembly of the rods (something you have to do) is more critical than you might have guessed. And to make the topic more intriguing, there's a very good chance that the practices a backyard Chevy engine builder uses to install steel connecting rods may be contributing to their early demise. The folks at Oliver Racing Parts have considered these factors, and much of their research may surprise you (in fact, some of this research may be absolutely contrary to what you once believed to be steel rod gospel). But before we look at how to handle steel rods, let's take a look at the basic numbers involved in selecting the right rods for your Chevrolet.

Connecting Rod Math For Mechanics...

Mathematics plays an important role in determining which rod to use in a given engine. One of the first things you have to think about is the crankshaft throw. Crankshaft throw is an easy formula that takes little work on your part (or on your calculator's part), but requires consideration when determining connecting rod length. For the sake of comparison, let's assume we have a 383ci small-block stroker engine combination. The engine has a crankshaft stroke of 3.750 inches and features a standard-length small-block connecting rod of 5.700 inches. Here's the math:

Crankshaft Throw
Crankshaft throw =Stroke/2 =3.750/2=1.875

This figure is almost useless until it's plugged into another formula, such as the calculation of engine deck height. The variables required to determine the cylinder block deck height include: connecting rod length (5.700 inches), piston deck clearance (0.008 inch), crankshaft throw (1.875 inches). Compression height can either be measured or determined from the piston manufacturer's charts. This is where it gets interesting with a 383 stroker combination. In order to "fit" all of the pieces in a standard 350, the piston compression height must be changed (you need custom pistons for this application). If a stock piston was used, it would stick out the top of the block. A typical aftermarket 350 piston features a compression height of 1.55 inches. Meanwhile, a 383 piston with a 5.700-inch rod has a compression height of 1.425 inches.

Cylinder Block Deck Height
Cylinder block deck height =Crankshaft throw + connecting rod length + piston compression height + piston deck clearance =1.875 + 5.700 + 1.425 + 0.008 =9.008 inches

Chevrolet's blueprints indicate that the 350 powerplant should come finished with a 9.025-inch cylinder block deck height. When our figure of 9.008 inches is subtracted from the published specification of 9.025 inches, we see that the block should be capable of withstanding a milling process that removes 0.017 inch. Before rushing your Chevy cylinder block down to the local machine shop, remember that these figures are hypothetical only. Production tolerances in the cylinder block, connecting rods, and piston compression height, as well as the desired deck clearance, must all be carefully measured on your particular powerplant before making the decision to begin major cylinder block surgery.

Total deck height also is important where a custom-length connecting rod and special compression height pistons are installed in the engine. Many Chevy race engines use this practice, and it also can be beneficial in some street applications. It also proves useful in cases where the crankshaft is stroked (as in our case) or destroked for a specific application. Let's take a look at another reworked small-block powerplant with the following changes:

Stroke: 3.50 inches (350 crank offset ground to increase stroke from 3.48 inches)

Connecting rod length: 6.00 inches, center to center

Piston compression height: 1.250 inches

Desired piston deck clearance: 0.010 inch Crankshaft throw = Stroke/2 = 3.500/2 = 1.75 inches

Cylinder block deck height = Crankshaft throw + connecting rod length + piston compression height + piston deck clearance = 1.75 + 6.00 +1.25 + 0.010 = 9.01 inches

This combination will require minor cylinder block deck milling, but it goes to show how engine shops can juggle components to come up with a special combination.

Another point you should really think about is the actual "rod ratio" or l/r of the engine. Rod ratios change with both rod length and crankshaft stroke and, in some cases, can have a profound effect upon how the engine behaves.

Rod Ratio

Rod ratio = (Connecting rod length, center to center) ÷ crankshaft stroke

5.700/3.750 = 1.52

The figure determined in rod ratio calculations looks interesting, but can it possibly mean anything? You bet. Theories in regard to connecting rod ratios have been discussed, tested, and argued about for almost a full century, and there are probably more theories on the subject than there are connecting rod lengths. Currently, connecting rod ratios in the neighborhood of 1.8:1 have grown in favor in racing circles, and some people would have you believe that a Chevy engine can't function with a ratio under 1.5:1 or over 2:1. Sufficient evidence is available to disprove both of these ideas, but the varied information that has fueled the rod ratio debate is ample enough to fill a volume twice the size of this issue of Suffice it to say that changes in the powerplant rod ratio have an effect upon piston movement at both top dead center (TDC) and bottom dead center (BDC), which in turn creates changes in the effect of camshaft timing events and the timing of the intake and exhaust charges. As an example, a 302ci small-block works out like this when it comes to the l/r ratio: 5.7 inches (rod length) divided by 3.00 inches (stroke) equals 1.9:1. For the most part, the larger the l/r, the higher the rpm peak torque that is produced. In a mild street-strip application, a low l/r such as we have in our hypothetical 383 small-block is perfect; we want low rpm torque to move the vehicle (which likely isn't a flyweight package). For the sake of simplicity, let's leave the rod ratio debate at this point and use the formula for comparison's sake only.

The Management of Steel Rods...

OK. You have the math figured out and you order a set of aftermarket steel rods for your engine. What's the big deal with the "handling" of steel rods? Plenty. You see, the forces contained within a typical engine (Chevrolet or otherwise) are significant. As the horsepower potential of the engine increases, so do the forces, particularly those imposed upon the load-bearing components. And yes, Mr. Newton's laws have an effect upon your engine. Oliver Racing Parts explains: "If there is something in the range of 18,000 pounds of pressure attempting to lift the cylinder head off the block, then there must be 18,000 pounds of force trying to blow the crankshaft out of the block...Consider the 18,000 pounds of force every time the crankshaft rotates and the rod caps experience 'ovalation' as the crankshaft accelerates through its arc and changes the load direction."

Think about this pressure scenario for a second. What really keeps your Chevy engine together? In truth (and assuming that the basic hardware has been engineered and manufactured correctly), it's just a set of rod bolts. And in the end, rod bolts are nothing more than a series of spiral wedges designed to create enormous forces (which in turn, counteract the forces trying to dump the crank out the bottom of the block). Of course, precisely designed and manufactured fasteners are more than lumps of steel with threads on one end and a hex head on the other. Oliver points out that selecting the right bolt and then installing it correctly is a primary factor in obtaining maximum service from your connecting rods.

Fastener installation is a primary factor in connecting rod longevity, but unfortunately, there's quite a bit of controversy and misinformation out there when it comes to hardware. Some people believe a correctly tightened fastener requires the application of enormous amounts of torque in order to keep the bolt from loosening. That may have been true years ago, but bolts and studs that were acceptable yesterday might not be viable today. According to Oliver, for a powerplant to stay together, the fasteners used must provide repeatable clamping forces that are greater than the loads acting upon them. So far so good, but how do you really know when the clamping loads are enough?

Stretching The Limits...

Oliver offers clear insight into measuring clamping loads: "If you get the idea that properly designed fasteners are basically a solid spring, you will have no difficulty understanding the concept of clamping force. In most cases, the manufacturer's instructions provide specifications that will cause the fastener to reach 75 percent to 80 percent of its yield point, plus a calculated safety factor.

"Calculating the clamp load you are actually getting can be complex. A correctly designed bolt, for example, may require an undercut shank to control where the actual stretch occurs.

"Undercutting prevents thread deformation and the concentration of load in stress-sensitive areas. The diameter of the undercut can help you determine the clamp load achievable from a given bolt or stud."

How Strong Is Strong?

There's more to fastener strength than simply manufacturing bolts with undercuts on the shanks. You have to consider "tensile strength." Basically, the term "tensile" is a layman's term that actually refers to "ultimate tensile." In essence, the more you stretch a given material, the closer you bring it to its yield point.

As you can see, ultimate tensile is the absolute point where the effective clamp load decreases as the fastener continues to stretch. Oliver points out that failure occurs when the material exceeds its elasticity (also called modulus of elasticity) and the clamping force at the joint drops to zero.

There may be more here than meets the eye. Oliver advises that tensile is a function of many factors, one of which is hardness. "Materials that are heat-treated to higher Rockwell numbers also develop higher ultimate tensile, which would lead you to conclude that more is better. That might not be absolute. The answer is simply, not always."

Here's why: "Tensile is only part of the story. All materials react differently at higher hardness levels. Some become very brittle, greatly reducing their fatigue life. Others develop stress corrosion, a special form of hydrogen embrittlement in which the material is attacked while under stress. This occurs when the hydrogen in the air we breath penetrates the material. It's as simple as improper handling. Moisture from your hands can deposit minute amounts of salt and acid on the surface of the material, which starts the corrosion process. The corrosion remains on the fastener until stress is applied (from tightening). Hydrogen then attacks this corroded area and promotes more corrosion, which attracts more hydrogen. The process feeds on itself.

"There are several exceptions to this rule. One is some of the multi-phase stainless steels. These materials can be processed into the 265,000-psi ultimate tensile range and still remain very ductile. Best of all, this material is 100 percent corrosion-resistant. The down side of this deal is that you can look forward to spending around $28 per bolt."

Clamping Forces...

What about torque? Torque is defined as the amount of friction that must be overcome to cause a nut or bolt to turn. Manufacturers of most high-quality fasteners designed for racing or extreme performance applications will supply some tightening specifications, torque reference numbers, or stretch data. It's no secret that this information is usually listed in foot-pounds of torque. Oliver states that of all the effort applied to a given fastener, 50 percent of the torque is used to overcome the friction under the head of the bolt and 40 percent is required to overcome forces that have absolutely nothing to do with the job at hand. Tom Molnar of Oliver sums it up this way: "You can look at this as a torque wrench being only 10 percent accurate." Think about that for a second.

It is easy to see that friction is an extremely challenging problem because it has so many variables and, of course, is difficult to control. Oliver states that many fastener manufacturers recommend using the stretch method of measurement since the pre-load is closely controlled and is obviously independent of friction.

Oliver offers more insight into friction: "The friction for a particular installation can change from one application of torque to the next. That is, when a bolt is torqued for the first time, the friction is usually at its highest value. Each additional time the fastener is torqued and then loosened, the friction factor becomes smaller."

What's the big deal over thread lubricant? Thread lubricants are absolutely critical since they are the primary element when determining friction. In many racer shops, it is common to use good old-fashioned motor oil. Why? That's easy. It's readily available. Everyone has a partially full bottle of their favorite engine oil sitting in the corner of the garage. Unfortunately, there's a catch. When you use specially formulated, low-friction lubricants designed for a specific task, the required torque can be reduced as much as 20 to 30 percent. The reverse is also true. Oliver points out that if the recommended tightening specifications are based on the use of a special lubricant, the use of motor oil or other non-specified lubricant will result in insufficient pre-load. The folks at Oliver also note that while engine oil is a good hydraulic bearing material, it is a poor extreme-pressure lubricant. If it is used on bolts, the torque required will actually increase. This is due to galling, which makes the surfaces rougher. Essentially, the torque must be increased to compensate for the added friction induced by the non-specified lube.

There's more, too: The surface finish of the fastener also is a contributor to the friction factor. For example, a bolt with black oxide finish behaves differently than a fastener that has been polished. Because of this, it is very important to observe the tightening recommendations supplied with each type of bolt.

It is easy to see that without a method of accurately measuring the stretch, it is a simple process to exceed the yield of the material and essentially fail the fastener--long before the engine has been fired. Oliver adds this advice: "Is there a predictable way to accurately follow the manufacturer's instructions and end up with correctly installed parts? Absolutely--if you follow the principals of stretch. Because it is bolt stretch that provides clamping force, we primarily recommend the use of the stretch method. The second choice is the torque/angle method. Remember, check it twice."

The Oliver spokesperson points out that the company cannot take credit for inventing the torque/angle method of measuring fastener installation. "All of the major car manufacturers in the United States, Germany, and Japan, as well as all the diesel engine manufacturers, follow this method, not because it is easier, but because it more accurately pre-loads the fasteners."

The bottom line here is that torque does not measure bolt stretch. It measures friction. For any fastener to supply clamp loads high enough to keep the parts bolted together, the fastener must be stretched the proper amount. That's why rod bolts should not be torqued. Instead, they should be "stretched."

Chevy Connecting Rod Tips...

You've figured out torque and stretch. Just when you think you have all of the factors sorted out, there's more information on steel connecting rods to consider. Check out the following Chevy rod tips. They may ultimately change the way you handle the connecting rods for your Chevrolet engine.

1.) Be sure to follow the old Chevy oil pressure proverb. Oliver recommends 10 pounds of oil pressure for every 1,000 rpm of engine speed. For example, if you run your mouse motor to 7,500 rpm, it needs 75 pounds of hot oil pressure.

2.) Measure bearing surfaces at least twice--once at the 12 o'clock position and again at the 6 o'clock position. Remember, too, that clearances vary according to the application. Oliver points out that typically you need 0.001-inch clearance for each inch of crank pin diameter.

3.) Measure wrist pin clearances and ask the rod manufacturer for a recommendation. Don't assume the out-of-box dimensions are correct. Oliver points out that, depending on the application, wrist pin clearance dimensions can range from 0.0007 inch to 0.0015 inch.

4.) Clean all reciprocating parts thoroughly. The idea is to remove all dirt and foreign oils. Spread the rod bolt assembly lube on the threads, and be sure to spread the lube under the head of the bolt prior to beginning the tightening sequence.

5.) Never use metal stamps to number connecting rods! According to Oliver research, metal stamps can disturb the roundness of the connecting rod bore. Instead, paint toolmaker's layout die on the rod and rod cap. Inscribe the numbers.

6.) Do not use rod bolts to draw the cap down the rod. The correct method is to locate the cap dowel sleeves into the counterbores of the rod. Then, very, very carefully, tap the cap into place.


Oliver Racing Parts
Grand Rapids, MI

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