Connecting Rods 101 – Part 2

Wayne Scraba

19 Apr 2018

Click Here to Begin Slideshow In our last issue, we began our examination of connecting rods. If you spin back the pages on your browser, you’ll recall we looked at rod materials – various types of steel, aluminum and titanium. We also touched on the advantages of forgings. In this segment we’ll cover the forces at work within a power plant and how they affect connecting rods. We’ll also investigate clamping loads of fasteners along with the tensile strength of fasteners. There’s a bunch here. Check it out: The Forces Contained Inside a Typical Engine… The forces contained within an engine (race or otherwise) are considerable. As the horsepower potential of the engine increases, so do those forces, particularly those imposed upon the load bearing components. Tom Molnar reminds us how Mr. Newton's laws affect an engine: "If there is something in the range of 15,000 pounds of pressure (PSI of combustion pressure times the square inches of cylinder) attempting to lift the cylinder head off the block, then there must be 15,000 pounds of force trying to blow the crankshaft out of the block." Tom continues: “Peak combustion pressures for a good n/a race engine are in the 1,000-1,200 PSI range. When you then multiply this by the square inches of the bore, a 4.500-inch bore piston has a surface of 15.904 inches squared, which means the push on the rod/crank is 15,904-19,085 pounds of force. When horsepower goes up, the force on the rods may not go up. The reason for this is, cylinder pressure is at the maximum at peak torque, then as RPM continues to go up, the cylinder pressure goes down. Since horsepower is a calculated number, the HP continues to go up due to more power pulses per minute. Also, if the crank is changed to one with a longer stroke, the same amount of push on the pistons and rods will normally be a higher power output because there is more leverage to rotate the crank. This is why we do not rate rods by power. There are simply too many variables that effect power.” There's more, too, from Molnar’s supposition: "Consider the 15,000+ 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. Ovalation takes place at TDC of the exhaust stroke and has very little to do with combustion pressures.” Take the time to ponder this pressure scenario: What really keeps the 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 when you really break it down, rod bolts are nothing more than a series of spiral wedges designed to create enormous forces (which, in turn, counteract the forces trying to ditch the crank out the bottom of the cylinder block). There’s no question - precisely designed and manufactured fasteners are more than chunks of steel with threads on one end and a hex head on the other. Selecting the right bolt and then installing it correctly is a primary factor in obtaining maximum service from your connecting rods. This is why Molnar only uses ARP bolts that are custom made for them. These bolts make them an upgrade over what many sell as an upgrade. The installation of a fastener is a primary element when it comes to connecting rod longevity, but regrettably, there's quite a bit of controversy and misinformation out there when it comes to hardware. There are people who 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 which were acceptable yesterday might not be viable today. According to Molnar, in order for an engine 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? Clamping Loads… Molnar shares some very 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% to 80% of its yield point which allows for a 20-25% safety factor. "Calculating the clamp load you are actually getting can be complex. A correctly designed bolt, for example, will have 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." Rod Bolt Ultimate Tensile Strength… 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, which actually refers to "ultimate tensile." In essence, the more a given material is stretched, the closer you bring the material to the yield point. Ultimate tensile is the absolute point where the effective clamp load decreases as the fastener continues to stretch. Molnar notes that failure occurs when the material exceeds its yield point; the clamping force at the joint drops to zero. Modulus of elasticity (Young's Modulus or stiffness) for a carbon/iron material is approximately 30 million pounds. Modulus does not change when the material is yielded. Interestingly, modulus does not change with hardness either. A piece of steel will have the same stiffness at 24Rc or 60Rc. There might be more here than meets the eye. Molnar tells us 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." This is the reason 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 may develop stress corrosion, which is a special form of hydrogen embrittlement in which the material is attacked while under stress. This occurs when the hydrogen in the air we breathe 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). “Due to the way bolts are processed today, we do not see much hydrogen embrittlement anymore - but corrosion can still be an issue, especially with carbon/iron bolts at high hardness. A bolt with an undercut diameter of 0.360-inch has a cross section of 0.1018-inches squared. If this bolt is stretched to the point it provides 18,000 pounds of clamping load, the undercut area will be subject to 176,817 pounds per inch squared. If corrosion (simplified as rust) starts, it reduces the cross section, making the unit loading go up. Since the load is now higher, the corrosion will now advance, causing less cross section, increasing the unit loading. It feeds on itself until the bolt fails completely resulting in catastrophic engine failure. This is not a bolt failure and was (in this case) caused by simply improper handling of the bolt.”

Connecting Rods 101 - Part 2 — Click Here to Begin Slideshow In our last issue, we began our examination of connecting rods. If you spin back the pages on your browser, you’ll recall we looked at rod materials – various types of steel, aluminum and titanium. We also touched on the advantages of forgings. In this segment we’ll cover the forces at work within a power plant and how they affect connecting rods. We’ll also investigate clamping loads of fasteners along with the tensile strength of fasteners. There’s a bunch here. Check it out: The Forces Contained Inside a Typical Engine… The forces contained within an engine (race or otherwise) are considerable. As the horsepower potential of the engine increases, so do those forces, particularly those imposed upon the load bearing components. Tom Molnar reminds us how Mr. Newton's laws affect an engine: "If there is something in the range of 15,000 pounds of pressure (PSI of combustion pressure times the square inches of cylinder) attempting to lift the cylinder head off the block, then there must be 15,000 pounds of force trying to blow the crankshaft out of the block." Tom continues: “Peak combustion pressures for a good n/a race engine are in the 1,000-1,200 PSI range. When you then multiply this by the square inches of the bore, a 4.500-inch bore piston has a surface of 15.904 inches squared, which means the push on the rod/crank is 15,904-19,085 pounds of force. When horsepower goes up, the force on the rods may not go up. The reason for this is, cylinder pressure is at the maximum at peak torque, then as RPM continues to go up, the cylinder pressure goes down. Since horsepower is a calculated number, the HP continues to go up due to more power pulses per minute. Also, if the crank is changed to one with a longer stroke, the same amount of push on the pistons and rods will normally be a higher power output because there is more leverage to rotate the crank. This is why we do not rate rods by power. There are simply too many variables that effect power.” There's more, too, from Molnar’s supposition: "Consider the 15,000+ 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. Ovalation takes place at TDC of the exhaust stroke and has very little to do with combustion pressures.” Take the time to ponder this pressure scenario: What really keeps the 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 when you really break it down, rod bolts are nothing more than a series of spiral wedges designed to create enormous forces (which, in turn, counteract the forces trying to ditch the crank out the bottom of the cylinder block). There’s no question - precisely designed and manufactured fasteners are more than chunks of steel with threads on one end and a hex head on the other. Selecting the right bolt and then installing it correctly is a primary factor in obtaining maximum service from your connecting rods. This is why Molnar only uses ARP bolts that are custom made for them. These bolts make them an upgrade over what many sell as an upgrade. The installation of a fastener is a primary element when it comes to connecting rod longevity, but regrettably, there's quite a bit of controversy and misinformation out there when it comes to hardware. There are people who 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 which were acceptable yesterday might not be viable today. According to Molnar, in order for an engine 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? Clamping Loads… Molnar shares some very 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% to 80% of its yield point which allows for a 20-25% safety factor. "Calculating the clamp load you are actually getting can be complex. A correctly designed bolt, for example, will have 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." Rod Bolt Ultimate Tensile Strength… 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, which actually refers to "ultimate tensile." In essence, the more a given material is stretched, the closer you bring the material to the yield point. Ultimate tensile is the absolute point where the effective clamp load decreases as the fastener continues to stretch. Molnar notes that failure occurs when the material exceeds its yield point; the clamping force at the joint drops to zero. Modulus of elasticity (Young's Modulus or stiffness) for a carbon/iron material is approximately 30 million pounds. Modulus does not change when the material is yielded. Interestingly, modulus does not change with hardness either. A piece of steel will have the same stiffness at 24Rc or 60Rc. There might be more here than meets the eye. Molnar tells us 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." This is the reason 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 may develop stress corrosion, which is a special form of hydrogen embrittlement in which the material is attacked while under stress. This occurs when the hydrogen in the air we breathe 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). “Due to the way bolts are processed today, we do not see much hydrogen embrittlement anymore - but corrosion can still be an issue, especially with carbon/iron bolts at high hardness. A bolt with an undercut diameter of 0.360-inch has a cross section of 0.1018-inches squared. If this bolt is stretched to the point it provides 18,000 pounds of clamping load, the undercut area will be subject to 176,817 pounds per inch squared. If corrosion (simplified as rust) starts, it reduces the cross section, making the unit loading go up. Since the load is now higher, the corrosion will now advance, causing less cross section, increasing the unit loading. It feeds on itself until the bolt fails completely resulting in catastrophic engine failure. This is not a bolt failure and was (in this case) caused by simply improper handling of the bolt.”

Molnar incorporates rod bolts manufactured by ARP. These are 7/16-inch ARP 2000 fasteners. ARP2000 is alloy steel that can be safely heat treated to a higher level, producing a greater strength material than 8740. While 8740 and ARP2000 share similar characteristics, ARP2000 has a tensile strength of 220,000 psi. Stress corrosion and hydrogen embrittlement are typically not a problem, providing care is taken during installation.

Molnar Technologies maintains strict dimensional specifications on their connecting rods. Case-in-point: The journal ends are honed to +/-.000050 (fifty millionths).

In the manufacturing process, the taper, bell mouth and barrel are machined to less than 0.001-inch, while the wrist pin bushing is honed to + or - 0.0001-inch.

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