Connecting Rods 101 – Part 3

Wayne Scraba

26 Apr 2018

Click Here to Begin Slideshow Over the last couple of issues, we’ve dug deep into what makes for a strong, race-worthy connecting rod. Thanks to Tom Molnar (Molnar Technologies), we’re given the opportunity to learn about rods, materials and bolts from a person who has been a leader in the race industry for decades. With this segment, we’ll connect the dots right to the end. But first, let’s start with the torque required to actually tighten a rod bolt. You might find it’s not all what it might seem to be: Overcoming Friction … Torque is defined as the amount of friction that must be overcome to cause a nut or bolt to turn. Manufacturers of most quality fasteners designed for racing 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. Molnar states that of all the effort applied to a given fastener, 50% of the torque is used to overcome the friction under the head of the bolt and 40% is required to overcome the friction between the mating threads. This means that of all the torque applied, 90% is required to overcome forces that have absolutely nothing to do with the job at hand. Tom Molnar sums it up this way: "You can look at this as a torque wrench being only 50% accurate." Dwell on that for a moment. It's easy to see that friction is an extremely challenging problem because it has so many variables and, of course, is difficult to control. Molnar notes that many fastener manufacturers recommend using the stretch method of measurement since the pre-load is closely controlled and is obviously independent of friction. Tom offers more information with regard to fastener 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 changes due to the mating surfaces of the bolt flange, rod spotface and threads changing, which means the proper torque is a moving target.” Fair enough, but one also must consider thread lubricant. Thread lubricants are absolutely critical since they are the primary element when determining friction. In many racer shops, it's common to use good old-fashioned motor oil as a lubricant. Why? That's easy. It's readily available. Everyone has a partially full bottle of their favorite engine oil sitting alongside a workbench. Unfortunately, there's a problem with this approach: 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%. The reverse is also true. 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. Engine oil is a good hydraulic bearing material, but it is a poor extreme pressure lubricant. If it is used on bolts, the torque required actually increases, 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 is also a contributor to the friction factor. For example, a black oxide finish on a bolt behaves differently than a fastener that has been polished. Because of this, it is very important to closely observe the tightening recommendations supplied with each type of bolt. It's 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. Tom 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." Finally, Tom points out: “Another bolt myth is when a certain HP level is reached, the bolts need to be upgraded. Changing bolt types does not make the rod stronger. Since power is created when the combustion pressure pushes on the piston and the only load a bolt sees is a tension (pulling) load when the piston reaches TDC on the exhaust stroke, the bolts do not know if the engine is making 100 HP or 1,000 HP. This high tension pulling load is based on piston / wrist pin / ring weight, stroke and RPM.” In the end, you can see there is a huge collection of “secrets” when it comes to the connecting rod part of the bottom end equation. And while plenty of connecting rods look alike (at least at first glance), there are many differences between them. For a closer look (and even more info), check out the accompanying photos.

Connecting Rods 101 - Part 3 — Click Here to Begin Slideshow Over the last couple of issues, we’ve dug deep into what makes for a strong, race-worthy connecting rod. Thanks to Tom Molnar (Molnar Technologies), we’re given the opportunity to learn about rods, materials and bolts from a person who has been a leader in the race industry for decades. With this segment, we’ll connect the dots right to the end. But first, let’s start with the torque required to actually tighten a rod bolt. You might find it’s not all what it might seem to be: Overcoming Friction … Torque is defined as the amount of friction that must be overcome to cause a nut or bolt to turn. Manufacturers of most quality fasteners designed for racing 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. Molnar states that of all the effort applied to a given fastener, 50% of the torque is used to overcome the friction under the head of the bolt and 40% is required to overcome the friction between the mating threads. This means that of all the torque applied, 90% is required to overcome forces that have absolutely nothing to do with the job at hand. Tom Molnar sums it up this way: "You can look at this as a torque wrench being only 50% accurate." Dwell on that for a moment. It's easy to see that friction is an extremely challenging problem because it has so many variables and, of course, is difficult to control. Molnar notes that many fastener manufacturers recommend using the stretch method of measurement since the pre-load is closely controlled and is obviously independent of friction. Tom offers more information with regard to fastener 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 changes due to the mating surfaces of the bolt flange, rod spotface and threads changing, which means the proper torque is a moving target.” Fair enough, but one also must consider thread lubricant. Thread lubricants are absolutely critical since they are the primary element when determining friction. In many racer shops, it's common to use good old-fashioned motor oil as a lubricant. Why? That's easy. It's readily available. Everyone has a partially full bottle of their favorite engine oil sitting alongside a workbench. Unfortunately, there's a problem with this approach: 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%. The reverse is also true. 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. Engine oil is a good hydraulic bearing material, but it is a poor extreme pressure lubricant. If it is used on bolts, the torque required actually increases, 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 is also a contributor to the friction factor. For example, a black oxide finish on a bolt behaves differently than a fastener that has been polished. Because of this, it is very important to closely observe the tightening recommendations supplied with each type of bolt. It's 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. Tom 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." Finally, Tom points out: “Another bolt myth is when a certain HP level is reached, the bolts need to be upgraded. Changing bolt types does not make the rod stronger. Since power is created when the combustion pressure pushes on the piston and the only load a bolt sees is a tension (pulling) load when the piston reaches TDC on the exhaust stroke, the bolts do not know if the engine is making 100 HP or 1,000 HP. This high tension pulling load is based on piston / wrist pin / ring weight, stroke and RPM.” In the end, you can see there is a huge collection of “secrets” when it comes to the connecting rod part of the bottom end equation. And while plenty of connecting rods look alike (at least at first glance), there are many differences between them. For a closer look (and even more info), check out the accompanying photos.

On the piston side of the rod, a precision bronze wrist pin bushing is incorporated. Molnar includes a pair of pin oilers drilled from the bottom side up in the beam.

The use of appropriate assembly lube is extremely important when dealing with connecting rod fasteners. Once the rods are properly cleaned to remove all dirt and foreign oils, spread the rod bolt assembly lube on the threads. Be certain to spread the lube under the head of the bolt prior to beginning the tightening sequence. There is more information on the proper method of lubricating and tightening connecting rod bolts in the text.

This engine was buttoned up by way of a common clicker torque wrench. The reality is, there are much better methods for obtaining the proper torque on something like a connecting rod bolt. Using a stretch gauge or using torque-angle is far superior.

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