Torque and Friction

For any fastener to function properly, it must be stretched. It’s the material’s ability to be placed under tension and then try to pull back to its original length (sort of like a rubber band) that provides the clamping load we need. Different materials react differently to the stress of stretching under different conditions, so it’s important that the fastener is designed exactly for the load it must bear.

Depending on differences in friction, torquing a bolt with a properly calibrated torque wrench to a specific reading does not always produce the same amount of stretch. If a fastener is over torqued and stretched too far, it won’t be able to pull back and is ruined. This is known as exceeding its yield strength.

This is why it’s important to remove friction from the equation as a variable by using some type of lubricant. You also need to make sure all the threads on both the fasteners and in the bolt holes are clean and free of debris. You can usually do this with your engine block by using a thread chaser or even gently running a thread tap through the holes.

Motor oil is a commonly used lubricant because it’s readily available, and your engine is going to be filled with it later on, anyway. But it isn’t necessarily the best option for getting consistently accurate torque values. If low friction is desired in order to install the fasteners with less torque (thus, less twisting deformation) you can use special, low-friction lubricants. Don’t worry, using a lubricant won’t cause a fastener to come loose, but it also has the added benefit of reducing the chances of two materials galling and sticking together. So lubricant will not only improve your ability to build an engine, but also makes teardown a lot easier.

With special lubes, the torque necessary to achieve the proper bolt stretch can be reduced as much as 20 to 30 percent. Of course, the opposite is also true: If you lubricate a fastener with a low friction lubricant and then torque it to a value designed for motor oil, the chances are probably good that you’ve just exceeded that fastener’s yield strength. So it’s always important to match the torque applied to the lubricant used. Manufacturers, like ARP, often provide a chart of proper torque values with their fastener kits.

Another factor when it comes to torquing bolts is that some lubricants, like moly paste, can actually polish the threads as the fastener is tightened down. So even if you use the same torque value, the second time you tighten that same bolt you will experience greater rod stretch. ARP’s engineers have done extensive testing on this phenomenon and developed a lubricant specifically for fasteners.

It’s called “Ultra Torque” and it’s exclusive to ARP. A small package is provided with most bolt kits and it has several valuable properties. First, Ultra Torque is amazingly consistent. It doesn’t polish the threads of either the fastener or the block so even if the bolt is installed and removed multiple times, the same torque value will achieve the same result in terms of bolt stretch. The lubricant is also quite sticky and won’t wipe off, so it will still be on the threads during teardown. And finally, it has no metal in it (believe it or not, most moly-based lubes do) and it’s compatible with motor oils, so it won’t harm your engine’s lubrication system.

Measuring Stretch and Rod Bolts

Now that we’ve spent all that time saying that studs are preferable to bolts, we’ll reverse course while talking about connecting rods and tell you that cap screws (a fancy name for a bolt) are preferable over the old-school bolt and nut assembly when it comes to securing the big end of the rod.

Stock-style rods use a bolt that is pressed into the body of the connecting rod. After the rod cap is installed, two nuts are used to fasten the cap to the rod. The cap screw design is better because it eliminates the weight of the nut. In this design, the cap is properly located on the rod with two dowel pins. After the cap is in place, bolts extend through the cap and thread into the body of the rod to pull the cap down securely.

Rod bolts are also the most highly stressed bolt in any race engine. And because of that, it’s most critical that they are installed optimally. For most applications, it’s good enough to simply install the bolts or nuts on the studs with a specific lubricant and a torque wrench. But trying to hit the right amount of stretch gauging only by the amount of torque applied to the fastener can be a bit hit-or-miss.

As discussed earlier, variables—such as the type of lubricant used, the slickness of the threads, debris caught between the threads, and even the accuracy of the torque wrench—can affect the actual amount of stretch you get. But by using a bolt stretch gauge you can bypass all of these pitfalls.

The only time you can measure bolt stretch is on the rare occasions you can reach both sides of the bolt after it has been installed. This means it won’t work when threading the fastener into a blind hole, such as head bolts into the deck of the block. But this isn’t a problem with rod bolts, so the practice works well.

Using a dedicated rod bolt stretch gauge is easily the most accurate, and easiest, way to measure bolt stretch. And as long as you’re using the same bolt in the same rods with the same lubricant, it’s usually only necessary to double check the amount of stretch against your torque setting on the first couple of bolts. After that it’s pretty safe to put away the stretch gauge and depend only on the torque wrench.

Begin by installing the rod bolt in your gauge to measure its length. Most rod bolt manufacturers, like ARP, engineer divots into both ends of the bolt to make centering it up in the gauge easier. Once the bolt is in the gauge, zero out the reading.

Now, install the bolt in the rod and cap (it can be installed in the engine), make sure your lubricant is properly applied, and begin torquing the bolt in 5-pound increments, measuring the stretch as you go. The correct amount of stretch for almost all rod bolts is between 0.006- and 0.007-inch, but you may want to check with the manufacturer to be certain.

Once you get the proper amount of stretch, check the amount of torque required, and then double check your findings by repeating the process on a second bolt. If they are consistent, you can finish the rest of the bolts with just the torque wrench.

It should be noted that while building a race engine you’ll have the rods apart probably three or more times while they are honed, fitted with bearings and assembled. Checking the stretch for the first couple of bolts is a great idea at each stage of the process, not just final assembly.

If you worry that a rod bolt may be failing, take it out and check it in your bolt-stretch gauge while it’s in a relaxed state. If it’s 0.001 longer than its original length, you should consider it a failed bolt. We all hate throwing away a bolt over a thousandth of an inch—and possibly putting your engine build on hold while you source a replacement—but look on the bright side: You may have just saved yourself from a costly engine failure.

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