ARP Bolt Talk: What Is Torque Spec And Why Fastener Torque Matters

For years ARP has explained bolts as being like little springs. Sounds a little weird, but for a bolt to properly hold, it needs to stretch a little. Once the fastener is tightened to a predetermined torque spec, that stretch holds pressure against the components that make up the joint being fastened. This spring-like stretch holds everything securely together.

What Is Torque Spec?

Most fasteners that are used to hold together important joints like cylinder heads to the engine block have specified tension values and procedures to ensure the joint stays tightly connected. If you don’t have the proper stretch on a fastener, it could loosen with vibration or heating cycles. If a bolt is stretched too much, it could break or weaken due to metal fatigue.

torque spec

Measuring bolt stretch with a bolt stretch gauge is one of the best methods of checking preload.

The tension applied to a fastener is measured by the amount of torque (twisting force) that is utilized to fasten it. Using a specified torque on components that must be held secure without warping, like the cylinder heads we mentioned above, is critical to evenly secure the components.

Torque is determined by multiplying the force applied by the distance from the point of the application of that force. This is an important point to understand because torque wrenches are calibrated by length.

torque spec

The measured length in the space defined as “A” is the effective length of the torque wrench. Adding an extension or crow’s foot that changes the length, changes the torque value at the fastener.

This becomes an issue when a crowfoot wrench or socket extension is added to the torque wrench and the distance from the fastener and the spot where the force is applied on the handle of the torque wrench has changed. In practical application, the torque applied to a fastener could actually vary if the user’s hand on the torque wrench is placed higher or lower on the handle.

Why Does Fastener Torque Matter?

Tightening fasteners to torque spec creates tension as the two or more components are fastened to resist pulling apart or separating. Once the fastener is holding the joint tight enough to prevent the components from sliding or moving, additional torque is used to stretch the fastener so it acts as a solid spring. As we’ve already determined, this stretch provides the clamp that holds the joint together.

Not all fasteners are made the same (i.e. with the same material). This is where fastener grades come from. To prevent material failure, the clamp load should never exceed the tensile load. Selecting the right fastener for the job is critical in creating a joint that will last.

A common example is when an enthusiast adds a power adder like NOS or supercharger. Looking to save a few pennies, the stock OEM head bolts are employed. This creates a situation where the fasteners may experience material failure when the tensile load is greater than the clamp load capability of the bolt. The fastener could stretch to the point of failure.

When this happens, the bolt is often blamed for the failure when it was simply not the proper strength range for the upgrade. When components are upgraded to create more power, upgrading the fasteners to these components should also be considered. This applies to engine and suspension components alike.

torque spec

With “L” representing the effective length of the torque wrench, “A” represents the measured length of the adapter. Using the formula (TA x L) divided by (L + A) the new setting (TW) is the accurate fastener torque with the adapter. Because of the added length and leverage, this new setting will be displayed as a lower number on the indicator.

How To Determine Proper Torque Spec

It is easy to understand how proper torque spec is vital to the function of the bolt. Proper torque can be determined by several factors. We’ve mentioned bolt stretch several times already and alluded to material failure when a bolt is stretched beyond its elastic limit. Fastener material is probably the most significant factor in determining proper torque.

There are many applications where fasteners are there to make sure the bolt is snug. For applications like valve covers, water pumps, and other accessories, tight enough is just good enough. Some mechanics refer to this as “German Torque (Guttenuff)” or use the calibrated elbow torque on the “uggadugga” scale. These are typically fasteners that get reused many times without experiencing a problem.

Many critical bolts are made of steel, which resists stretching more than some other materials. Another factor that is often overlooked by many enthusiasts is the substrate material being held in place by the fastener.

Ideally, the fastener material and substrate material will be similar enough to share the joint preload. A steel bolt should be used with components that resist compression at the same rate to balance the clamping pressure.

Another major factor in determining proper torque spec is bolt diameter and class. Combined, the size and material define a fastener’s tensile strength. Many bolts are graded by industry standards like SAE J429 which covers the mechanical and material requirements for inch-series fasteners used in the automotive industry. ISO 898 governs the standards for metric-size bolts.

torque spec

If you must use an adapter or extension and do not trust your math skills, you can simply line the adapter at 90 degrees and take out all the arithmetic work.

Hitting The Right Torque Spec

Correct torque involves a lot more than just slapping a socket on a torque wrench and cranking away till you hear a click. With the friction and resistance created by different fasteners, materials used in the fastener and the receptacle, cleanliness of the threads, lubrication used, and other factors, the torque between two identical fasteners can differ by as much as 30-40 percent. Even more in some cases. With all these variables, getting the correct torque spec applied sounds like a coin flip.

torque spec

Before we are ready to call it a hit or miss function, there are some things that can be done to make torquing fasteners a more precise action. Companies like ARP understand installation pre-loads (fastener stretch) are much higher in engine installations and drivetrains these days. They spent a lot of time and money to develop their own assembly lubricant, ARP Ultra-Torque, to help with consistent, repeatable, and accurate target preloads. The best feature of the Ultra-Torque lube is an accurate preload can be achieved on the first torque cycle! The days of cycling through the torque procedure three times to achieve proper preload are over with Ultra-Torque assembly lube.

According to ARP’s published technical documentation: “The [standard] torque method is sometimes inaccurate because of the uncertainty in the coefficient of friction at the interface between the bolt and the rod. This inaccuracy can be minimized by using the lubricant manufactured by ARP.”

ARP constantly reminds everyone to read and follow the specifications provided with each package of fasteners.

Almost every expert agrees, engine and drivetrain bolts should never be installed with dry threads. In the past, thread locker, silicone sealant, engine oil, or assembly lube have all been used as recommended for specific installations. Some of these are still called for in certain installations. ARP is very adamant about reading the instructions that come with each particular fastener. Even if you have done the procedure a thousand times, ARP recommends reading the instructions each time as specifications sometimes change due to improvements.

 

Torque Wrenches And The Torque Process

For many enthusiasts, the torquing process seems simple. You grab a torque wrench with the appropriate socket and torque a bolt to the pre-determined torque spec. If that is you and all you are looking for is getting somewhere in the same galaxy as the specified torque, then by all means … roll with that procedure, but understand there is so much more involved in the process.

Now that you understand the importance of getting the proper preload on a fastener, the next step is understanding the different types of torque wrenches available and how to use them. Torque wrenches are precise measuring tools and should be treated as professional measuring instruments. Ensure it is stored in a protective box and protected from shock.

Don’t be fooled by the length of a torque wrench. Most of them are longer for leverage but they are not a breaker bar. A torque wrench is a tightening instrument and should never be used to loosen bolts or fasteners.

Because some torque wrenches are longer and have more leverage, it is easy to apply pressure with one hand in a smooth, continuous motion. Most torque wrenches have an indicator mark showing where to grip the wrench for the best results. Do not use an extension or cheater bar on the handle of a torque wrench as this impairs the correct torque signal.

torque spec

Over-tightening a bolt can have disastrous results.

It is easy to over-tighten a fastener, even with a torque wrench. Use caution to avoid over-tightening a bolt when the set torque spec is reached. Even a “click-type” torque wrench, which is designed to prevent over-torquing, can accidentally over tighten a fastener. If this happens, loosen the bolt with a ratchet or wrench, then reapply the proper torque spec to the fastener.

Torque Wrench Care

We have already mentioned that torque wrenches are precision measuring instruments that should be respected and treated with care. In addition to storing the device properly, there are a couple of things to keep in mind. If you are using an adjustable “click-type” torque wrench, reset the torque wrench to the lowest value before putting it away. This releases the spring pressure and prevents fatigue. Never set a “click-type” torque wrench to zero as the internal mechanism requires a small amount of tension in order to prevent components from shifting and reducing their accuracy.

Most companies involved in automotive maintenance along with quality fastener companies like ARP advocate regular frequency calibration for torque wrenches. This helps with torque accuracy. In fact, ARP has offered free torque wrench testing at NHRA events and has for many years. According to ARP’s PR agency, “it’s not uncommon to see an error of 35 percent or higher.”

ISO standard 6789 covers the construction and calibration of hand-operated torque tools. This standard sets the re-calibration for torque wrenches at 5,000 cycles of torquing or 12 months, whichever is the soonest. The American Society of Mechanical Engineers (ASME) standard echoes the ISO standard for re-calibration. These standards are repeated by tool manufacturers and distributors like Chicago Pneumatic. Not to mention, if the tool has been dropped or damaged, it must be sent to service immediately.

Where To Go For More Information

If you would like more information on bolt torque or ARP products, visit them online at www.arp-bolts.com. They have a full tech section published at technical.php. ISO 6789 can be previewed here. Details on ARP’s Ultra-Torque assembly lube can be found here.

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
Read My Articles

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