Critical Torque: Fast Facts About High-Performance Fasteners

We’ve all heard that a chain is only as strong as its weakest link. We learn that lesson in our high-performance cars too, don’t we? As we improve one part of the car, another part is suddenly pushed past its capabilities and either stops performing correctly, or breaks. Add engine power, and the transmission or clutch fails. Upgrade the transmission, and the driveshaft or rear axle goes South. Get the entire drivetrain up to snuff, and then the tires spin effortlessly. Upgrade the tires, and the suspension gets tweaked or you learn what wheelhop means. You get the idea!

It’s a similar battle inside your engine. Once you start adding power, you find that weak point that’s simply incapable of keeping up with the program. Most of us choose to build complete engines after bolt-ons reach their limit, and once that bridge is crossed, the weak link almost always becomes the hardware holding the engine together.

The fasteners used when the engine is assembled at the factory are selected for the power level of the base design. Once we begin upgrading an engine to produce more power, or even to produce the same power for a longer duration of time (like an endurance racing engine, or a marine application), it’s time to bring higher-strength bolts, studs, nuts, and security hardware into the game.

We have heard stories of builders using stock replacement rod bolts, or even re-using hardware during a rebuild, but we could never do it ourselves. We’d certainly never recommend reusing any main bearing or connecting rod fastener. Upgrading to higher quality, higher strength fasteners is the kind of insurance we happily pay for. It’s easy enough to find premium bottom-end hardware, but should we go with bolts, or studs? How should they be installed? What about the lubricant on the threads? We understand that a shift in hardware will generate a lot of questions, and we’re here to answer as many as we can. We teamed up with the experts at Automotive Racing Products (ARP) who specialize in high performance fasteners, and they shared some solid technical information and installation tips with us.

While they may not share the spotlight with other aftermarket parts, the fasteners used in this rod are not only a selling point of the item, but are a key part of its power-holding ability.

Rolled Threads vs. Cut Threads

There are two ways threads can be formed on a fastener: rolling and cutting. The rolling thread process involves use of dies to press the threads’ form into the fastener. You can imagine the effect on the grain of the metal as a result: it gets forced into curved waves as the threads form, and the cold rolling of the material by the hardened dies forces it into a more stable, strong, and resilient fastener.

Cutting threads, by comparison, removes material and leaves actual microscopic tears in the metal surface of the fastener. Not surprisingly, these small rips and defects in the threads provide a place for cracks and breaks to begin. Creating stress risers is not a good idea for a critical performance application. That is why ARP recommends that you insist on rolled threads for your high performance hardware, and leave cut threads to non-critical fasteners.

Fastener Materials

Over 90-percent of all fasteners are made of carbon steel. There’s a good reason for that – it’s affordable, and can be tailored to maximize a wide range of attributes through simple alloy changes. The adjustment of carbon content, and factors like heat treating can make the fastener more durable. The addition of manganese, silicon, copper, molybdenum, nickel, or chromium in small amounts will alter the capabilities of the fastener, and can improve its capacity to be heat treated. This opens the door to a wide range of strength-to-ductility combinations. ARP also offers more exotic materials (like titanium) for more exotic applications where weight is a factor (and budget is not). They also offer incredibly high strength materials that are capable of over 300,000 psi of tensile strength.

Stainless Steel

Stainless steel is also an iron-based alloy, but it contains at least 10.5-percent chromium. The addition of the chromium alters the surface of the fastener and makes the material corrosion resistant. Naturally, there are many grades of stainless steel, as other ingredients can be added, and the percentage of chromium can be altered. If corrosion resistance or appearance is a concern (like when the fasteners are being used on the outside of the engine, like intake manifold bolts for instance) then stainless steel fasteners are a viable option.

By altering its elemental contents, the many stainless alloys have a much higher corrosion resistance, meaning they do not require a protective coating. That means they can be polished to a high shine for a stunning visual effect.

Heat Treatment

The process where a material is brought up to a very specific temperature and allowed to cool at a specific rate, in a very specific pattern is known as heat treatment. Heat treatment changes certain characteristics of metals and alloys. This procedure can make them more suitable for a specific application. The structure of the material means that heating it can alter the physical property of the material, and impact strength, hardness, and even wear resistance of the alloy. By heating and cooling in specific ways, the grain structure and grain size can be impacted, and make the material take on desired characteristics, and shed undesirable ones.

Fasteners can be further refined through hardening and/or tempering processes, but the additional costs involved put such work into a pretty extreme category. We just want really strong bolts and studs to hold our cranks and heads to the block, right?

Heat treatment is no simple matter. Besides the intense heat required, if you overheat or underheat the material, allow it to cool too slowly or too rapidly, or just do things in the wrong order, you can create a fastener that is as brittle as glass, and weaker than untreated material.

Bolts Vs. Studs

There are two main styles of fasteners available. A bolt has a fixed fastener head and single threaded shaft, while a stud has not fixed fastener head, and is, at its core, a threaded shaft which uses a separate nut to provide clamp load, and are generally regarded as an upgrade to bolts for certain applications. There are several advantages to upgrading to studs. The primary reason for this is because studs provide more accurate and consistent torque loading, which is critically important in a true high performance engine.

We want all the bottom-end fasteners and cylinder head fasteners to be loaded as equally as possible, and studs make this more consistent by relying on the finer threads used to torque them down. For example, when the stud is installed into the block, the coarse threads are used on the hole tapped into the block, which is the same one the factory bolts were screwed into and torqued down with.

On the left is a big-block Chevy head loaded for bear with a full complement of ARP rocker and head studs to secure everything in place. Note the chamfered washers used under each 12-point nut. On the right is the same big-block Chevy head, this time with ARP bolts. ARP head bolts are still a huge step up from anything the factory ever offered, and in some applications, they may be preferred over studs. However, if you can run a stud, we'd recommend it.

But, when the studs are being tightened to a final torque specification, the nut on the fine threads is used. Since the threads are finer, they have larger stress areas and are stronger in tension. Because the helix angle is smaller on finer threads, they permit a higher degree of accuracy and consistency from fastener to fastener.

A more accurate torque reading can be achieved when a finer thread is used, and the finer threads will deliver a torque position closer to each other as a result. Additionally, studs really aid in the alignment of components when you assemble them. There’s not any question everything is lined up properly when studs are used.

But, due to limitations beyond the fastener itself, there are many situations where a stud simply cannot be used to replace a bolt. For instance, if you know you’ll be removing the cylinder head while the engine is in the car (particularly in a V8 application), the use of studs means that many components under the hood will have to be removed before that head will come off. Think about the brake master cylinder, or the A/C system on the firewall.

You might be better off pulling the whole engine just to replace a head gasket in such a situation. Also, in some cases, the nuts atop the studs may interfere with the valvetrain (like rocker stands) or require some machining to clear the heads themselves. So, while a stud is always going to be the preferred fastener in a high stress application (like the cylinder heads or main bearing caps), we might not always be able to use them.

Precision Machining

The fasteners you choose for your precision engine must be held to the same kind of high tolerances that every other part is held to. The threads must be consistent, and critical areas (like the radius under the bolt heads, and the chamfers in the washers) must be both correct in a dimensional sense and consistent from part to part.

From the perfectly-cut threads, to the step on the shank, to the radius under the head, this single bolt showcases all of the precision design and machining required in a high performance fastener.

Bolt Finishes and Coatings

All carbon steel (non-stainless) ARP fasteners leave the factory with a black oxide coating. This is a simple and effective corrosion protector, and is not expected to last the life of the fastener. It’s not unusual for black oxide-coated fasteners to show some rust as they age. They can be cleaned, inspected, and re-used without detriment in most cases. Check with ARP if you have any doubts about fastener re-use. Of course, stainless steel fasteners are used without a coating, and they can be polished for show.

The black oxide coating is not intended to be a permanent corrosion solution, but if you keep the fasteners oiled until just before you install them, the coating will last as long as possible.

Prior Preparation Prevents Poor Performance

Purchasing premium quality hardware is a huge step toward improved security in your high-performance engine, but it’s not the end of the story. Preparation prior to installing the fasteners is incredibly important, as is the actual installation itself.

The use of a fastener involves two surfaces interacting: the threads on the fastener, and the threads in the hole. Both of them must be cleaned, as any foreign object or debris that exists between the two different thread surfaces will potentially damage them. Dirt in the threads means that the torque will not be accurate or identical to all of the other fasteners. Consider what a small chunk of metal, left in the threaded hole from the machining processes, will do to a fastener.

Clean all of the fasteners and all of the holes prior to assembly. We’ve used everything from wire brushes to compressed air to clean out bolt-holes, and we’d have to recommend the use of chaser taps too. These types of taps are designed specifically to clean the threads in bolt holes, and proper use of all these techniques and tools will give your fasteners the best-possible chance to do their job.

Before installation, take the time to clean every bolt, nut, stud, and washer carefully. The fasteners are machined parts, and must be cleaned prior to being lubricated or sealed for installation.

Lubrication

Properly lubricated fasteners will torque more consistently, which makes a great lubricant a critical part of the assembly procedure. While some people will use motor oil, or extreme pressure grease, the engineers at ARP developed a lubrication product specifically for this purpose called ARP Ultra-Torque. Use of this lubricant, in conjunction with proper cleaning and proper tightening, will result in very consistent performance from fastener-to-fastener and torque-cycle to torque-cycle. We’ve tested and used it a number of times before, and it really is all that it’s cracked up to be.

Security

Many of us are familiar with liquid thread lockers, like those made by Loc-Tite and Permatex. These products are effective in their various forms, and if the manufacturers’ instructions are followed, they can provide reliable fastener security. There are several different colors of thread locker, and different formulations can be had within a given color family, so they are not a one-size-fits-all product, and should be used carefully.

Additionally, there are mechanical means of holding fasteners in place, and this is most-necessary on hardware affected by heat and high stress. While lock washers and cotter pins might be the easiest way to hold a fastener in place, nothing beats the effectiveness of safety wire. If you know how to properly install it, safety wire (also called lock wire) loops through holes drilled in the bolt heads or nuts and literally ties the fasteners together so they cannot loosen up.

Sealing

It’s common practice for head bolts (or studs) to be threaded into a wet water jacket. This requires the fastener to be properly sealed to prevent any leakage. ARP makes its own excellent sealer, and by following the instructions during installation, it’s proven to be an effective leak preventative.

To ensure the optimal performance from these premium fasteners, they must be installed properly. This means a clean fastener, installed into a clean hole, with the correct lubricant or sealant (shown). If all the right steps are followed with every single fastener, the maximum benefit of the engineering inside them can be realized.

Torque

Measuring how tight a fastener is literally calculating how much that same fastener is being stretched based on its resistance to turning. It’s a very critical dimension, since having all the fasteners delivering identical pressure and clamping force on a given component (like a cylinder head or connecting rod cap) means the load is equally distributed. Any variance between torque figures means one fastener will be carrying a greater percentage of the load than any other, and this can become hazardous to the component being held (like a cylinder head, which can warp or leak under such conditions) or the fastener itself (like the connecting rod bolt).

There are three common ways to measure torque: using a torque wrench, measuring the angle of rotation (from a given point; typically from where the fastener is hand tight with zero preload), or by measuring the length of the fastener while tightening it for an actual stretch dimension. Of these three methods, the measurement of stretch is considered the most accurate, but access to both ends of the fastener is rarely available.

A good torque wrench that has been recently calibrated/certified for accuracy is perfectly sufficient. Also keep in mind, that when using a torque wrench, the actual clamp load created can be altered when a fastener is installed “dry” (without lubricant, sealant, or thread locker on it) or “wet,” and the manufacturer’s torque guidelines should indicate the condition of the desired torque measurement.

The team at ARP prides themselves on maintaining tight tolerances and repeatable measurements on all their products. Make sure to inspect every single fastener prior to installing it into your engine. If you see an inconsistent thread, or a chamfer that doesn’t match all the others, make certain to replace those parts. The smallest variance can make a big difference in clamping performance. You wouldn’t accept it in your piston rings or roller lifters, so don’t accept flaws in your critical hardware either.

As you can see, on an already-wide engine, like this Ford 5.0L Coyote, head studs can add considerable room needed to remove the heads in the engine bay. In an application such as this, a head bolt might be a better option, if the engine wasn’t coming out of the car.

Summary

Your engine is only as good as the hardware holding it together. It’s critical to invest in quality hardware, and then to take the time to install it correctly. To make all of these tasks easier, ARP has done the homework required. They have pre-packaged hardware kits available for the more popular engines, which not only ensures you have premium fasteners from top to bottom, but also makes it easy to assemble, since the kits are well organized and clearly labeled. They have also engineered premium lubricants and sealants to complement the hardware, which will help your investment in top-quality hardware pay off.

Fastener Facts

Fastener diameter can be measured either as a size number (i.e. #4 or #10; usually found on smaller machine screws) or as a direct measurement (i.e 3/8-inch or 8mm).

  • The Thread Diameter is the measurement of the outer diameter of the threads of a fastener. It is also called the “major diameter.”
  • The Shank Diameter is the measurement of the shank before the threads are cut into it. It is typically the same as the thread diameter. Both nuts and washers are sized by the shank diameter they fit. For example, a ½-inch nut fits a ½-inch bolt, and a ½-inch washer will fit that same bolt. The diameter of all common hex bolts, machine screws, studs, and socket head screws, is the shank diameter. This measurement is expressed in inches for US bolts and millimeters for metric bolts. Because this is approximately the same as the thread diameter, the thread diameter measurement can be used for fully threaded fasteners.
  • The Root Diameter is the measurement of the fastener shaft inside the threads – imagine if the threads had been shaved off, and what is left would be the root diameter. This is also called the “minor diameter.”
  • Thread pitch is defined as the distance between the threads, measured in millimeters. It’s how metric fasteners define how many threads they’ll use over a given distance. This changes in a linear fashion with the size of the fastener, so smaller metric fasteners have a lower thread pitch (as a result of their finer threads) and larger metric fasteners have a higher thread pitch specification because their threads are coarser. A 1.0mm thread pitch is finer than a 1.5mm thread pitch.
  • American (inch measurement) fasteners use threads-per-inch (TPI) as a defining factor. There are two common thread families: coarse and fine. Simply put, this is the number of threads cut into the fastener over the distance of one inch. For example, a 1/4-inch coarse (UNC) thread would be 20 TPI, while fine (UNF) would be 28 TPI, while a 5/16-inch fastener would be 18 TPI and 24 TPI, respectively.

Article Sources

About the author

Scott Parkhurst

Beginning his career in the U.S. Air Force, Scott transitioned from hands on positions assembling air craft and racing engines when he became Tech Editor of Popular Hot Rodding. He was later Editor of Engine Masters and is a published author.
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