In-depth Look At What Puts ARP Up On Top


It’s hard to imagine a world without bolts. Bolts and nuts are literally what holds nearly everything we use together, especially things that will have to come apart eventually. While bolts are used in limitless applications, they are most certainly not all created equal. Some applications allow for a standard lower grade bolt while others, such as high performance engines, need to be able to withstand immense forces.

ARPbottomsideofnutIn 1968, racing enthusiast Gary Holzapfel started noticing a pattern in his friends’ broken down performance vehicles. He found that the majority of the failures were caused by a fastener that simply couldn’t withstand the forces it was being subjected to. As an experienced fastener designer and maker for a leading aerospace subcontractor, Holzapfel began manufacturing custom hardware to solve these problems. His company, ARP (Automotive Racing Products) started out as a garage workshop and grew into the highly diversified and world renowned fastener manufacturer it is today.

Superior quality material and design allow ARP to manufacture fasteners that greatly exceed stock strength. Chris Rashke, ARP

Based in sunny southern California, ARP has several locations in Santa Paula and Ventura over 170,000 combined square feet dedicated to design and production of their fasteners. Covering around 4,000 products, the ARP line up provides solutions for OEM replacement parts and more exotic high performance applications such as Formula 1, NASCAR, NHRA and of course, diesel power plants. Follow along as we put ARP and their fasteners under the magnifying glass!

What is the purpose of a bolt?

In the shortest terms possible, bolts are designed to hold things together without permanently bonding them. To must be able to maintain the clamping pressure in an application after years of use. Bolts aid in the ability to make improvements or repairs without the complete replacement of the unit. Most automotive manufacturers design their fasteners for standard conditions, though ARP has products for those pesky factory blunders that we often find. As automotive enthusiasts, we tend to push our rigs well beyond what the engineers intended, requiring an upgrade to the hardware, so that a failure is much less likely.

Understanding The Numbers

Definition of terms:

Stress: Stress is the force per unit area that a material is experiencing. (10 lbs hanging from a 1″ diameter rod, the rod would experience 12.7 psi (10/pi*0.5^2) of internal force (excluding the weight of the rod)).

Strain: Strain is a measurement of how much the material stretches or grows.

Yield Strength (YS): The amount of stress a material can handle before it starts to permanently deform.

Ultimate Tensile Strength (UTS): The maximum stress a material experiences.

The number manufacturers quote is the UTS not YS. Most engineering formulas use UTS, so it is the more relevant number in engineering applications. For the normal guy, these high numbers are hard to understand.

So, ultimately what does it mean if the bolt can handle 200,000 psi? Without doing some calculations there is no direct relation for us. We just need to know that the higher the number, the stronger the material. Even if you back out the area of the bolt to determine the force required to break, that only tells you when it will break. Headgasket failure may happen before that, because the material stretched before it broke. The resulting stretching, could have allowed the headgasket to fail.

The big key here, higher number equals better bolt.

Studs or Bolts?

Bolts and studs ultimately provide the same purpose. Bolts allow for tighter spaces, making installations of components like cylinder heads easier within the confines of an engine compartment. When torquing down a bolt, however, it is put under different stresses than a stud. Both a stud and a bolt are under a normal force (stretching along the length), but bolts also experience a twisting forced called torsional shear. The combination of these two forces reduces the maximum amount of clamping force. A stud made out of the same material as a bolt will be able to handle more clamping force before it fails.

Currently there is a trend with OEMs to use what is called torque to yield bolts. Yielding happens when a bolt begins to stretch past its elastic region. Most material can stretch some amount and when let go, will return to its pre-stretched length. Once a bolt has yielded, it will no longer return to its original length. There is a chart in the side bar that shows two different types of bolts. One is a brittle bolt and the other is a ductile bolt.

Brittle bolts really don’t yield, they just fail, but ductile bolts will start to stretch before they break. If you have ever been tightening a bolt and then all of a sudden there is very little resistance and the bolt is still turning, the bolt just yielded. For some ductile materials, there is a point where more stress can be had after the yield point. This is the concept of a torque to yield bolt.

Many cylinder head bolts are torque to yield, making them a one time use product. If you ever need to remove the head and reinstall it, you’ll need to get new bolts. With a premium bolt or stud like ARP, if it hasn’t yielded, then the fastener can be reused. The key is to follow the proper torque settings as they are built to withstand a torque rating well below their yield point.


How does ARP select the material they use?

ARP uses several different materials depending on the application. ARP2000 covers the majority of moderate to extreme builds. ARP2000 has a martensitic crystalline structure and it has been quenched and tempered. (Bottom line, it has very good harness and toughness characteristics after it has been processed.) This stainless steel was initially developed for use in steam power plants and it was designed for high temperature environments. It can safely be treated to a higher level, giving a much greater strength than 8740 chrome moly. It is tempered above the “temper brittle zone” (between 500° and 700°F) temperature range and has a strength between 200,000 and 220,000 psi. Stress corrosion and hydrogen embrittlement (a process by which metals become brittle and fracture after exposed to hydrogen) are generally a non-issue as long as they are kept free of moisture and are properly lubricated.

When engineering a new product, ARP goes through many tests to ensure their product will far exceed the strength requirements of its application. Once the product has been developed and is in production, testing continues to ensure the highest quality parts are being produced. The image on the left is a tensile test. A sample is put into the test jig and pulled until it fails.

For a higher strength rating, Custom Age 625 + material is used. ARP 625+ is known as “the ultimate fastener material available today.” This newly formulated super-alloy demonstrates superior fatigue cycle life, tensile strength, and toughness. The ARP Custom Age 625+ bolts provide complete resistance to corrosion and oxidization making it the ideal material of choice in the high strength, super-alloy applications. ARP was the first to utilize the Custom Age 625+ material in manufacturing and testing procedures. Typical tensile strength of a Custom Age bolt or stud is 260,000-280,000 psi.

As a testament to our commitment to quality, we became ISO 9001 and AS 9100 registered.

With steels, as the strength goes up, the overall toughness decreases. At too high a strength, the metal ends up being too brittle for some applications. Threaded areas are much more brittle due to the inherent design of threads. A tool steel that can be heat-treated to 350,000 psi would make a terrible bolt because of the threads. “Metals are like a spring, put a load on them and they will stretch. Double the load and they will stretch double,” said Chris Rashke, Director of Sales and New Products for ARP. For connecting rods, this is important. When measuring the stretch, you are really measuring the load. Measuring the load and stretch of a given size and configuration of a bolt will indicate how much load is stretching the bolt.

materials chart


When manufacturing their bolts and studs, ARP uses a very efficient “cold heading” process. (Cold heading is usually done at room temperatures, which is much cooler than most metal forming processes.) ARP holds several patents on cold heading procedures for higher nickel and cobalt based alloys. Starting with a softer material in wire form on a spool, the production process work hardens the bolt head and under-head area to the desired hardness. Using a process called power extrusion, the shank is reduced and hardened giving the entire bolt even strength and hardness from the edge to edge.

There is quite a bit of difference between hot working a material and cold working the material. Cold working requires a lot higher forces and more power. The end product difference can really be seen in the effects of work hardening. Lower temperatures produce more work hardening resulting in a stronger part. ARP does not use the typical aerospace hot heading method because the hardness immediately under the head ends up softer and heat treatment processes are not able to bring the partially annealed area back to full hardness.

What does adding a lubricant do?

Installation pre-loads are the amount a fastener is stretched during installation. ARP Ultra-Torque lubricant aids in a more consistent and repeatable pre-load. Ultra-Torque is metal free and will aid in achieving a targeted pre-load on the first torque cycle. Lubrication, also, prevents seizing and galling on all threaded fasteners as well as preventing rust and corrosion. The chart below shows how important it is to use Ultra-Torque. Most assembly lubricants fall short of providing the proper fastener pre-load. ARP went through an extensive two year testing period to measure the torque applied and the resulting pre-loads while they were developing Ultra-Torque.

ARP Ultra-Torque Fastener Assembly Lubricant is used on both bolts and nuts to help provide a much more consistent pre-load. Each kit comes with a packet or two.

ARP, also, tested three of the industry standards while they developed their Ultra-Torque. Each of the four lubricants were tested in the same fashion and conditions for an accurate comparison between them. The Cycle Test had a targeted pre-load of 18,000 lbs for each test. One stud was used to test each lube through ten consecutive pulls. The torque applied to the stud was 120 lb-ft. and lubricant was re-applied after five consecutive cycles (tighten/loosen). The use of oil resulted in a very sharp incline through the torque cycle starting far below the 18,000 lbs objective at just below 14,000 lbs. Moly and EPL fared a bit better but still had a 3,000 lbs differentiation between the first and last pull. ARP Ultra-Torque lubricant stayed within its nearly spot on 18,000 lbs first pull through the entire 10 pull series.

A Consistency Test was also performed using the four different lubricants. The Consistency Test is equally, if not more important than a Cycle Test considering it compares a series of bolts in relation to each other when used together, like a cylinder head for instance. 10 studs were used for each lube using one pull on each stud at 120 lb-ft and the target pre-load was again set at 18,000 lbs. Each of the other three lubes ranged wildly through the pattern. In addition to having more than 2,000 psi swing in most instances, the average pre-load was far below the desired 18,000 lbs. The ARP lubed bolts again stayed within a 5% range of each other.  Uneven and/or low pre-loads can cause head gasket or major bearing failure.

There are countless opinions on the web regarding fastener assembly lubes and whether or not they should be utilized. Each application may be different, though ARP recommends use of their Ultra-Torque lube with all of their fasteners. Ultra-Torque Fastener Assembly Lubricant is available in four sizes ranging from a small blister pack to a 20 ounce brush top bottle.

ARP fasteners go through a very specific production process and the parts are regularly checked for accuracy.

When should you upgrade your fasteners?

Some engine applications require a fastener upgrade straight from the OEM factory. For custom builds and enhanced engines it will depend on what components are being used, though it’s pretty clear that ARP fasteners have many benefits that far outweigh the pros of using a stock bolt or stud. When in doubt, upgrade. Even if you are not in doubt, the use of a performance bolt will increase the strength and durability of your assemblies.

What products does ARP offer?

ARP has a full-color catalog on their website that is free and downloadable. The entire book covers their expansive product line in great detail, with clearly marked sections. They have applications that cover just about every aspect of any vehicle’s driveline and powertrain. Intake specific fasteners, flywheel and flex plate bolts, stainless steel hardware as well as a full line of diesel specific products. Head stud and main stud kits are available for all the major diesel engines, as well as several of the not so popular power mills. Rod bolts, valve cover bolts and harmonic damper bolts are all available for Cummins, Duramax, and Power Stroke engines, allowing enthusiasts to take their street machine to the next level, without worry of fastener failure.

ARP is a one stop shop for all critical hardware needs. Growing from a one man shop to over 280 employees, the company has built its business around premium quality products. Racers around the world rely on ARP every time they line up for competition. ARP is a family company three generations of Holzapfels work the factory floor every day and they pride themselves on building the best products possible. For more information on ARP and their product line, please visit ARP’s website.

Article Sources

About the author

Jake Headlee

Jake's passion started at a young age wrenching on cars with his Dad. Obtaining that glorious driver's license sparked his obsession with grease and horsepower, and the rest is history. Soon, he was a general mechanic and suspension specialist, and currently designs and modifies products for the off-road industry. Jake enjoys rock crawling, desert racing and trail running, and writing in his spare time.
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