ARP’s Secret Fastener Technology Lab


Why ARP Fasteners Are Your Best Choice for High Performance Engine Building

by John Baechtel

arp boltsARP fasteners have long been a staple of the performance and motorsports industries, but few people know the lengths ARP goes to to provide fasteners that exceed aerospace quality and performance in almost every aspect. The term state-of-the-art is widely used, but it is standard operating procedure at ARP and they are constantly seeking to rise beyond it with fasteners that far exceed requirements while providing the insurance that engine builders demand for their highly boosted and exotic fueled engines. Manufacturing quality is job one at ARP, but advanced research and testing procedures follow accordingly.

The company employs the most sophisticated testing and evaluation equipment available to ensure the quality of every fastener that leaves its facility. A large part of that philosophy is performance testing conducted to verify the mechanical properties and tolerances of components. To accomplish this ARP incorporates a full complement of Schatz USA advanced testing equipment at its main manufacturing facility in Santa Paula, California along with an identical setup in its main lab in Ventura, California.

ARP bolts


ARP boltsSchatz precision test equipment is used to establish and verify torque versus preload relationships for each type of fastener and it adheres closely to the internationally recognized standard ISO 16047 which determines the tightening characteristics of mechanical fasteners. The testing procedure determines the total friction coefficient, thread friction coefficient, the friction coefficient of the head or nut bearing surface, preload force at the yield limit and the breaking point force. By adhering to and exceeding these values, ARP ensures the fastening integrity of all its fasteners. This fixture can test any bolt, stud, nut or combination thereof for any of these performance qualities.

"The International Organization for Standardization (ISO) is an independent, non-governmental international standard-setting body comprised of representatives from various national standards organizations."

Mechanical and performance properties are also investigated in accordance with ISO 2320 which describes recognized testing and associated limits for prevailing-torque nuts. Schatz describes prevailing torque as important protection against loss of the nut and the ability to prevent loosening of the nut if it is not properly tightened. ARP thus quantified the varying degree of clamping force loss incurred by the continued reuse of nuts on cylinder head studs, main studs, rod bolts and other high stress fasteners that rely on nuts to ensure maximum clamping force. Tests conducted in association with noted engine builder Kenny Duttweiler verified the potential for a 20% or more loss in clamping force with used nuts depending on the condition of the nut and how many times it has been previously torqued. Imagine that much loss of clamping force on your cylinder heads with maximum boost in the cylinders and maybe a little detonation too. Scary.

Torque/Angle Transducers

ARP boltsInstrumentation within the test jig measures the required test parameters to verify the performance and repeatability characteristics of components being tested. The shaft of the torque/angle transducer is outfitted with strain gauges and the sensor signals and supply voltage are couple via very low wear slip rings. The transducer shaft is fitted with an incremental encoder disc with 360 apertures passing through a dual optical barrier sensor for very high resolution. The torque angle-pulse signal is further conditioned via two phase-offset signals corresponding to the direction of rotation.

Preload Force/Thread Friction Torque Transducer

arp boltsThe sensor heads of the preload force transducer and the thread friction torque transducer are fitted with high resolution strain gauges coupled through a full Wheatstone bridge circuit that measures the unknown resistance by balancing it against the known sides of the circuit. The thread friction torque transducer is braced against the preload force transducer with an axial bearing that permits thread friction torque to be independently measured. The housing unit supports the use of assorted adapters and perforated plates that allow various sizes of bolts and nuts to be tested as required.

Drive Motor

ARP boltsA powerful drive motor and controller are required to provide the twisting force necessary to test fasteners accurately. Through a combination of available gearboxes, the DBL series of synchronous servo motors offer a torque range up to 35,000 Nm at speeds up to 3000 rpm for the horizontal test device. The accompanying power and control unit can be powered via a 3-phase AC supply with a range of 230 to 480 volts. The controller permits the very precise movement of the torque shaft to ensure optimum accuracy throughout the operating range.

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Assortment of adapters, braces and fittings required to test a broad
range of ARP
nuts, bolts and studs for performance quality.

Instrument Controller

ARP boltsThe measurement control unit and associated software package controls, acquires and analyzes the specified testing regimes. It analyzes threaded fasteners under practical real world conditions. Measurement and control tasks are executed in real time with near real time plots graphically displayed on the monitor screen for instant evaluation and/or manual override if limits are exceeded. Robust software provides extensive personalization so the operator can specify the required analytical functions and reports.

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The monitor screen displays all recorded parameters together for easy
analysis and test adjustments. Large monitors are used so multiple
people can easily view them.


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Typical test printouts for analysis. Tests are also stored in the main database.


arp boltsOne of the most important things verified by this testing regime is the potential loss of clamping force possible with used nuts. While nuts are manufactured to exacting standards with premium materials they nonetheless offer a minimal amount of material to generate the required clamping force. Regardless of whether you're talking about a main stud (right) a head stud, or a rod bolt, the importance of consistent clamping force cannot be overstated. When properly lubricated and torqued they can easily maintain design strength until disassembled. When re-torqued , deformation occurs and the threads engage differently with the stud.

arp boltsARP nuts are manufactured from premium material. They are checked for concentricity and to ensure that the threads are centered in the nut. When the threads engage during torquing, they deform according to the friction contact with the engaging threads on the stud thus deforming the nut which works fine until you release it and re-tighten it over and over again during maintenance or rebuilds.

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Larger main stud nuts are tougher and less likely to fail, but clearance issues could ensue if the nuts relax or do not pull up to spec. Head stud nuts are more critical as even the slightest relaxation may cause you to lose a head gasket.


To accommodate a range of requirements, ARP offers up to 3 or 4 different nut sizes for various studs in both hex head and/or 12-point configuration depending on the application. For example 7/16-20 nuts come in 1/2, 9/16 and 5/8-inch diameters with the associated change in size and height. See accompanying chart for other sizes.

Distortion and loss of clamping force are the unavoidable consequence of the torquing process with prevailing torque nuts. Making the nut larger helps, but the threads remain the same size and they still deform as they lock together. Some cylinder head nuts are physically smaller because they have to clear adjacent objects such as valve springs or rocker arm gear. The ARP design process takes maximum advantage of the material properties to suit the application, but sometimes a nut simply has to be physically smaller than the ideal. This make it even more susceptible to clamping force decay due to multiple re-torquing. ARP further addresses the problem by offering a range of nut sizes for each stud size, thus the engine builder can choose what works best for his application.



arp bolts
This assortment of 8740 12-point and hex head nuts are all 7/16-20 threads for different small block Chevy applications. The more robust nuts on the left are for main studs while the smaller nuts on the right fit small block head studs. They range from 1/2- to 9/16- to 5/8-inch nut sizes and all have different head dimensions to suit specific requirements.


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The main stud nut on the left is a bit heftier and thus less likely to succumb to clamping force decay due to excessive torque cycles.

ARP's advanced lab equipment helps identify these issues and also helps to verify the proper torque and lubricant for every application. A new set of nuts is cheap insurance against the problems you could face if a head gasket lets go or a main stud loses clamping force and the bearing clearance changes. By using new nuts on all your assemblies you eliminate a critical failure point that may help save your engine whether its a crate motor or a Pro Stock engine. The stud will not fail, but when the nut loses its clamping power your engine enters the danger zone.

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Within the same thread size the difference in nut configuration can be substantial. This is easy to see with the 1/2- and 9/16-inch nuts on the left and the view of the load bearing surfaces on the right. For special applications consult your ARP rep.


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Any full race rebuild is a good candidate
for new nuts on all studded fasteners.

ARP stud kits are specifically designed for each application and the nuts are appropriately matched. Some kits are specified for clearance issues such as the proximity of valve springs, spacers or rocker arm gear. Smaller nuts specified for these applications are designed to provide the same level of performance but they are more susceptible to clamping force loss when reused multiple times. New nuts are good insurance in this case. ARP tests these components extensively to ensure their integrity and suitability for the task when new. When's the last time you ever heard of an ARP failure in normal operation? Still, the company's responsible posture is to identify potential misuse problems and alert racers to them so they can avoid possible issues. It should also be noted that we are talking about potential problems, not common occurrences.

DRCE nitrous engine

The chaotic nature of nitrous oxide combustion events and subsequent cylinder pressure spikes provides one of the severest tests of a fastener's clamping ability.

So what does it all mean? Well, if you're building a naturally aspirated street performance engine and you re-torque the nuts a few times, you're not likely to have any problems. In most cases you will not generate the RPM loading or the cylinder pressure to compromise the clamping force of the nuts. Millions of rebuilt engines are certainly testament to that, but if you run a lot of engine speed or power adders such as nitrous, supercharging or turbocharging its a different story. Sustained loading is considerably higher particularly in racing applications. In any of these scenarios, new nuts are always advisable.

Common practice in the aerospace industry requires new nuts on almost everything and so should you. Nuts are surprisingly inexpensive so it's pretty much a no brainer. Make it your smart policy on all your performance and racing engine builds. You can bulk buy nuts from ARP so you always have a new supply on hand. For a new 2016 catalog and a direct line to detailed ARP info click the ARP link located above this story.


Automotive Racing Products
1863 Eastman Avenue
Ventura, CA 93003
( 800)826-3045