Head Studs Versus Head Bolts

ARP’s factory Tech Representatives are often asked which is better, cylinder head studs or head bolts. The answer depends on the application, power level, and installation. On many street-driven vehicles, where master cylinders and other items protrude into the engine compartment obstruction, and limited access may require the use of head bolts so that the cylinder head removal can occur with the engine in the vehicle.

For most increased performance applications head studs are the recommended product to ensure maximum performance levels and trouble-free engine operation.

The Use of ARP Head Stud Kits Provides These Benefits:

• Using head studs will make it much easier to assemble an engine.
• Racing engines that must be serviced frequently and quickly will benefit from stud kit installation.
• Cylinder head and gasket alignment are assured.
• Studs provide a more accurate and consistent torque loading.
• Head studs in an engine can be a significant decision that affects the way the motor runs.
• Head studs are typically threaded on both ends and often have an unthreaded middle section of the shaft.
• Studs do not have a head, whereas head bolts always do.
• Ease of engine assembly and disassembly is a significant benefit.
• Enables the ability to handle higher boost and compression levels.
• It can be a significant factor in increasing overall engine performance.
• Head studs are also thermally treated at high pressure to improve structural strength.
• Head Studs are carefully shaped to allow the production of high-quality fasteners.
• The machining of a head stud requires a series of cuts to produce a straight and balanced fastener.
• Studs can be roll threaded before or after heat treatment.
• Studs have a higher mechanical strength and stress tolerance than head bolts.
• ARP manufactures Higher-end head studs that are specially designed within exacting tolerances and strength ratings for high-power engines.
• These head studs provide secure positioning of the head gasket and cylinders with near-perfect alignment.
• Stud Kit installation translates into higher engine reliability and a lower chance of head gasket failure.

Why are Head Studs better than Head Bolts:

When you use head bolts to secure the cylinder head the fastener is being “twisted” while it’s torqued to the proper reading.  The head bolt is reacting to two different forces simultaneously.

A Head Stud installation should be completed in a relaxed mode. Never install a head stud by cranking it into the block by using a jammed nut. If everything is right, the stud should be installed finger tight. When applying torque to the nut, the stud will stretch only on the vertical axis. Remember, an undercut shorter stud will have a rate similar to a longer, standard shank stud. The head Stud provides a more even clamping force on the head. Head Studs allow the head gasket to compress upon initial torquing, make sure studs and bolts are re-torqued after the engine has been run.

KLM Performance has recently expanded our online catalog to offer a more comprehensive collection of ARP Automotive Racing Products for your motor vehicle. If you don't see what you or have a specific product question or need, please feel free to contact us. Shop for great deals on head stud kits, main studs, rod bolt kits, ring compressors, driveline bolts, accessory bolt kits, and ultra torque lube. Most of our ARP products are in stock and ship for free with no handling charges.

ARP designs and manufacturers it is head studs with gasket and cylinder alignment in mind.  These studs are typically more expensive than the equivalent head bolt models. Torque Efficiency During engine assembly or maintenance, a bolt must be installed by torquing it into place. Due to the head bolt's design, it has to be rotated into its slot to engage the threads and secure them into place. This process creates both twisting force and a vertical clamping force, which means that when the cylinders within the engine combustion chamber begin accumulating load, the bolt will both stretch and twist. Because the bolt has to react to two different forces simultaneously, its capacity to secure the head is slightly reduced, and it forms a less reliable seal in high-powered engines. By contrast, a head stud can be tightened into place without any direct clamping force applied through the tightening. The head stud can be threaded into a slot up to finger tightness or the degree to which it would be tightened by hand.

Afterward, the cylinder head is installed, and a nut is torqued into place against the stud. The nut torque provides the clamping force, rather than the torque of the fastener itself, and the rotational force is avoided entirely. Because the stud is torqued from a relaxed state, the pressure from the nut will make it stretch only along the vertical axis without a concurrent twisting load. The result is a more evenly distributed and accurate torque load compared to that of the head bolt. Head Stud installation ultimately translates into higher reliability and a lower chance of head gasket failure.

What is the difference between an ARP Head Stud and an ARP Main Stud?

The Head Stud is used for the installation of the Cylinder Head or Heads. The Head Stud holds the cylinder head in place on the engine block. The ARP Head Stud is what keeps the cylinder head in a constant state of tension to prevent head gasket failure. Heads studs work on the top of the motor as the connection point between the cylinder head, head gasket, and engine block.

The Main Stud is used for the installation of the Main Caps; The Main Caps hold your engine crankshaft on the lower end of the motor.

What Alloy Steel do I need my ARP Head Stud Kit to be manufactured from?

ARP Head Stud Kits are produced from multiple grades of alloys to meet your exact needs. Here are the particular head stud strengths offered by the Automotive Racing Products. These are the head stud alloys currently available for your engine.

8740 Chrome Moly: Until the development of today’s modern alloys, chrome moly was popularly considered a high-strength material. Now viewed as the only moderate power, 8740 chrome moly is seen as sturdy steel, with adequate fatigue properties for most racing applications, but only if the threads are rolled after heat treatment, as is the standard ARP production practice. Chrome-moly is classified as a quench and temper steel, that can be heat-treated to deliver tensile strengths between 180,000 and 210,000 psi.

ARP2000®: ARP2000 is an alloy steel that can be safely heat treated to a higher level, producing a more significant strength material than 8740. While 8740 and ARP2000 share similar characteristics – ARP2000 is capable of achieving a clamp load at 220,000 psi. ARP2000 is used widely in short track and drag racing as an upgrade from 8740 chrome moly in both steel and aluminum rods. Stress corrosion and hydrogen embrittlement are typically not a problem; providing care are taken during the installation process.

Custom Age 625 Plus®: This newly formulated super-alloy demonstrates superior fatigue cycle life, tensile strength, and toughness – with complete resistance to atmospheric corrosion and oxidation. ARP is the first to develop manufacturing and testing processes for fasteners with Custom Age 625+. Best of all it is less expensive and expected to soon replace MP-35 as the material of choice in the high-strength, super-alloy field. Typical tensile strength is 260,000-280,000 psi.

Titanium: ARP now offers special order fasteners made of an alloy (Ti6Al-4V) that is specially heat-treated (a process developed by ARP's own Russ Sherman) and provides superior strength to other titanium alloys employed in racing and aerospace. The material has a nominal tensile strength of 180,000 psi and is very corrosion resistant. The main advantage of titanium, of course, is its weight – which is about 40% lighter than a comparable fastener made of steel. Head studs and accessory bolts are ideal applications for this lightweight material. 

All ARP Head Stud Kits are proudly MADE IN THE USA!

ARP’s product line contains thousands of part numbers and has expanded to include virtually every fastener found in an engine and driveline application. If you don't see the product, you are looking for please feel free to contact us at 203-210-7200 for more information.