ARP Bolts & Head Studs

More and more Diesel Power Products customers are running large single turbos or compound turbo set-ups. Many of these turbos are capable of upwards of 50PSI of boost pressure with the right fueling. At these pressures the stock head bolts on our light duty diesel trucks are prone to stretching which can lead to head gasket failure. Head to block sealing is critical and that's where ARP Diesel Head Studs come in.

You should know that ARP uses a premium grade 8740 alloy that is rated far superior to "aircraft" quality. Then, each stud is precisely heat-treated to 220,000 psi. Following heat-treat, each stud is centerless ground to make it as close to perfectly concentric as possible. This procedure involves about ten very slight cuts and results in an exceptionally straight part. It's important to note that lesser quality studs are not even centerless ground—the material is thread rolled in bar stock form (mostly before heat-treat, when the material is easier to machine). ARP studs are thread rolled after heat-treat, which gives them about 1000% (that's ten times) better fatigue strength than those studs that are threaded prior to heat-treat. Because ARP studs are manufactured to such exacting tolerances, you will note that gaskets and cylinder heads literally glide into position and are perfectly aligned—something that won't happen with inferior quality head studs.

ARP HEAD STUDS AND HEAD BOLTS

What really is the difference between a head stud and a head bolt you may be wondering. From the factory, all engines have head bolts. As nearly anyone has seen, a “bolt” is a threaded shaft with a hexagonal head attached. These bolts thread into the respective hole and the bolt head is then torqued down on the head(s) with a great deal of the force being applied to the thread themselves. In contrast, a “stud” is only the threaded shaft that is first installed into the block. Once the stud is installed onto the block, then the head gasket and head are placed. Finally, the head stud washer and nut are placed and torqued. Due to the change in orientation of where the force is applied, a head stud will not stretch as a head bolt will commonly do, as well as the superior metals used to make ARP head studs compared to stock head bolts.

SHOP ARP STUDS AND MORE FROM DPP!

While most diesel performance enthusiast know ARP for their heads studs, they actually have a wide range of fasteners available that are far superior to their stock counterparts. From harmonic damper hardware to rod bolts, flexplate bolts, and so much more. With ARP, you can rest assured that you’re installing the best hardware on the planet.

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We get a lot of calls and e-mails each day with various product, vehicle, and installation questions. Some are completely off the wall and can even catch us off guard at times, but many others are fairly routine, so we thought we'd post some of the most popular questions regarding ARP bolts and studs that can hopefully be a good resource for you.

Q: Can you install head studs without removing the head(s)?
A: That depends. If your stock head gasket is still intact, then you definitely can. This process would involve removing one bolt at a time and installing one head stud at a time. Now, it is possible that you could damage the existing head gasket during this process so we'd always heed caution when going this route, but its been accomplished successfully enough times that we can say yes, it will work.

Q: Where is ARP located?
A: ARP is located in Ventura, California where all product is manufactured (not imported).

Q: What does ARP stand for?
A: ARP stands for American Racing Products.

Q: What is the difference between ARP's 425 and 625 series head studs?
A: The short answer is that the 625's are the superior product, but both are far superior to a stock head bolt. The 425 series, which is the most common, is built from grade 8740 alloy and heat-treated to 200,000 psi. In contrast, the 625 series is manufactured from proprietary Custom Age 625+ material that has a minimum rating of 260,000 psi tensile strength.

Q: Do I need to re-torque my head studs?
A: If you follow the ARP installation instructions, there should be no need to do a re-torque. However, it may be necessary under certain circumstances if the gasket manufacturer’s instructions require it, particularly if a fire ring has been installed. ARP recommends not doing a re-torque on a hot engine.

Q: Do I need lube on my bolts or studs?
A: ARP recommends using ARP Ultra-Torque lube to ensure an even, accurate clamp load and to prevent thread galling. This is particularly important for stainless steel fasteners. The lube should be used under the head of the bolt or the bearing surface of the nut and on the threads, unless a thread sealer is used.

Q: Can I use Loctite or any thread locker instead of ARP Ultra-Torque assembly lube?
A: Yes, you can. Some engine builders use blue Loctite on the stud end that goes into the block. Do NOT use Loctite on the nuts for the head studs or main studs. Always ensure that the threads are clean prior to applying any lube. When using Loctite, make sure you assemble the parts before the Loctite cures. You can use Loctite instead of ARP Ultra-Torque, but do not use them together.

Q: Does the chamfer on the inside diameter of the washer go up or down?
A: The chamfered side of the washer goes up, towards the head of the bolt. The chamfer is there to create clearance for the radius between the bolt shank and the bolt head.

Q: Do the head studs only go in hand, or finger tight?
A: Yes, cylinder head studs are installed only hand tight. Other than the use of an allen wrench on the hex broached into the end of the stud (to ease installation, not to apply torque), use no tools to seat them in the block. However, it is extremely important to ensure that the studs are fully bottomed out in the hole in the block and not hung up on damaged or corroded threads in the block preventing the stud from being fully seated. This is often indicated when the stud threads extend past the deck surface.

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