ARP Fasteners Guide: Why You Need Head Studs & Main Bolts

ARP Fasteners Guide: Why You Need Head Studs & Main Bolts

When it comes to the structural integrity of a forged engine, the most expensive connecting rods and pistons in the world are useless if the fasteners holding them together fail. Founded in 1968 by aerospace fastener specialist Gary Holzapfel, ARP (Automotive Racing Products) was born to solve a critical motorsport problem: catastrophic engine failures caused by subpar bolts.

Today, ARP is the undisputed gold standard in high-performance engine fasteners, trusted by championship-winning teams across Formula 1, IndyCar, NASCAR, and NHRA drag racing. Manufactured entirely in-house in California, ARP produces aerospace-grade head studs, main bolts, rod bolts, and driveline fasteners.

Looking to secure your engine block? Check out our complete ARP Fasteners collection here!

The Problem: Factory Torque-to-Yield (TTY) Bolts

Most modern OEM head bolts and main bolts are Torque-to-Yield (TTY) fasteners. This means they are engineered to permanently stretch when torqued at the factory. When you introduce higher boost pressures, aggressive ignition timing, or higher RPMs, the extreme cylinder pressures force these already-stretched factory bolts to yield even further.

  • On the top end, stretching head bolts cause cylinder head lift, allowing combustion gases to blow out the head gasket.
  • On the bottom end, stretching main bolts cause cap walk, allowing the crankshaft to shift, destroying the main bearings.
  • Furthermore, TTY bolts are strictly one-use only. If you remove your cylinder head for maintenance, the factory bolts must be thrown in the bin.

ARP Head Studs vs. Head Bolts: Securing the Cylinder Head

When researching top end fasteners for your build, you will often hear the terms "head bolts" and "head studs" used interchangeably. While both components do the exact same job, securing the cylinder head to the engine block, they are distinct mechanical designs.

To ensure you are ordering the correct hardware for your application, here is the exact difference between the two:

  • Head Bolts: A head bolt is a single fastener. It slides down through the cylinder head and threads directly into the engine block, applying clamping force as the head of the bolt is tightened down. While factory engines use stretch-prone torque-to-yield bolts, ARP manufactures ultra-high-tensile replacement head bolts for builders who want a massive strength upgrade while maintaining the factory fastening style.
  • Head Studs: A head stud is a threaded rod. It is threaded loosely into the engine block first. The cylinder head is then lowered over the protruding studs, and clamping force is applied using a separate washer and nut on top.

Why do engine builders upgrade head studs?

  • Pure Clamping Force: When installing a standard bolt, the fastener twists inside the block's threads. This creates friction that leads to inaccurate torque readings. A stud remains completely stationary while you tighten the top nut, eliminating twisting forces and delivering perfect, uniform clamping pressure across the entire head gasket.
  • Engine Block Protection: Repeatedly threading steel bolts in and out of a delicate aluminum engine block will eventually strip the threads. Because studs remain seated in the block during tear-downs, your block threads are protected.
  • Perfect Alignment: Studs act as fixed guide pins. When installing the top end, you can simply slide the head gasket and the heavy cylinder head straight down over the studs, guaranteeing perfect alignment on the first try.

ARP Main Studs & Main Bolts: Securing the Rotating Assembly

Your crankshaft is subjected to violent, violent rotational forces. The main bearing caps are the only components keeping the crank secured to the block. Under high RPM and heavy load, factory main bolts flex, allowing the main caps to shift side to side, a destructive phenomenon known as cap walk. This misalignment instantly destroys main bearings and can snap the crankshaft.

Upgrading to ARP Main Studs provides a massive increase in clamping force, locking the main caps rigidly to the block. This enhances overall block rigidity, maintains perfect crankshaft alignment, and eliminates bearing wear caused by cap distortion at high RPM.

The ARP Manufacturing Difference

What makes an ARP fastener virtually indestructible? Thread rolling after heat treatment. Standard engine bolts have their threads cut into the metal before the heat-treatment process, which creates brittle stress risers. ARP heat-treats their aerospace alloys first, and then physically rolls the threads into the hardened metal. This painstaking process improves the fatigue strength of the fastener by a massive 1,000% over standard hardware.

ARP isn’t just an aftermarket brand; it is a foundational requirement for motorsport engineering. Whether you are building a reliable 400bhp daily driver or a 1,000bhp track weapon, upgrading your top and bottom end with ARP head studs, main bolts, and rod fasteners is the ultimate insurance policy for your engine.