Did you know that right now in some laboratory hidden away from sight there are engineers looking at ways to glue the cylinder heads to the block in your next passenger vehicle?

It may sound crazy but the reason is because bolts are expensive, especially when you’re building more than a million vehicles a year. It may make rebuilding such a beast a bit of a hassle—or impossible—but that’s hardly a concern compared to cutting costs and maintaining profits in the short term.

Of course, that’s the possible future, and at Circle Track we’re worried about the real present where the only things getting glued are the stickers on the valve covers. The forces working on a modern race engine are beyond just about anything else out there, and if you don’t use the proper fasteners—and install them correctly—you’re just about guaranteed an engine failure. That’s why a company like ARP can practically become a household name in the motorsports industry producing only threaded fasteners.

In engine building for racing, one bolt isn’t the same as any other. For example, consider that on an engine revving at 7,000 rpm the piston travels the length of the cylinder bore and changes direction 233 times every second. At peak piston speed that translates to 68 feet per second!

Now consider that the only thing holding the rod and piston assembly in place are those two small rod bolts. If one should fail, you’re looking at a catastrophic engine failure and a repair bill that’s probably worth a couple of house payments.

Science Class

The reason high-performance bolts are so much stronger than either the usual Grade 5 or Grade 8 bolts you get at the hardware store is because there’s a lot more engineering and technology that goes into creating them. This is particularly true when it comes to how the threads are actually created on the bolt or stud. On most bolts, the threads are simply cut out from the material on a lathe. So the voids between the threads are cut away. After that, the bolts are dumped into big bins and heat treated.

On the other hand, ARP creates its bolts through a very different process. Beginning with an extremely high-grade alloy (8740 for all you metallurgists out there), each stud is hung vertically in a rack where it goes through a very precise heat treating process. Using a rack ensures each stud sees the same temperature, unlike bolts that have been dumped into a bin before going through heat treat. Because in the bin process the bolts in the middle of the pile are insulated by the outer bolts. That doesn’t mean that the bin process is bad—it just isn’t up to spec for the highest-quality possible engine bolts.

After the heat treatment process each stud is centerless ground to ensure concentricity. This step is almost always exclusive to the highest quality fasteners. Most lower-grade bolts are simply thread rolled while still in bar stock form and declared good enough.

After that, it’s finally time to create the threads, but instead of cutting, the threads are actually created by “rolling” the studs so that the material is compacted instead of cut away. Doing this after the heat treating process is complete makes rolling the threads very hard on the manufacturing equipment, but ARP says it can create bolts and studs that are 10 times stronger than bolts or studs that are cut prior to the heat treat process. Now you can begin to see why these fasteners are a bit more expensive.

Bolts and Studs

You may also notice that in many of the engine tech articles you see in Circle Track—either build projects we do ourselves or photos from race engine builders’ shops—the cylinder heads and main caps are secured with studs and not bolts. This isn’t an accident.

A stud is a lot like a bolt except it doesn’t have a head like a traditional bolt. Instead it’s threaded on both ends, and a nut is used to secure whatever is being fastened in place. The reason studs are often preferable to bolts is because they are capable of providing better and more consistent clamping loads, especially when installing cylinder heads. What this means for you is the reduced chance of suffering a blown head gasket.

When tightening a bolt, you’re actually creating two different forces. The desirable one is a vertical clamping load. But the fact that the bolt has to spin as the clamping load is applied also creates a twisting force in the block. This can create distortion in the cylinder bores and also is much harder on the threads in the block.

But using a stud eliminates this twisting force that can be so detrimental. This is because the stud is threaded in place with no clamping load—it’s spun into position with no torque and the stud stays in a relaxed state. The clamping load isn’t applied until the cylinder head is in place and the nut is threaded on and tightened. There is a twisting force on the nut, but for the most part, the stud only sees a clamping load along its vertical axis. The result is torque is more accurately applied to the studs so each is more consistently stretched and the clamping load across the cylinder head is more consistently distributed.

But that’s not the only advantage studs can have over bolts when it comes to engine assembly. For example, having studs in place makes it easier to properly align both the head gaskets and the cylinder heads. Studs are also easier on the threaded holes in the block because you’re not pulling against the threads in the block to pull the stud into place. The torque isn’t applied until after the threads in both the bottom of the stud and the block are fully engaged.