High Performance Bolts and Assembly Processes - The Science Of Stuck

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.

Torque and Friction

For any fastener to function properly, it must be stretched. It’s the material’s ability to be placed under tension and then try to pull back to its original length (sort of like a rubber band) that provides the clamping load we need. Different materials react differently to the stress of stretching under different conditions, so it’s important that the fastener is designed exactly for the load it must bear.

Depending on differences in friction, torquing a bolt with a properly calibrated torque wrench to a specific reading does not always produce the same amount of stretch. If a fastener is over torqued and stretched too far, it won’t be able to pull back and is ruined. This is known as exceeding its yield strength.

This is why it’s important to remove friction from the equation as a variable by using some type of lubricant. You also need to make sure all the threads on both the fasteners and in the bolt holes are clean and free of debris. You can usually do this with your engine block by using a thread chaser or even gently running a thread tap through the holes.

Motor oil is a commonly used lubricant because it’s readily available, and your engine is going to be filled with it later on, anyway. But it isn’t necessarily the best option for getting consistently accurate torque values. If low friction is desired in order to install the fasteners with less torque (thus, less twisting deformation) you can use special, low-friction lubricants. Don’t worry, using a lubricant won’t cause a fastener to come loose, but it also has the added benefit of reducing the chances of two materials galling and sticking together. So lubricant will not only improve your ability to build an engine, but also makes teardown a lot easier.

With special lubes, the torque necessary to achieve the proper bolt stretch can be reduced as much as 20 to 30 percent. Of course, the opposite is also true: If you lubricate a fastener with a low friction lubricant and then torque it to a value designed for motor oil, the chances are probably good that you’ve just exceeded that fastener’s yield strength. So it’s always important to match the torque applied to the lubricant used. Manufacturers, like ARP, often provide a chart of proper torque values with their fastener kits.

Another factor when it comes to torquing bolts is that some lubricants, like moly paste, can actually polish the threads as the fastener is tightened down. So even if you use the same torque value, the second time you tighten that same bolt you will experience greater rod stretch. ARP’s engineers have done extensive testing on this phenomenon and developed a lubricant specifically for fasteners.

It’s called “Ultra Torque” and it’s exclusive to ARP. A small package is provided with most bolt kits and it has several valuable properties. First, Ultra Torque is amazingly consistent. It doesn’t polish the threads of either the fastener or the block so even if the bolt is installed and removed multiple times, the same torque value will achieve the same result in terms of bolt stretch. The lubricant is also quite sticky and won’t wipe off, so it will still be on the threads during teardown. And finally, it has no metal in it (believe it or not, most moly-based lubes do) and it’s compatible with motor oils, so it won’t harm your engine’s lubrication system.

Measuring Stretch and Rod Bolts

Now that we’ve spent all that time saying that studs are preferable to bolts, we’ll reverse course while talking about connecting rods and tell you that cap screws (a fancy name for a bolt) are preferable over the old-school bolt and nut assembly when it comes to securing the big end of the rod.

Stock-style rods use a bolt that is pressed into the body of the connecting rod. After the rod cap is installed, two nuts are used to fasten the cap to the rod. The cap screw design is better because it eliminates the weight of the nut. In this design, the cap is properly located on the rod with two dowel pins. After the cap is in place, bolts extend through the cap and thread into the body of the rod to pull the cap down securely.

Rod bolts are also the most highly stressed bolt in any race engine. And because of that, it’s most critical that they are installed optimally. For most applications, it’s good enough to simply install the bolts or nuts on the studs with a specific lubricant and a torque wrench. But trying to hit the right amount of stretch gauging only by the amount of torque applied to the fastener can be a bit hit-or-miss.

As discussed earlier, variables—such as the type of lubricant used, the slickness of the threads, debris caught between the threads, and even the accuracy of the torque wrench—can affect the actual amount of stretch you get. But by using a bolt stretch gauge you can bypass all of these pitfalls.

The only time you can measure bolt stretch is on the rare occasions you can reach both sides of the bolt after it has been installed. This means it won’t work when threading the fastener into a blind hole, such as head bolts into the deck of the block. But this isn’t a problem with rod bolts, so the practice works well.

Using a dedicated rod bolt stretch gauge is easily the most accurate, and easiest, way to measure bolt stretch. And as long as you’re using the same bolt in the same rods with the same lubricant, it’s usually only necessary to double check the amount of stretch against your torque setting on the first couple of bolts. After that it’s pretty safe to put away the stretch gauge and depend only on the torque wrench.

Begin by installing the rod bolt in your gauge to measure its length. Most rod bolt manufacturers, like ARP, engineer divots into both ends of the bolt to make centering it up in the gauge easier. Once the bolt is in the gauge, zero out the reading.

Now, install the bolt in the rod and cap (it can be installed in the engine), make sure your lubricant is properly applied, and begin torquing the bolt in 5-pound increments, measuring the stretch as you go. The correct amount of stretch for almost all rod bolts is between 0.006- and 0.007-inch, but you may want to check with the manufacturer to be certain.

Once you get the proper amount of stretch, check the amount of torque required, and then double check your findings by repeating the process on a second bolt. If they are consistent, you can finish the rest of the bolts with just the torque wrench.

It should be noted that while building a race engine you’ll have the rods apart probably three or more times while they are honed, fitted with bearings and assembled. Checking the stretch for the first couple of bolts is a great idea at each stage of the process, not just final assembly.

If you worry that a rod bolt may be failing, take it out and check it in your bolt-stretch gauge while it’s in a relaxed state. If it’s 0.001 longer than its original length, you should consider it a failed bolt. We all hate throwing away a bolt over a thousandth of an inch—and possibly putting your engine build on hold while you source a replacement—but look on the bright side: You may have just saved yourself from a costly engine failure.