There are pursuits that stress the need for critical fasteners. Aerospace and energy industries are two obvious examples. The next most obvious answer, of course, is motorsports.
It should come as no surprise that most significant advancements in high-strength fasteners came in the 1940s as a result of World War II and then in the 1950s and 1960s as demand developed in aeronautical and space industries. Most of the fastener grades and materials on the market today were invented and in some ways perfected in those years.
Metallurgical Advances
“What’s happened more recently, I would say in the last 20 or 30 years, obviously the advancement of the super alloys and especially the nickel-base super alloys has continued to go further and further. And some of that is aerospace and defense, and some of that is energy, the oil and gas business,” said Shannon Strother, director of feature technologies with Point One Manufacturing, a critical fastener manufacturer in Rogersville, Mo. “A decade is really not very long from a fastener perspective, but over the last 20 or 30 years, those super alloys have continued to evolve and in some cases get stronger and even more corrosion and heat resistant.”
For fasteners P1 makes for racing applications, things like cylinder head and main studs, and connecting rod bolts, it begins with raw materials like 8740 steel heat-treated to a tensile strength of 190,000 psi and 6304 at 220,000 psi. The psi is what gives you an idea of the strength and the density of the material. It tells you about the performance of the metal. It doesn’t tell the whole story, but those are the numbers used most often.
Despite the advances in materials, some of the machinery that P1 uses to make critical fasteners is not modern. It was difficult for P1, essentially a startup fastener company, to find the tooling it needed. The machinery has been updated for today’s world, but in some cases the company’s tooling was made when Eisenhower was president.
“We really feel like we bought the best of the best we could in some of these cases. We have machines that were made in the 50s and 60s when they made these really heavy-duty machines to produce these fasteners like this,” Strother said. “And they might be modernized with CNC controls and things like that, but the underlying frame was made in the 50s or 60s in many cases just because that’s when this heavy iron stuff was produced, and there’s really not a better way to do it today.”
For example, the machine P1 uses to make nuts is overkill for a lot of applications, but necessary for larger fasteners. The cold-forming servo-controlled machine has six dies that take what starts as round wire of 8740 and it comes out as a 12-point hex with a hole in the middle that still needs to have threads cut.
The level of capital investment required to make nuts and bolts is astonishing, but the result of that outlay is that P1 has found its way into championship teams in Top Fuel and Funny car. Its fasteners are being used in sprint cars, air racing, aerospace and energy industries, offshore powerboats, motorcycles, NASCAR and Formula 1.
“The reality is it’s all very humbling, especially when you learn it in a vacuum. There’s a lot of reasons why there’s not a lot of guys doing this. Every step is difficult and every step is expensive to learn,” said Strother, emphasizing the passion that drives the company’s pursuit. “I know a lot of people would say that about a lot of different things, but these fasteners go through a lot of different processes and they’re all by hand and they’re all difficult to learn. They’re like their own individual piece of art. But I think the most difficult thing to learn is thread rolling. I mean, rolling threads after heat treatment on very high strength parts is very, very expensive even when everything’s going perfectly and it’s very, very unforgiving.”
Tooling doesn’t last as long as it would if the company were cranking out grade 5 or even grade 8 bolts for your local Ace Hardware. The process is expensive for parts that most people are not accustomed to paying a lot of money for. In raw numbers, there are several parts that P1 makes in which several dollars per thread is a real cost.
However, racers know better than perhaps anyone the frustration of a race lost due to the failure of a 50-cent part. If they could spend more to avoid frustrating losses, it’s money well spent, right? Ultimately, we are talking about nuts and bolts, so understanding a bit about how they are made can be helpful. What can be even more helpful is understanding the finer points of how we should use them.
A Matter of Friction
We all have a torque wrench in our toolboxes, and we all know many of the torque specs on our cars by heart, but is important to know that torque is a measure of friction, and what fasteners are really about is clamping load.
If more of the torque input is going to friction, then obviously you achieve less clamp load. As friction value increases, the clamp load goes lower. That is where thread lubricants come into play. For example, if you have two different lubrications and you use the same torque input, you will get two different results for the clamp load.
“It is a deep subject and then people are blown away when you start talking about how the torque wrench, it is kind of a crude tool for tightening, even though that’s what everybody in this business does,” Strother said. “I mean, it really is kind of a crude instrument. It’s just the most convenient choice. And so that’s why people use it.”
Two Types of Friction
For any fastener, be it a nut or a bolt, there are two types of friction. One is created by the thread. The other is created by the bearing surface, which is either the bottom side of the nut or of the bolt head. For its fasteners, P1 can separate the two values to determine the appropriate lubricant so that the end user can get the clamping load required for the application.
The formula for determining clamping load from a given torque spec is as follows:
T = KDP
In English, that is (T) torque input is equal to (K) friction multiplied by (D) nominal diameter of the faster multiplied by (P) clamp load. You can isolate any of those variables algebraically depending on which values you have or which values you are trying to determine.
Getting the optimal clamping force is why P1 includes proprietary thread lubricant and instructions in each box of fasteners, which allows the end user to replicate how the manufacturer intended for the fastener to work.
“A torque spec definitely is a tightening strategy that helps you achieve a specified clamp load. And obviously the clamp load is what we’re after,” Strother said. “Here at P1 we have a very sophisticated torque tension bench that measures in controlled environment how the fastener is performing. During a test, we gather thread friction, the rotation angle, the clamp load that’s being achieved, and the torque input all at the same time on our bench. So there are other ways you can develop a tightening strategy, but generally speaking for us it starts with a torque-tension instrument to confirm what we see as far as the design of the fastener.”
Most high-end fastener companies offer fastener assembly lube. P1’s is called Torquepoint and it allows the fasteners to deliver consistent torque values for precise and repeatable clamp loads, plus or minus 10 percent or less. It features high film strength to reduce friction and prevent galling on threads and under-head surfaces under extreme temperature and load.
Long story short, if you get a set of head or main studs, rod bolts, or any critical engine fasteners, do not discard the instructions. Those instructions are going to help you get the performance the fasteners were designed to deliver and reduce variations in clamp load.
Torque to Spec
Strother said that most people don’t realize fasteners must be elastic. If they’re not elastic, they cannot function when you’re tightening a fastener. Even if it’s a hardware-store fastener, it’s stretching the part, and manufacturers want to control that. Stretch is what creates the clamping force, not torque. `
As a fastener stretches, just like a rubber band, it “fights” more and more as it elongates, and that’s called elastic deformation. Manufacturers design them to be elastic so that when you take it out of load, it should return to its size. As long as a fastener is only in the elastic zone and it has no corrosion, it can be torqued and retorqued to spec over and over again as designed by the manufacturer.
Torque to Yield
Torque to yield is a way to intentionally stretch, also called yield, a fastener to get everything you can out of it. It’s basically just going to a hundred percent of a bolt’s stretch, and then a bit more of what it’s capable of.
When a fastener has been yielded, that means it is somewhat compromised. It has become what Strother called “plastically deformed.” It can still be functional, but it’s compromised to a certain degree. Because you don’t really know how much it’s been hurt, you don’t how to tighten it again, reliably. Its mechanical properties have now changed.
“The same bolt could be used for both strategies. In other words, torque to spec and torque to yield are both types of fastener tightening strategies as well,” Strother said. “There certainly could be a design that lends itself to making a better torque to yield bolt or torque to spec bolt, but in general, torque to yield versus torque to spec are just two different methods of tightening.”
Torque-Plus-Angle Strategy
Similar to torque to yield, a torque-plus-angle strategy is another common method of tightening, and probably one of the most reliable. This is another case in which the fastener can be used only once.
The idea behind this method that you torque the fastener to a certain specification, then tighten it further, measured in degrees of rotation. At the initial torque spec, forces are at work and the compression of the gaskets or the assembly already has mostly occurred. After that, you dial in, say 90 degrees of rotation, the angle.
“The angle is a precise mathematical calculation based on the thread pitch. Since we know what the thread pitch is, we can calculate that one revolution equals so much displacement of the fastener,” Strother said. “In other words, we can say a 90 degree rotation after 25 foot-pounds or whatever the number is, gets us to a certain clamp load and it gets it very reliably. We’re probably down to plus or minus 2 or 3 percent reliability now.”
The variability of the clamp load is greatly reduced, so the precision is theoretically worth sacrificing fasteners after one use.
“The torque-plus-angle is a strategy for tightening, but that’s a torque-to-yield application. We used torque and we used angle to achieve yielding the bolt in a very linear way or a very repeatable way. And so basically they’ve given you a strategy that takes the bolt beyond 100 percent of yield,” Strother said. “A lot of those torque-to-yield bolts take the bolt out to 110 or 120 percent of yield. And in those torque-to-yield designs, oftentimes the bolts are manufactured such that they yield very evenly in a very flat curve over a long, long duration. So in other words, if you were looking at the curve of clamp load, it would sort of go up and go up at a 45-degree angle or whatever and it would roll over and just be flat up until the point where it just is giving up and getting ready to break.”
How Bolt Stretch Occurs
We can talk ad infinitum about bolt stretch, but you have to wonder how it is even possible inside a threaded bore of the same diameter and pitch? In other words, where and how does stretch occur?
A coarser pitch or a more aggressive pitch will take less rotation to create stretch. Because it’s got a steeper helix, it rotates less to create more stretch. Conversely, a finer pitch, a fine thread will need more rotation to create bolt stretch.
“In a typical head stud, you might have 3 or 4 or 5 inches of head column or something like that that is being clamped, but even if the fastener is very short, the stretching is still occurring. That is a part of the design,” Strother said. “It’s not going to stretch as much or as far, but that stretching is still occurring, even if it’s only a quarter of an inch over a quarter of an inch area. We call that the tensile zone, but that’s the area that’s being affected, hopefully elastically during tightening.”
Studs vs. Bolts
As is so often the case in this field, answers to one question lead to more questions on another matter. Take cylinder head or main cap studs, for example. Why are they nearly always used in the highest-performance applications? Why or how are they superior to bolts?
Well, it goes back to the torque-tension bench that P1 uses to create the instructions to allow for repeatability and minimal variances in clamping force. Cylinder head bolts must screw into thread bores in the block that P1 had no control over. For example, the spot face on the cylinder head that the bolt tightens against could be poorly machined or damaged, or the threads in the block could be damaged or flawed, all of which can create variances in clamping force. That’s not what you want for containing massive cylinder pressures or securing the rotating assembly to the bottom of the block.
“Objectively, one thing that is better about a stud, and this is 100 percent true, not just somebody’s opinion, we get to control the friction much more closely to how it is in our lab. When you’re tightening a bolt in the block threads, the thread friction is occurring in threads that may or may not be made properly, that may or may not be damaged. And obviously we’ve never seen them before,” Strother said. “We can measure them here, but in the field, they’re difficult to clean, they’re difficult to inspect, they’re difficult to measure. But with a stud, obviously you drop that thing in there in the uncontrolled threads, and obviously that should go in by hand, but that’s really the only test we need to know, generally speaking, to make sure that we’ve got good engagement. As long as it goes in by hand and goes in smoothly, then the tightening is occurring on the nut side, which is by the way, identical to how we test those parts here in our shop. So with the stud, we can translate a tightening strategy to anywhere in the world and almost have 100 percent repeatability. If you follow our instructions and you use our washer and our nut and our stud and our lube, well, that’s exactly the same thing we did in our lab.”
Something to think about next time you pull on your torque wrench and hear it click.
Love the article! I haven’t gotten through it 100% so I’m not sure if this was touched on. But regarding lubrication of threads, it should be mentioned that for most fasteners on your vehicle, you shouldn’t lube threads (or bearing surfaces) unless the joint calls for it. I know a lot of folks use never-seize on lug nuts, brake hardware etc. This has some obvious benefits, but in a lot of cases torquing a lubed fastener to spec could be pushing it past yield and can be risky, as they were expected to be torqued dry and clean.
Most engine crankshaft, con rods and cylinder heads need some kind of lube for torquing.