Our cars are held together with a dizzying array of fasteners– all sorts of nuts and bolts, studs and washers. They may not be as captivating as the components they attach to one another, but don’t take them for granted. A chain is only as strong as its weakest link, and faulty fasteners have the power to …
1. What Makes Fasteners Deteriorate?
Photography Credit: David S. Wallens
In addition to fatiguing from the cyclical loading and offloading, ferrous materials can deteriorate over time due to stress corrosion cracking, rust, permanent deformation and/or galling. It’s good practice to not reuse old fasteners for any critical application without carefully and thoroughly inspecting them.
2. What Do Those Markings Mean?
Photography Credit: Bill Holland
Most quality commercial fasteners have markings on the head. Three lines on an SAE bolt indicate Grade 5, or a tensile strength of 120,000 psi. Some racing associations require at least Grade 5 bolts for attaching roll bars and the like. The metric equivalent is stamped “8.8.”
Six lines denote Grade 8 hardware, which has a nominal rating of 150,000 psi. Class 10.9 is the metric equivalent.
ARP stamps its initials on its bolt heads, and sometimes it also places identifiers regarding the materials and/or tensile strength. The company’s cylinder head, main bearing, connecting rod and driveline fasteners are rated at a minimum 200,000 psi–some are even rated as high as 280,000 psi.
Bottom line: If the head of a bolt is blank, beware.
3. Coarse or Fine Thread?
There are several reasons why fine-threaded fasteners are stronger than coarse, not the least of which is the larger minor thread diameter (better shear strength) and more threads (improved tension load). Fine threads also have less of a tendency to loosen because the thread incline is less steep. While fine threads are more easily tapped into hard materials and thin-walled tubes, coarse threads are better suited to softer materials, like aluminum and cast iron.
4. What do Fasteners and Springs Have in Common?
To achieve preload or clamping force, a fastener should be stretched a measured amount. A properly installed fastener works like a spring: The resulting “rebound” applies clamping force.
A typical 3/8-inch-diameter rod bolt made of 8740 chrome moly will need to be stretched about 0.006 inch to achieve a 10,000-pound clamping force. And, like a spring, if you don’t pull it very far, there’s little rebound; if you pull it too much, it will not return to its original length and shape–and will fail in service.
5. How Do Your Select the Proper Bolt?
If you want to order the correct fastener for a given application, you’ll need to consider at least eight dimensions. They include:
- underhead length
- thread length
- grip length
- thread type
- grip diameter
- wrenching
- head height and collar diameter You’ll also need to decide whether you want standard hex or 12-point heads.
6. Head Studs or Head Bolts?
Photography Credit: David S. Wallens
With bolts, torque is applied by twisting them into the block. Conversely, studs are installed fingertight into the block, and the clamping force comes from tightening the nut against the cylinder head and pulling up on the stud. Ergo, studs are much “easier” on the block.
Secondly, the use of studs assures proper head gasket positioning and easy installation of the cylinder head. To this end, ARP grinds its head studs with precision centerless grinding after heat treatment so they’re perfectly concentric and the heads literally drop into position. Lesser-quality studs are often not ground or are heat-treated after machining and threading, so installing the head can be a chore. In the past there have been clearance issues with the use of studs, but ARP hex-broaches its studs to facilitate easy installation and removal. Case closed.
7. What's Preload Scatter and How Can You Prevent It?
When you apply torque to a fastener, much of the energy is expended on overcoming friction in the threads, any load-bearing surface (the underside of the bolt head or nut and washer against whatever surface it’s tightened against) and, most importantly, the lubricant itself. (Note that moly, oil, diesel lube and the like all put up varying degrees of resistance).
As a result, the torque wrench may “click” at the desired setting–but it doesn’t mean the desired preload has been achieved. This is called preload scatter. The difference between actual and desired preload can be dramatic–as much as 30 percent.
Now picture uneven preload placed on adjacent bolts or studs in a cylinder head. Result: distortion of the cylinder bore and a negative impact on piston ring seal.
For decades, the only proven method for assuring consistent preloading was to cycle the fastener–torque, loosen, re-torque–up to a halfdozen times to mitigate the friction. Now there’s an easy way. Following extensive testing, ARP introduced their Ultra-Torque Fastener Assembly Lubricant, and they say it delivers 95 to 100 percent of the desired preload on the first–and any subsequent–pull of the torque wrench.
8. What are TTY Bolts and Why Replace Them?
For the past 30 years or so, most automobile manufacturers have built engines employing robotic devices and torque-to-yield, aka TTY, fasteners. Head, main and rod bolts are tightened to 95 to 120 percent of their yield, and anything past 100 means their material has stretched to the point of deformation. This is acceptable provided you don’t modify the engine and put extra strain on it, or service the engine and reuse the TTY bolts.
ARP engineers its fasteners to exceed factory specification loads by 25 percent or greater while installed at 75 percent of their yield strength, increasing the load-carrying capabilities and leaving enough of a margin to ensure the fastener remains reusable.
9. What's the Right Washer for the Job?
You won’t find split or locking washers in ARP’s catalog, because those pieces aren’t appropriate for most automotive applications. They’re only suitable for applying slight tension under extremely light loads–in a double-shear joint, for example, where tension on the bolt is only required to keep it in place.
Yes, you will find Special Purpose ARP washers, but these are made from premium chrome moly alloy, hardened and parallel-ground to ensure equal distribution of force. These should be used in high-preload applications. The General Purpose washers are softer and intended for light loads, like attaching accessories. ARP also makes Insert Washers that protect the tops of columns in an aluminum cylinder head from galling or collapsing in on the bolt or stud
10. What's the Best Method for Using Anaerobic Glues and Fastener Lubes?
For most high-preload applications like cylinder heads, main caps and connecting rods, fastener lube by itself is sufficient. If the bolt or stud is “wet”–meaning that it protrudes into a water passage–then you’ll want to use a sealer.
And for some applications, like flywheels, clutches and ring gears, most racers prefer to use Loc-Tite or a similar anaerobic glue. If you wish to follow suit, the tech team at ARP strongly recommends the following method:
Sequentially secure all fasteners by torquing to the required level first, using oil as the lubricant. Next, remove and clean the first fastener and thread. Then, apply Loc-Tite and promptly re-torque the bolt before moving on to the next one.
Why this procedure? Because while a group of fasteners is tightened, the anaerobic glue sets up quickly and can start to harden before the desired torque is applied to the last ones–which then throws off everything.
11. Why Replace OEM Driveline Fasteners?
The simple answer is that replacing OEM driveline fasteners with upgraded hardware provides an extra margin of safety at high-rpm operation.
Take wheel studs as an example. The stock ones are manufactured to commercial specifications: casehardened and not engineered to handle the extreme side loads seen during road racing. Quality aftermarket pieces designed to withstand 200,000 psi might be a smart move.
12. Why Should You Measure Your Rod Bolts?
Photography Credit: Bill Holland
Rod bolts are the most critical fasteners in an engine. Failure here can have catastrophic results. Most failures, according to ARP, come as a result of bolts that are either under-torqued or, in some rare cases, over-torqued.
The only sure way to determine the correct amount of preload is by measuring the stretch of each rod. To do this, keep a log of all rod bolts, measuring their length before installation and after disassembly. If the bolt has permanently stretched more than 0.001 inch, it has past its yield point and should be replaced.
13. How Accurate is Your Torque Wrench?
Photography Credit: Bill Holland
ARP has tested torque wrenches for competitors at tracks across the nation and found that a large percentage were inaccurate– some by as much as 30 percent. Obviously this can have a huge impact on obtaining the proper fastener preload. The moral of the story: Have your torque wrench tested periodically and handle it carefully. It’s a very delicate instrument.
14. How Do Stainless-Steel Accessory Studs Make Life Easier?
There are several important reasons why studs are preferred for attaching things like valve covers, oil pans, exhaust headers and, on inline engines, intake manifolds. Studs assure proper gasket alignment, guide the components into place, and provide compact wrenching.
Details count: ARP’s offerings have a rounded nut-starter nose to speed up installation time, and they’re made of a proprietary stainlesssteel alloy that’s very heat-resistant and won’t rust.
15. How Does Manufacturing Affect Strength?
Photography Credit: Bill Holland
Two bolts may look alike, but they can have dramatically different load bearing capabilities and service life. Four grades of 8740 chrome moly steel are typically used to make fasteners: commercial, aircraft quality, cold head quality (CHQ), and seamless defect free (SDF). Depending on the application, ARP uses only the more expensive CHQ or SDF steel.
Then there’s the method of heat-treating. Some manufacturers merely throw them in a basket and batch-treat. ARP has special racks that hold the bolts and studs in such a way that each part receives the exact degree and duration of heat and quench.
To save time and effort, some manufacturers also cut the threads before heattreating– when the material is softer and easier to form. ARP does it after heat treatment to aerospace MIL-S- 8879 specifications, giving the threads incredibly higher fatigue strength.
