Almost all metals are elastic and when flexed, bent or stretched, they try to return to their original shape. Knowing that metal is elastic is important to engine builders that deal with very tight tolerances between parts. If metal is elastic, and engine bolts are made of metal, you can correctly assume that engine bolts stretch as they are tightened.
We’re going to take a look at how to correctly select, setup, and torque engine fasteners to control critical measurements and clearances within the engine. When it comes to fasteners, a little knowledge goes a long way to keeping engine failures to an absolute minimum.
Heat treating the bolts to increase hardness can range from an hour and a half to over eight hours depending on the intended application.
All About Fasteners
The topic of fasteners is a very large and sometimes complex subject. There is an entire industry built around fasteners with lots of very well educated people and sophisticated equipment in labs that work on fastener issues.
The industry’s professional association, The Bolting Technology Council, holds routine meetings and seminars concerning fastener technology. We are not fastener engineers and if you are reading this article, you’re probably not either, but having a good working knowledge of the nuts and bolts of this business is important in building solid and dependable high performance engines.
To help us get a handle on the subject, we enlisted the help of Chris Raschke at Automotive Racing Products (ARP). Raschke explains how intricate and precise race car fasteners actually are, “Most commercial and aerospace houses don’t hold the tolerances that we need to hold here for race car parts,” says Raschke.
Threaded fasteners allow for the removal and reassembly of parts where other types of solid fasteners are a single use item. With the wide range of applications where threaded fasteners can be used, there are basically two categories that fasteners are divided into, critical and non-critical.
Most commercial and aerospace houses don’t hold the tolerances that we need to hold here for race car parts. - Chris Raschke
Raschke also identifies there are different categories of fasteners, “We’ve always been known for our connecting rod bolts and our head and main studs, but we make quality fasteners in a variety of different shapes and forms. From bumper to bumper.”
High stress and high load areas like connecting rods, main bearings and head bolts or studs are examples of critical fastener areas. These critical fasteners generally have exact tightening specifications and procedures whereas the non-critical fasteners have relaxed tightening specifications.
Examples of non-critical fasteners are pan bolts, timing cover bolts and valve cover bolts. Because non-critical fasteners do not require a detailed tightening procedure, we will limit our focus to critical fasteners in this article.
Shot peening the bolts to relieve stress risers.
How Fasteners are Made
The factors involved in threaded fastener designing range from determining the fastener load to geometric configuration that prevents metal fatigue. In addition to design features, material selection, testing and quality control weigh heavily into the manufacturing of fasteners.
Raschke explained the basic process ARP Fasteners are made, “the cold formed fasteners start with a wire coil of material, based on the clamping requirements of the fastener, that goes into a machine that straightens the wire and forms the fastener head.”
The material used in the wire coil can be anything from steel to chrome moly or stainless steel.
“Some materials are coated before they enter the forming machinery depending on the requirements of the material. For example, stainless steel gets a copper coating to prevent galling,” says Raschke.
The ARP manufacturing process
Cutoff from wire or bar stock
Heading - hot headed or cold headed based on the material and spec
Machining - undercut if called for in the spec
Fillet Rolling - if specified
Shot Peening - to remove stress risers
Metal Finishing - Black Oxide coated, or in the case of Stainless, polishing
Packing - bolts are coated to prevent surface corrosion prior to packing and storage
After the heading operations, Raschke explained that the fasteners may go through “aging or heat treating depending on the hardness requirements of the fastener.”
During the heat treating process, “temperatures as high as 1550 degrees are used from a one and a half hour treatment to as long as an eight hour heat treatment exposure.”
After the fastener comes out of heat treating or aging, the batch of fasteners are shot peened to remove scale and make the surface smooth, then routed through tumblers to deburr the fastener. Centerless grinding technique is used to prep the fastener to get threads rolled into the fastener blanks.
According to Raschke, “rolling the threads into the fastener blanks is forging the threads into the material, not cutting material away to create threads. This produces a better, stronger more uniform thread.”
Raschke calls the rolled thread pattern a “J thread” and cautions against running a die over the fastener. “If a thread is damaged, throw it away and get a new fastener. Running a die over the threads weakens the clamping ability of the fastener.”
It’s All About Clamping Force
It’s all about clamping ability for fasteners. Every procedural step at ARP is in place to improve the ability of the fastener to provide a specific clamping load. After the threads are rolled into the fastener, spot checks are done on the batch for fatigue testing and tensile testing.
ARP bolts and studs have threads that are rolled into the material.
Raschke explains that nothing is left to chance, “all the calibration for the machines and testing equipment is done in-house as well as the research and development of new fasteners.”
From the material selection all the way to the oxide coating process, ARP fasteners are manufactured to provide consistent load clamping to desired specs. Jeff Kibler, ARP research and development technician, explained “consistency and repeatability is the key in any application where fasteners are used.”
Kibler is an expert on preload in fasteners. Each ARP fastener is designed for a specific preload, which can be undermined by the method used to torque the fastener. According to Kibler, “proper lubricant helps the torquing procedure become more consistent and repeatable.
“We’ve done tests on different lubricants from motor oil to moly based, and the ARP Ultra-Torque assembly lubricant is the only lubricant that produces a consistent preload in multiple cycles. This is true to within a plus or minus 5% for each cycle,” he added.
Tightening fasteners to their required preload is critical for proper performance. “If a fastener is not tightened properly, the fastener will not apply the required preload and may become susceptible to failure,” says Raschke, adding “if a fastener is overtightened, it is also susceptible to failure by exceeding it’s maximum yield point.”
There are three generally accepted methods employed to determine how much tension is exerted on a fastener:
Using a torque wrench
Measuring the amount of stretch
Torque angle (rotating the fastener a predetermined amount)
Destructive load testing to failure is one of the many spot checks that ARP does to ensure quality control.
It’s widely accepted that measuring the amount of stretch of a fastener is the most accurate method, however, stretch can only be measured with the use of specialty type gauges or expensive ultra sonic measuring equipment.
For most auto enthusiasts, measuring stretch is only practical for measuring the stretch on connecting rod bolts where it is possible to monitor the overall length of a fastener as it is being tightened.
Jeff Kibler checking main bearing measurements on a Ford Coyote engine using ARP fasteners. (Photo by Bill Holland)
Since most fasteners are installed blind and can’t be accessed from both ends to monitor stretch, using a torque wrench is the most common approach for the majority of assembly work.
Simply stated, the relationship between torque and clampload is friction. When you tighten a fastener with a torque wrench, you are measuring the amount of friction that is generated by the mating surfaces in the joint as the clampload is increased.
The value of the friction in the joint cannot be predicted or measured, but it can be minimized. Using assembly lubricants like ARP’s assembly lubrication stabilizes the torquing procedure making it “consistent and repeatable.”
A fastener must be stretched a specific amount in order for it to function properly. The amount of preload you can achieve from a bolt or stud depends on the material, it’s ductility, the heat treatment, and diameter of the fastener.
Every fastener has a yield point, or point where the fastener is overtightened and stretched too much. “As a rule of thumb, if you measure a fastener and it’s 0.0005-inch longer than it’s original length, it should be replaced,” said Raschke.
Raschke explained that “when using a stretch gauge it’s best to measure the fastener prior to starting and monitor overall length during installation. When the bolt has stretched a specified amount, the correct preload has been applied.”
Using a Torque Wrench
Kibler explains what happens during the torquing operation with a torque wrench, “friction is at its highest value when the fastener is first tightened. Each subsequent time the fastener is torqued and loosened, the amount of friction lessens. Eventually the friction levels out and becomes fairly consistent for all following repetitions.”
Friction is affected by the surface finish of the fastener itself, as well as the receiving threads. Black oxide behaves differently than a polished fastener,” says Raschke, “so, it’s important to follow the torque recommendation with each fastener kit.”
Using a torque wrench to get the correct preload on a fastener is more than just dialing in the preset value and tightening until the wrench clicks.
Almost every engine builder is aware of the burrs and debris in the bolt holes that can affect bolt and stud torque but where the do-it-yourself garage engine builder tends to go wrong is using cutting taps instead of thread chasers to optimize the receiving threads before installation.
“There’s a very real problem of burrs and debris in the bolt holes that can significantly affect the amount of torque required to achieve the recommended preloads,” says Raschke, “All bolt holes should be thoroughly cleaned using special chaser taps.”
Cleaning the receiving threads with a thread chaser helps to achieve the correct preload.
Most torque wrenches used in garages today are the click type torque wrench. You simply adjust the wrench to a predetermined “preset” value and use the torque wrench like a ratchet applying rotational force until the wrench “breaks away.” Conventional wisdom is to use a torque wrench within 50% to 75% of it’s range.
These type torque wrenches depend on a calibrated spring for operation and they tend to lose accuracy when you operate them below 20% or above 80% of their rated range. If you taken precautions to select the right fastener, a high quality assembly lube like ARP’s Ultra-Torque, and cleaned all the receiving threads, you will want to tighten the fastener with confidence by operating the torque wrench within it’s most accurate range.
Bill McKnight, Team Leader of Training at MAHLE/Clevite explained how preload works on engine bearings, “the split bearing halves are slightly larger than an exact half, this extension is called crush height.
When the split bearings are snapped into place in the housing, as the bolts are tightened the bearings compress like springs. The resulting force holds the bearings tight and prevents them from spinning in the housing bore.” Without the proper preload, it’s possible for internal engine components to fail quickly and destroy an engine.
Fasteners and Torquing Methods Make a Difference
We’re pretty satisfied that selecting the correct fastener, manufactured for the intended clampload, and properly installed is the difference between a professional engine build and the amateur engine assembly. It’s simply not enough to buy top shelf engine components without considering how these components are attached to the assembly.
Professional engine builds begin at the ground level with proper selection of fasteners and good torquing techniques. Overlook these fundamentals and you may have overlooked your chance at building an exceptional engine.