Torque values and threaded fastener clamping loads: Part I: From clamping loads to tightening steps
Text and photos by Mike Mavrigian
In a manner of speaking, bolts can be compared to rubber bands. Once the underside of the bolt head makes contact with the parent surface, the head can’t enter the threaded hole, so additional rotation of the bolt head causes the bolt shank of a high-tension bolt to begin to stretch. The objective is to reach the ideal point where this stretch provides the needed clamping force to properly secure the component being installed.
When tightened properly (to specification), the bolt has stretched within its designed elastic range.
When the bolt is loosened, the “metal memory” elasticity allows the bolt to return to its free, uninstalled length. That’s why we can think of a bolt or stud as a rubber band. If the bolt is under tightened, and does not enter its elastic range, it won’t provide its designed clamping force. If overtightened beyond its elastic range, the bolt can enter a yield point, and can permanently weaken. If there’s no elasticity, the bolt can’t do its job in terms of providing clamping force. It’s just sitting there, filling a hole. Taken a step further, if overtightened beyond its elastic range, it can shear. We’ll delve into this in greater detail, but hopefully you get the drift.
Bolt or stud diameters are based on the load required for component clamping performance. That’s why 1/4-inch or 6 mm bolts may be used in one location, and 3/8-inch bolts or 10 mm bolts in another, etc. A smaller-diameter bolt requires less torque value to achieve ideal clamping load, and a larger-diameter bolt requires more torque value to achieve ideal clamping load.
Although not a perfect analogy, you can somewhat view threaded fasteners as “fuses.” The diameter is based on the requirement for the specific job, just as the amp rating of a fuse is based on the requirement for a particular circuit. Taking advantage of a threaded fastener’s clamping load potential isn’t a matter of guesswork. Especially for critical fasteners, such as any involved in the brake system, steering system, suspension, engine, transmission, differential and wheels, all threaded fasteners must be tightened to their specific-application torque value. If you don’t pay attention to torque values, it’s like buying a set of pistons and sticking them into cylinder bores without measuring oil clearance, or like building a front suspension without measuring any wheel angles. It’s just not a good idea.
How do you tighten a bolt or a nut? While many folks hand-tighten by “feel,” the more astute rely on a monitoring device such as a torque wrench. Unfortunately, when most folks use a micrometer-type (click type) torque wrench, they simply adjust the wrench handle to the desired force (if the wrench is the micrometer type), slap a socket onto the wrench, stick it over the bolt head and tighten until they hear a click, without regard to the variables that may enter the picture.
First of all, is the torque wrench accurate? If it’s a cheap one, or if it’s been banged around in the shop for years, it’s probably out of calibration. Secondly, is the fastener being tightened lubricated properly? Third, is the clamping force being created suitable for the diameter and type of metal?
As I just noted, when a bolt is tightened far enough to enter its elastic range, it begins to stretch (it’s this stretch that creates the needed clamping force). What many folks don’t realize is that quite often as much as 90% of the torque applied during tightening is used, not to create clamping force, but to overcome friction. Friction occurs between mating threads, and between the underside of the bolt head (or nut) and the parent material of the object being installed.
Excess friction can occur if galling or “thread seizing” takes place. This is especially common with threaded fasteners made of alloys such as aluminum, stainless steel and titanium. If galling occurs (at any level of severity), this will make your torque readings inaccurate, since the galling effect will add significant friction at the thread mating area. This will result in a severely under-tightened fastener since your torque reading or indicator click will take place well before your desired clamping load is reached.
Several factors can affect fastener tension, including type of material, material hardness, lubrication (or lack thereof), fastener hardness, surface finish/plating, thread fit and tightening speed. So, the next time you tighten a bolt, consider the many factors at play. It’s not as simple as tightening until you hear a click.
In order to give the threaded connection the best chance of performing properly, pay attention to the following:
1. Make sure the threads (both male and female) are clean.
2. Make sure the threads are in good condition, and free of deformation or burrs.
3. When necessary, apply the required lubricant to the threads before assembly (this may involve engine oil, molybdenum disulphide, an anti-seize compound, or an anaerobic thread locking compound, depending on the situation.
4. Keep your torque wrenches clean and calibrated. Depending on their amount of use, consider sending your torque wrenches out for recalibration once per year. Also, store your torque wrenches in a safe place. Don’t toss them around the shop. They’re delicate instruments that deserve proper care.
5. When tightening, whether using a common hand wrench or a torque wrench, slow down! Fast tightening creates friction and heat at the thread area, which can lead to thread galling. Fast tightening can also lead to inaccurate tightening, as the torque wrench must overcome the increased friction.
6. When you reach the torque limit (your desired torque value), approach this slowly and watch the needle or feel for the click. If you tighten too fast, you may pull the wrench past the pre-set limit (unknowingly adding a few more foot-pounds of torque).
7. Whenever space/access permits, always try to use a calibrated torque wrench to tighten any threaded fastener. This will ensure that you not only reach the desired clamping load per fastener, but also that each fastener on a given component is tightened at the same amount, distributing the clamping load evenly. Obviously, there are applications where the use of a torque measuring device isn’t practical (when snugging down a few small bolts to secure a radio bracket, etc.), but for any engine-assembly or chassis application, it’s mandatory that fasteners are tightened to a specified torque value. This isn’t just theoretical mumbo-jumbo... it’s vital!
The use of a torque wrench or other torque value or fastener stretch monitoring device is absolutely necessary for anything engine related (not only main caps, cylinder heads and connecting rods, but intake manifolds, carburetors, throttle bodies, water pumps, oil pumps, rear seal housings, oil pans, timing covers, valve covers, exhaust manifolds or headers, power steering pump brackets, etc.).
Always take the time to look up the torque value for the specific component’s fasteners, and adhere to the specs. Tightening anything on the engine by “feel” is more often than not the root cause for annoying fluid or vacuum leaks. If you always follow specified torque recommendations, you don’t need to wonder, if a problem arises during vehicle operation, if you under-tightened or over-tightened a fastener. Routinely following specified torque values removes this variable from the fault-finding equation. The skilled engine builder/assembler never guesses about anything. Under-tightening or over-tightening can and does lead to problems, ranging from the mildly annoying to the most severe (blown head gasket, rod bearing failure, etc.). Don’t guess!
While we’re on the subject, I feel the need to address wheel fastener tightening specifically. Wheel installation is the most critical aspect for any vehicle. If a water pump fails, the engine overheats. When an intake manifold leaks vacuum, the engine runs badly and throws DTCs. But when a wheel falls off of the vehicle, this results in crash damage and the potential for loss of life. Please get out of the habit of whamming wheel nuts with whatever air gun is handy. Snug the fasteners, and finish by tightening to spec with a torque wrench. Under-tightened wheel fasteners will cause the wheel to loosen. Over-tightened wheel fasteners can be stressed beyond their designed elastic limit and can snap off. Unevenly tightened wheel fasteners can easily result in warped wheels and rotors, creating vibrations and brake pedal pulsation. This is especially important with alloy wheels and today’s thin-hat brake rotors that are more sensitive to clamping loads.
Yes, it takes a few minutes longer to open a manual or to click on a website to locate the torque specifications, but it’s worth the effort. Remember: We’re automotive technicians, not “grease monkeys.” When we’re installing wheels in the shop, we’re not in a race where seconds count. Take the time to do it right.
How much torque is needed?
We need to modify our thinking in this regard. It makes more sense to consider what load is required instead. After all, torquing to a specific value is simply a means to an end. The load is the important factor. Installation torque is simply a factor that needs to be considered when trying to achieve a specific load.
When torque is applied to a nut or bolt head, most of the input is spent in overcoming friction. At the end of the process, 85% to 95% of the energy transferred through the wrench has been lost. In other words, the clamp load itself may only represent 5% to 15% of your effort.
Because of this frictional loss factor, slight variations in the frictional conditions can result in huge changes in the resulting preload. Variables include surface roughness, surface finish, lubricant, load range reached, dimensions, temperature and torquing sequence.
As you can see, it’s just as important to ensure consistent friction conditions as it is to seek a consistent torque. If the dimensions and surface finish are fixed factors, the preload target range will depend on the lubricant and the tightening method.
The greater the friction, the higher the torsional stress in the fastener body. Since torsion is a function of the imposed friction, a given material reaches its yield strength sooner when the friction is high as opposed to low. During tightening, the apparent yield strength drops by 10% to 20% from the yield strength measured in tensile.
As a consequence, when regular preloads are required, good quality engine oils are sufficient for thread, washer, under-nut and under-head lubrication. In addition, the relatively higher friction will prevent any loosening under vibration.
Choose lubes carefully
When ultra-slick lubes are used, they require high preload to prevent backing-off under vibration. For this reason, be aware of under torquing and make sure that your fastener is not losing clamp during service, which can result from compression of the mating part or stretching of the fastener.
Tightening steps for cylinder heads
In order to optimize your results, following are steps to consider when tightening cylinder heads:
Pressure should be well-distributed along the joint before significant loading is applied. Toward this end, it’s recommended that the first tightening step is made at 20% to 30% of the final desired torque value. This should prevent localized damage due to excessive pressure. In other words, perform the tightening process in multiple steps, rather than tightening a bolt to the final value in one step.
Each additional tightening step should guarantee balanced and progressive loadings. After the first preload step, try to keep the torque targets consistent with the angle of turn needed to reach that torque. For example, the second step could be achieved in approximately two 60-degree turns, and the last step in a single 90-degree turn. This will ensure good repeatability.
Most applications can be achieved in three steps, but more steps can be used if you’re dealing with a great deal of compression.
If you opt for a multi-step procedure, make sure that the steps are not too close to each other. Static friction is more difficult to overcome than dynamic friction, which means that if the steps are too close to each other, the wrench might click before even moving the nut or bolt head. In other words, don’t try steps that are only a few ft.-lbs. apart.
Always follow the tightening pattern, or sequence that’s specified for the particular heads and block. If you’re in a bind, it is generally accepted that cylinder head fasteners will be tightened in a spiral pattern, starting at the center and gradually working outboard. As you’re looking at the top of the cylinder head, visualize a spiral pattern that starts at one of the center fasteners. The spiral will intersect the opposite row center, then as the spiral widens in diameter, each successive fastener location is tightened. That may sound confusing at first, but if you take the time to study the bolt hole layout of the cylinder head, I am sure you will begin to understand what I mean.
In Part II of this article, we will take a look at torque-to-yield, torque-plus-angle, threaded fastener tips and torque wrenches and their use.