If you’ve ever had a flat tire and visited a repair shop like Discount Tires, you’ve probably noticed the technician using a power tool to loosen the lug nuts. But have you ever wondered why they then switch to a hand wrench when tightening the nuts back on?
Let me tell you why.
He is using a torque wrench to ensure that the lug nuts are properly tightened without over-torquing them.
Over-torquing fasteners can lead to frustrating problems like stripped threads, sheared bolts, damage to surrounding components, etc.
In this article, I’ll explain how tight is too tight, the consequences of over-torquing, and ways to avoid and fix it.
What is Over Torquing?
To understand what over-torquing is, let’s see what torquing means:
When you tighten or remove a fastener like a bolt, for example, you apply force to the bolt’s head to make it turn. This force is called “rotational force” or torque, and it’s measured in lbs-ft or Nm depending if you are measuring it using the imperial or the metric system.
This unit comes from this basic equation: T = r*F
Where T (Torque) equals r (in this case, it would be the length of the wrench) multiplied by F (which is the force applied to the wrench).
In this example, 20 pounds of force are applied one foot from the center of the bolt, resulting in a torque of 20 lb-ft.
- Torquing is tightening a nut, bolt, or other fasteners by applying a specific amount of torque. Most service manuals include the torque specs of each component to ensure that they will be properly tightened, not too loose or too tight. One of the key factors for calculating the torque specs of an assembly is the material strength of the fasteners and parts’ inserts.
- Over-torquing is applying too much rotational force or torque to a fastener, exceeding its material strength.
- Torque can be applied both clockwise and counterclockwise, so it’s important to avoid using excessive torque while adjusting a fastener and when you are unfastening it. For example, a frozen or stuck bolt may require more torque than it was designed to withstand to be released.
Why is Over Torquing Bad?
Over-torquing is bad because it puts excessive stress on the nuts, bolts, screw holes or inserts, and other components weakening, damaging, and even breaking them. Besides damaging the fasteners, over-torque can also damage the parts they hold.
Below you will find a list of the possible consequences of over-torquing:
1. Damage to Fastener Head
Over-torquing can damage fasteners’ heads, making them difficult or impossible to engage with a proper tool. This usually happens when the threads get stuck, and the fasteners or their heads are built from a weaker material than the tool used to handle them. While the fastener can rotate freely, the heads are safe; the problem is when the fasteners stick.
The most common types of head damage are:
- Rounding: hex (for hexagonal) nuts and bolts’ heads deform and lose their sharp edges, preventing a tool like a socket or a wrench from having a proper grip and turning them.
- Stripping: this happens when screws’ heads are over-torqued or worn. Philips, Allen, and Torx screw heads have slots inside their heads. Applying excessive torque may produce damage or deformation, which can even make the slots disappear, making the screw impossible to turn using a proper tool.
- Cam-out: excessive torque can make the tool’s tip slip out from the fastener’s head leading to damage or deformation. This usually happens when working with screws or small bolts and nuts.
2. Tool Tip’s Damage
When working with hard fasteners, over-torque can damage the tip of the tool you use to tighten or loosen them. For example, sockets can round or break, and screwdrivers’ tips can strip, bend, or even break.
3. Twisted or Bent Bolts
Applying excessive force to a bolt may twist it or bend it. In simple words, a bolt can’t handle more torque than its material can support without being damaged.
- Twisted Bolts: are over-torqued bolts which have been deformed around their rotational axis. Twisted bolts can expand laterally, becoming very hard to turn despite being weakened, or can lose clamping force.
- Bent Bolts: bending can be a consequence of over-torquing, but it can also be a consequence of over-torque (excessive rotational force) plus a misalignment of the tool applied to the bolt and the bolt’s head. Misalignment adds lateral force that helps to create excessive stress beyond the bolt’s material strength and can also prevent the bolt from seating properly, which can result in a bent bolt.
Fasteners stretch as you fasten them. The torque you apply is converted into clamping force (tensile force), and as the bolt’s threads bite into the nut or threaded hole’s material, they stretch. They should recover their length once you stop applying torque, but if they receive excessive torque, they won’t recover their original shape and will become permanently stretched.
5. Fastener Breakage
This is one of the most significant consequences of over-torquing. Fasteners can brake at any point; for example, studs can get broken anywhere, including the threaded section, and the screw heads can snap off. Something important is that sometimes, the fastener won’t brake as it’s being manipulated; it will brake while it’s “in place”, which can be very dangerous.
Image: A broken car wheel stud as a result of over-torquing.
Fasteners are designed to endure a certain tension, also called “tensile load”; they stretch and recover their regular length. The capacity to endure tension without breaking is called “tensile strength”, and it’s related to the material’s strength.
Fasteners’ materials can withstand a certain amount of tensile strength before deforming permanently and eventually snapping. The maximum force that they can withstand before becoming unable to recover their original state is called the “yield point”. Once the force applied is bigger than the fastener’s yield point, they suffer permanent damage. Applying force above the yield point will end up breaking the fastener.
6. Fastener Thread Damage and Galling
Over-torquing can stretch or twist a fastener. The fastener could lose clamping force or stick if the thread stretches or deforms.
The thread also can be damaged because of over-torquing, producing galling. This happens when the fastener’s material transfers to the surface of the screw hole’s thread, making them adhere once against the other. Materials like stainless steel, aluminum, and titanium are prone to galling.
7. Stripped Screw Holes
Screw holes’ thread can also be stripped due to over-torque. This often happens when the fastener’s material is harder than the screw hole’s thread material.
Sometimes the workpiece’s screw thread can be ripped off by the fastener, for example, when hard bolts are fastened into soft metals like aluminum.
In some cases, both parts suffer some kind of damage, but stripped screw holes are harder to fix than just replacing a fastener. Galling is also often a product of over-torquing, which can damage the workpiece screw thread.
8. Over Torqued Wheels Can Cause:
- Shaking and vibration due to the wheel’s uneven or improper tightening can lead to off-centered wheels and prevent the wheel from sitting properly against the hub.
- Warping or distortion of brake discs, leading to steering wheel shaking and shuddering when braking. It can also damage other braking and suspension components.
- Stripped wheel nuts and wheel nuts threads and potential deformation or braking of the studs. It’s important to note that deformed wheel nuts can loosen up or even break due to vibration.
- Damage to the wheels themselves. Alloy wheels, for example, are usually softer than wheel nuts which are often made of harder materials. These can produce cracks in the wheels’ screw holes which can extend to the rest of the wheels’ surface. Besides, loose or improperly tightened wheels can distort or warp over time.
9. Blown Head Gaskets
Cylinder heads must be adequately fastened to the engine’s block to ensure the engine’s correct operation. Head gaskets seal the gap between them and prevent leaks of compression, gases, and lubricant. Over-torquing the head cylinder head can damage, deform, or crack the head gasket preventing it from doing its job properly. Over time, over-torqued cylinder heads can lead to catastrophic failure.
10. Damage to Parts and Mechanical Components
Over-torquing the mounting bolts of some components, like control arms and tie rods, can break them or affect their performance. Besides, over-torquing can increase components’ tear and wear, decreasing their lifespan.
11. Excessive Friction to Moving Parts
Over-torquing moving parts generate more friction between the parts being held, which can cause deformation, damage, and excessive friction. This can limit the movement of the components resulting in a loss of efficiency which can lead to bigger problems.
Is it Better to Over or Under Torque?
Both practices are wrong and can be not only harmful to the fasteners’ and the components being held by them, but they also can be very dangerous. It’s like choosing the best of two evils.
There is plenty of information about the consequences of over-torquing above. Now let’s have an insight into the consequences of under-torquing:
- Under-torqued parts are not clamped enough, leaving the parts loose.
- Loosen fasteners can allow excessive play between the parts being held, leading to excessive friction and decreased performance. Besides, as the pieces move freely, the fasteners are exposed to unpredictable forces which can damage, break, or snap them.
- Even when under-torquing won’t damage the fasteners, threads, and components as they are being assembled or disassembled, they can eventually suffer the same kind of damage once they are working. In that sense, over-torquing is a little more predictive than under-torquing because a snapped bolt head, for example, it’s a sign that something is wrong.
How to Avoid Over-Torquing?
The best way to avoid over-torquing is to use a torque wrench to give a fastener the last turns to ensure it meets the manufacturer’s torque specs. If you are unsure of the torque specs or you are in a hurry, always make the final adjustments by hand using a ratchet or a breaker bar so that you can feel the force you are applying.
When using a torque wrench, ensure the parts are clean, rust-free, and dry; dirtiness and lubricants can affect the torque wrench readings.
If you don’t have access to the torque specs of an assembly, you can try figuring it out by reading the bolt’s head marks and using your common sense. The marks on the head of a bolt have helpful information like the bolt’s strength. You can also find charts that will give you an idea of how much stress a bolt can handle depending on its material, thread size, and other useful information. It’s important to remember that when dealing with fasteners, tight not always means better.
Remember that over-torque can be caused in both directions. Try to don’t apply excessive force to remove it to prevent potential damage.
How to Fix an Over-Tightened Bolt?
Always replace over-tightened bolts; they may seem visually ok, but if they were pushed over the yielding point, they might fail at some point. It’s the best way to avoid unnecessary headaches.
Sometimes, over-tightened bolts can be hard to remove. You can try applying penetrating oil and letting it work before trying again. You may need an impact wrench or impact driver to remove the screw without excess torque applied to the fasteners.
If the head is too rounded or snapped off, you may have to drill a hole in the middle of the stud and use a bolt extractor to remove it.
Once you remove the bolt, check the threads for damage. Clean any debris or damaged material from the screw hole’s threads and make sure everything is ok. If the threads are damaged, you will need to fix the threaded hole. You can try using a chemical thread filler if the damage isn’t too serious and the component isn’t too critical.
If the damage is really bad, you can carefully enlarge the hole and use a threaded or spring insert to fix it. There are many models of threaded inserts available. These inserts work great on soft metals like aluminum and are great for fixing oil pans’ screw holes, for example. Sometimes, you can even fix engine blocks’ screw holes. If the damage is severe, or you are unsure, it’s always asking for expert advice.
David Castillo is an automotive industry expert specializing in vehicle electronics and stand-alone fuel management systems. He has over 20 years of experience and owns a car repair garage and tuning shop.
David still runs his shop but is now more focused on pre-purchase car inspections and writing for FinePowerTools.
Back to Contents
- What is Over Torquing?
- Why is Over Torquing Bad?
- 1. Damage to Fastener Head
- 2. Tool Tip’s Damage
- 3. Twisted or Bent Bolts
- 4. Stretching
- 5. Fastener Breakage
- 6. Fastener Thread Damage and Galling
- 7. Stripped Screw Holes
- 8. Over Torqued Wheels Can Cause:
- 9. Blown Head Gaskets
- 10. Damage to Parts and Mechanical Components
- 11. Excessive Friction to Moving Parts
- Is it Better to Over or Under Torque?
- How to Avoid Over-Torquing?