Torque wrench buyers guide and how to use a torque wrench
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This is a basic article about fasteners, how to use a torque wrench, types of
wrenches, and some good brands.
Table of contents
Torque wrench introduction
Basic notes about fasteners
Using a torque wrench
Torque wrench types
Torque wrench brands
Final notes
Torque wrench introduction
A torque wrench measures the torque on a fastener. Torque is a measurement of twisting force. There are other methods of tightening fasteners, such as through tension or bolt stretch but this is beyond the scope of this article and not used in your VW manual. A torque wrench is similar to a normal wrench but you should only use it for tightening. Do not use a torque wrench for loosening. When looking at torque specs, always check to see if it's in ft-lbs, in-lbs, Nm (Newton meter)! Also, the unit of measurement is "ft-lbs", not ft/lbs, that's foot divided by pounds. Lb-ft is the unit when referring to the amount of torque an engine produces.
All critical components must be tightened with a torque wrench because improper torque leads to damaged parts. Some parts call for "x" torque, some call for torquing in stages of increasing force, and some should be torqued in stages even though it's not specified. For example, pressure plate bolts, head bolts, or camshaft retainer cap bolts should be always torqued in stages to avoid warping. Threads should be clean and fasteners should be dry except when specified. Some VW head bolts should be lightly coated in oil and let dry before torquing. Using lubricant will change the torque values, so check to see if the VW specs call for oiling the bolt.
Disclaimer: I'm not an engineer so please suggest additions and corrections to the information below. This is a basic article about using a torque wrench on a car and avoids discussion of things that an engineer would point out such as thermal expansion, tensile strength calculations, or bolt stretch as a measurement of torque. Have a question or wrench brand recommendation? Post your comment in the myturbodiesel forums.
Basic notes about fasteners
When any fastener is torqued, it stretches a little bit. For
simplicity, this article uses the term bolt for all fasteners and assumes metal
bolts. Think of a bolt like a coil spring. If you use too
little force it's floppy and doesn't hold. If you use too much
force the bolt stretches too much and fails to return to it's original
shape, like an overstretched slinky. The
majority of clamping force is at the head/shoulder of the bolt and the first
1/3 of the threads. As a rough estimate, about 40% goes into the head, 40%
goes into the threads, and 20% goes into stretching the bolt. If you look at the below picture, you can see how the
failed bolt was pulled by force between the first 1/3 of the threads and
shoulder. Metal can stretch but the elastic limit of the bolt below has
clearly been exceeded. The spot where the bolt stretched has also reduced
in diameter.
Stretch bolts
Even a stretch bolt should never look like that! Some bolts used on your TDI are stretch bolts that should only be torqued to their final value
once. Adding to the problem is that techs at the dealership might overtorque
or reuse them, further weakening them and bringing them close to their failure
point. If you do not replace them, this can lead to serious damage!
The cylinder head bolts, "dogbone" pendulum transmission mount bolts, and engine mount bolts are examples of stretch bolts used on some generations of VW. Dealers sometimes reuse them and it holds, but it's also possible (and it has happened many times) to have the engine fall out of the engine bay, resulting in thousands of dollars of damage and possible injury. The engineers who designed your car, the service manual, and good practices all call for replacement of these single use bolts when removed, so I suggest always replacing them as called for in the manual.
Why use stretch bolts? To the best of my understanding, tightening a bolt close to it's yield point produces a more consistent torque, especially across temperature and vibration stresses. When you tighten the stretch bolt to the specified torque, it comes close to it's yield point. When close, it sometimes feels as if the bolt isn't getting any tighter, sometimes you don't feel anything odd. But if you continue to tighten it, you will notice a drop off in resistance. It's possible that the bolt was overstretched and is no longer usable. In your VW service manual, if a bolt says to tighten to "x" torque and then turn another 1/4 turn or 90o, it's definitely a stretch bolt. This brings it to the point where it begins to stretch, then stretches it a little bit. This method of torquing, the torque turn method, can be more consistent than just going to a certain torque. "The angle of rotation of a threaded fastener is more directly proportional to the clamp load than the measurement of torque...Because of the additional expense of angle measurement, the torque-turn methods are usually reserved for only the most crucial fasteners in an assembled product" - 2007 Quality Magazine.
Unless otherwise noted, make sure that the threads are clean and dry. You should not apply lubricants such as WD-40 or oil to a bolt because these decrease the resistance to the fastener. You should also clean any damaged threads and in the case of a machined tapered fit, like the camshaft sprocket, the taper should be free of burrs or dirt/oil. "Threaded fastener analysis has shown that most of the energy applied goes into overcoming the friction that exists under the head of the fastener and in the threads...Together these two sources can typically be as much as 80% to 90% of the applied torque energy, leaving as little as 10% of the energy to transfer into clamp load...the presence of dirt on the threads can increase the amount of friction....a drop of oil under the head of a fastener can reduce the friction and allow more energy to flow into the clamp load, which can overtighten..." - 2007 Quality Magazine. In other words, rust and corrosion, or WD-40 and oil on the threads can throw off your torque readings. In some cases, such as the VW head bolts, a very thin coat of oil is called for. In this case, the oil is being used as a friction stabilizer.
Bolt stretch or bolt tension is an accurate way of measuring fasteners but this is beyond the scope of this article and this method is not used in the VW manual.
VW wheel lug bolts
As an example of lubricating bolts is the VW lug bolts. The lug bolt factory torque spec on your VW TDI is 81 ft-lbs. When tightening the lug bolts, I suggest a thin dab of anti-seize for bolts on the brakes or suspension. This will prevent seizing of the lug bolts because they would otherwise rust solid and become impossible to remove in an emergency. However, this lubrication does change the torque spec that you should use to a lower value, anywhere between 10% and 40% lower has been suggested. Personally, I feel that 75 ft-lbs on the VW lug bolts is low enough since many people go even higher when applying anti-seize and do not report lug bolt failure. The greater evil is loose or seized lugs when using hand tools and seized bolts/nuts when using impact wrenches, not lugs failing due to slight overtorquing. Retighten the lug bolts after a few days of driving since they can loosen.
Using a torque wrench - general tips
When tightening a fastener to the final value with a torque wrench, you should make a smooth, controlled motion until the final value is reached. The reason why is because of the difference between static friction and kinetic friction. In most cases, static friction is higher than kinetic friction but the relationship changes as torque value increases. Stopping and re-tightening will change this relationship and possibly change the final value. If you are jerking the wrench, this can throw off the perceived torque spec.
As a rough rule of thumb, you should also use the wrench on the part that is being rotated. For example, if you are tightening a nut and bolt, counterhold the bolt and tighten the nut. If you are tightening wheel lug bolts, tighten the bolts instead of turning the wheel and holding the wrench steady!
When tightening (or loosening) head bolts, camshaft cap bolts, clutch pressure plates, etc., do it in stages. For example, tighten it hand tight, then 25% on each bolt, then 50% on each bolt, then 75%, etc. This keeps the object being tightened level and avoids warping.
As a rule of thumb, torque wrenches are most accurate in the middle of their acceptable range and least accurate at the top and lowest 10%. For example, if a wrench is rated for 10 ft-lbs to 100 ft-lbs, it's least likely to be accurate below 19 ft-lbs and above 91 ft-lbs. Of course a well calibrated wrench should be accurate within a certain percentage throughout the whole rated range but you never know! Because of this, an assortment of torque wrenches is a good idea for a well stocked garage. If you were to buy some wrenches, I would suggest a low range one that includes the in-lbs scale up to 20 or 25 ft-lbs and one for about 10-100 or 150 ft lbs. This covers most auto use fasteners, a third wrench for higher values can be useful for some less used fasteners.
Universal joints - don't use them with a torque wrench but you can use a socket extension. Just make sure that when using extensions, to keep the wrench perpendicular to the fastener and avoid slanting the wrench. Crow's foot extensions change the lever arm length and add leverage, causing you to underestimate actual torque. If you must use an extension that changes the angle of the wrench, use it at a 90o angle to get an approximate torque and use your best judgment, it's still better than nothing.
If you are using a micrometer type wrench, reset the torque wrench to the bottom of the range after you are done using it. If you don't set it to the lowest range (not necessarily 0), it will wear out the spring inside by taking a set and losing its calibration. After letting a micrometer type torque wrench idle for an extended period, you should set it to the middle of the rated range and exercise it a few times. It can also be damaged if you use it to loosen bolts. Most torque wrenches are for tightening, not for loosening. An exception is the dial/beam type wrench since it's just a bar but they should not be maxed out to either end of the scale to prevent any possible damage.
If find that your micrometer type wrench has been stored improperly and it's urgent that you use it, exercise it a few times to try to reset the spring as much as possible. Tighten a spare fastener to 25%, 50%, 75%, 100% of the wrench's rated range. Let it rest for a few minutes, exercise it back down, and let rest again. Obviously you should store your tools properly but in an emergency, this will do.
Torque wrench types
The most common types of torque wrenches are the beam, click micrometer, click split beam, dial, electronic, and digital. Here are some of their pros and cons. I recommend the split beam, then the micrometer type for auto use. Regardless of which type you get, it should be calibrated to check it's accuracy.
The beam torque wrench (below left) is the most basic and reliable type of torque wrench. If
you don't exceed the metal fatigue strength, it will also be accurate for a long long
time. It has almost no moving parts and if the needle gets bent you can
just bend it back to 0. It's generally the least expensive too. The
problem is that you have to view the pointer needle at a 90o angle or
else you get parallax, or viewing error. It's also the hardest to use in
tight spaces where you might not be able to see the pointer needle at all.
I generally avoid this type of wrench since you often can't see the scale,
especially in auto use, and
I'd rather spend my money on another wrench. There is also no way to
prevent slight overtorquing since it relies on user precision.
The dial wrench (above right) is also a good design. Like the beam wrench, it's simple and reliable but due to the size, you can't use it everywhere. It's similar to a beam wrench in that it uses a torsion bar but it reads on a round dial instead of a scale. Like the beam wrench, it can be hard to see the dial when used on autos.
The click micrometer style torque wrench is the most common type of
wrench. To adjust the torque setting, you unlock the handle and turn it to
set the scale on the handle with the scale on the shaft. There
are a few variations on internals, but they are similar in function. When
you tighten the handle, you are turning the handle on an internal screw shaft.
This shaft adjusts the main spring preload which is holding the ratchet head
(with a ball detent between the spring and the head). When the force on the ratchet head overcomes the spring force the
ball detent breaks away and releases the head, hitting the inside of the housing and making the click noise. This
click noise lets you hear and feel when the torque setting has been reached and lets you
use the wrench where you can't see the numbers. Because of the
strength of the click depends on how much force is being overcome, a low torque
setting will generally make a smaller and softer click than a higher
setting. This clicking breakaway also prevents overtorqueing (as long as
you do not continue to turn the wrench) since it releases the force. Below is
a simplified illustration
that helps show how it works. Cordless drills use a similar spring
loaded ball/detent clutch just behind the chuck to limit torque at the
bit. When you exceed a set value, the balls release the torque and it makes
a rapid clacking noise.
Below is a click type
micrometer wrench (lower), with a split beam wrench (upper). These are the
two types I recommend for auto use. Note the
pivot just below the head on the micrometer wrench. A good site which
shows the internals and disassembly of a micrometer style craftsman wrench is http://www.tegger.com/hondafaq/torque_wrench/index.html
.
The main disadvantage of a click micrometer wrench is that it can be sensitive to getting dropped and you must reset the spring to the lowest torque setting after each use or after about 1/2 hour, whichever is longer. If you haven't used the wrench in a long time, it's also suggested to set it to 50% of the wrench range and exercising the spring a few times before use. This is because the spring fatigue or the non-linear properties of the main spring. Another disadvantage is that it can take extra time and effort to turn the adjustment handle since you are working against the pressure of the main spring. It's especially tiring when you are trying to set a heavy torque setting and are working against a heavy spring. They are also less accurate in the counterclockwise direction vs. counterclockwise.
The split beam click torque wrench is also a commonly used
wrench. I think it's the best design for a few reasons, mainly
durability and ease of use. It has fewer moving parts and you don't have to reset it to the
lowest setting after each use or exercise it before using. You also don't
have to turn the handle against the force of the main spring when setting the
torque spec. There's a small knob on
the side which you can turn with a light finger touch. It's position is
locked under the guard/lock. Due to the design, they
only measure in either counterclockwise or clockwise but not both. Here is
a picture of a split beam type on left, micrometer style wrench on right.
An electronic torque wrench is like one of the above but with an electronic readout or torque selection. It is not to be confused with a digital strain gauge torque wrench.
The digital strain gauge torque wrench measures torque via a strain gauge. A strain gauge uses electrical current variation when deflected to measure torque. I don't use these due to cost. Instead of a click, they use a light or beep to indicate that the torque has been reached.
Torque wrench brands
| My personal opinion is to spend a little more,
the equivalent of 1 tank of
gas, and get a quality tool. Note - this article was written in 2008,
if the picture to the right shows current gas prices, then tools are cheap
by comparison! Remember that a failed fastener on a critical engine
component can result in $1000's of dollars in damage! Cheap torque wrenches from H---- F---- work but don't rely on them for accuracy. If you just need something to torque the lug nuts and don't want to spend much, it's better than the "loosey goosey, whoops, stripped" method. With lug nuts, consistency between the torque nuts is important and a cheap wrench shouldn't be too far off. The mechanics at your local tire shop sometimes overtighten the nuts even worse, so using a cheap torque wrench is better than nothing. Forget about using them on any important component like timing belts or head bolts. Any wrench that doesn't come with a custom certificate of calibration with the tool serial number is not something you want to use on the most critical components. I would just spend the money on another wrench. You may also have trouble having it calibrated because some torque wrench calibration services won't calibrate these cheap wrenches. It also doesn't make sense to spend $60 to calibrate a $25 wrench. |
 |
The Craftsman torque wrenches used to be higher quality but
they have some issues with the current designs. Their torque wrenches
have a plastic handle with the scale painted on them. Users report that as
soon as you wipe
the numbers with auto type chemicals, the painted numbers will come off. The
locking rings on the handle have also had some poor reviews due to
failing. The calibration is supposed to be decent but the
plastic handle and locking ring dissuaded me from keeping it. In
addition, the handle's scale on mine wasn't perfectly lined up with the shaft's scale
so I had to unscrew it and turn it back to the lowest setting to be really sure
where the marks
were lining up. Although craftsman sockets and most tools have a
lifetime warranty, the basic craftsman wrenches come with only a 90 day warranty and no warranty on the
calibration. Pictured below is the "microtork" wrench that I tried and was
unsatisfied. I own mostly Craftsman tools due to their value and lifetime warranty
but I do not like their torque wrenches or hydraulic floor jacks. I
don't mind breaking a socket or ratchet but I won't use a leaky floor jack
or an inaccurate torque wrench, both of which craftsman seems to be putting out
lately. For the price of 1 tank of fuel, you can buy a quality
torque wrench that comes with a custom calibration certificate and should last longer.
The Snap On tools are high quality but very expensive. You can expect a custom certificate of calibration and the tool will last a long time with care. One way to get a good tool at a cheap price is to buy a used wrench off ebay and calibrate it. One seller even sells wrenches that are recently calibrated and he guarantees them too. I would be wary of a micrometer type wrench that has been drop kicked by United Package Smashers but if the tool is packed well, it should be fine.
CDI wrenches are now known as "CDI by Snap On". CDI used to make the wrenches for Snap On until they were purchased by Snap On. It's basically the same exact torque wrench with a different ratchet head. This is a good deal if you want a new wrench. They should come with a custom certificate of calibration.
Precision instruments used to only make wrenches for Snap On and have been making torque wrenches since 1938. Their split beam wrenches are the "old style" Snap On torque wrenches. They should come with a custom certificate of calibration. They make both micrometer and split beam wrenches and I feel that they are a good price. The split beam wrench in the pictures above is a precison instruments wrench. I prefer this brand and type because I think a split beam wrench is better to use than a micrometer style wrench.
SK and Matco have good reputations but I don't have experience with them.
Final notes
The ugly truth is that the mechanic's techs at your local garage often don't use torque wrenches on many components. While there is some margin of error built into fasteners, there are also lots of stripped or loose fasteners. Remember, a torque wrench is measuring only torque applied on the wrench: bolt hardness, size, dirty threads, lubrication, measuring torque instead of bolt tension or stretch, variations between wrenches, etc., all cause variations in force.
You've probably cursed over tightened oil filters before, this is only the tip of the iceberg. Sometimes they can get away with it but oftentimes, a timing belt fails, a bolt gets stripped, and various components fail, resulting in thousands of dollars of damage when it could have been avoided. Hearing about TDI engines that fell out of the engine bay due to reused and overtightened motor mount bolts or failed timing belts is a tragedy since it's avoidable and many times, due to cover ups or lack of consumer knowledge, causes great financial losses to the consumer. At least in those cases there is often a financial remedy, if you're reading this, you're probably working on your own car and you are your own warranty. On my own car, I always use a torque wrench on anything that is important. The responsibility is in your hands, so make you've stuck a torque wrench in it!
Additional links:
calibrating your torque wrench: http://home.jtan.com/~joe/KIAT/kiat_3.htm
another link on calibrating a torque wrench
Have a question or correction? Post your comments in the myturbodiesel forums.