Bolts, Torque, and Torque Wrenches

There are guys who work on bikes who never use a torque wrench. Those are the same guys you often see walking up and down the road looking for parts that fell off. Or they may be waiting alongside the road for someone to haul them to town to rebuild something major after some other part came unglued because they stripped the threads when they bolted it up.

A torque wrench is the tool necessary for precision assembly of anything that uses threaded fasteners. While you can educate your wrist to get close enough on some things like spark plugs and bodywork bolts you really ought to use a torque wrench when dealing with critical fasteners. This article will tell you something about bolts and torque, and why torque matters. It will also give you my opinion on torque wrenches for the amateur in his home shop - and there are those who disagree.

Although they don't look like it, bolts are springs that work by being stretched, put under a strain. That spiral of threads on the bolt is actually a wedge wrapped around the shank. When you tighten a bolt down, the threads work to stretch the shank by wedging the end of the bolt away from the head.

It is the pull exerted by the shank attempting to return to its normal, unstretched state that makes the threads grip one another, holding tight by friction. Torque, while measured as a twisting force, is actually just an indirect measure of the force you are applying to stretch the bolt. It is not a direct measurement of the "pull' being exerted by the bolt.

If you tighten a bolt too tight one of two things can happen. Most common is that you strip the threads of the softer metal. This is not to be confused with stripping the threads by cross-threading the bolt or nut. That's an entirely different situation.

No, brute force is what strips threads the way we are concerned with here. You apply a twisting force so strong that the threads on the bolt or in the threaded hole (that's all a nut is; a movable threaded hole) can no longer stand up to it, so they shear off from their parent. With that, the bolt spins in the hole, and no longer exerts adequate clamping force on the parts to be held together. Failure of the joint generally follows soon after as the bolt pulls out of the hole. How quickly that happens depends on the load exerted (Straight pull is the worst. If the load is a shear across the joint it may stick together for a long time.) and the vibration level the joint is subject to.

The other, less common thing is that you stretch the spring too far. When you exceed its elastic limits it no longer attempts to pull back to original length. When that happens, the threads on the bolt are no longer bound as tightly by friction as they need to be to the threads in the nut or hole. While the bolt won't just pull straight out of the hole, it will be likely to work its way out, or the nut to work loose.

Having nuts and bolts stay together is not the only reason that we want a certain amount of torque applied to them. Gasket surfaces also need to be held together by a certain amount of force, so that they can resist penetration by whatever it is that the gasket is supposed to restrain. A head gasket, for instance, that does not have adequate clamping force exerted on it will eventually blow out because the mating surfaces on either side do not provide enough grip to support it against combustion pressure. That clamping force is exerted by bolts pulling a part down against the gasket, and the amount of pressure exerted is measured indirectly as torque on the bolts.

We also want a specific amount of clamping pressure other times as well, such as when assembling a tapered fit joint, or on wheel bearings.

Torque figures published for bolts in generic tables are useful. They are intended to give you an indication of what amount of torque you need apply to assure that they will stretch within their design limits to stay fastened, and no more. Those tables do not address how much torque the receiving metal will stand up to, nor do they acknowledge that the torque they show may be too much load on a gasket.

What that means is that it is not safe to use that published generic figure if you are tightening a specific fastener on your engine. For instance, the maximum torque for a ¼" grade 5 fastener may be X if it is going into a grade 5 nut or grade 5 steel part. However, if you are turning it into an aluminum part it may well strip the aluminum threads before it reaches the value published for the bolt itself. So go by what the shop manual says for THAT PARTICULAR APPLICATION.

To generate a reliable reading from a torque wrench the threads in both pieces, the receiving surface, and the underside of the bolt head must be clean, no grit. The biggest variable in tightening down a fastener is not the wrench, but the fastener. If the threads are dirty, or just plain rough, or if the underside of the bolt head is rough or smooth, or if the surface it bears on is rough or smooth, the torque value may fluctuate up to 25% from the actual strain you want to exert upwards on the bolt. In other words, a rough surface may cause your torque wrench to indicate a higher value than is actually being put on the bolt to stretch it. Look here for some in-depth discussion of the subject.

As to lubrication, well, see what the manual says. Usually a figure given for U.S. equipment supposes a dry assembly unless it says otherwise, but not always. The following link is to a table that gives generic torque values for dry and lubricated threads.

You need to find out for sure, because lubing the threads and bolt head will definitely result in you generating a higher stress on the bolt than what the torque wrench tells you that you are generating. That way, friend, lies stripped threads.

Thread locking compounds such as Loctite have a lubricating effect. I have never seen figures published figures on how much, but any time I use Loctite I cut back on the published figure by 10% UNLESS the book says to use Loctite on the fastener. This may give some engineer a headache, but it has worked for me for a long time.

You may also see in your manual a demand along these lines: "Torque down to X lbs/ft of torque, then turn an additional ¼ turn." You'll often find that for head bolts, especially on Harleys. When it calls for that, do it. The engineers have figured out that the method will, in that particular case, be the most reliable way of achieving the necessary torque on the bolt.

When tightening down groups of bolts in an assembly you should do it in the order specified by the book. If there is no specific order set forth, work in a criss-cross pattern. If it is a long piece, like a Harley primary drive cover, start with the two screws closest to the center, then work your way to the ends, criss-crossing as you go.

Usually, it is better to approach max torque in two or three stages. For instance, if the instructions call for 22 lbs/ft, then I like to go through the tightening pattern at about 12, then 18, then the final value. Then I do a circuit around the edge at the highest torque value just to make sure I got them all.

When working on an assembly, particularly a gasketed assembly, it is more important that the torque be as close to identical on all fittings than that you try to get EXACTLY a book figure. Harley quotes torque figures in ranges, like "12 to 14 inch/lbs." In cases like that, I shoot for the middle figure (13), and try to get 13 on every fastener. I'd be just as happy to have 12 or 14, but it ought to be the same on all fasteners in the assembly. Again, the Harley primary cover comes to mind. Mistightened screws are a major source of leaks from the outer primary cover.

BMW, being German, quotes one precise figure. It's a Kraut thing, and truthfully, I always thought they ought to have better things to do than be that precise. Fasteners work ok when torqued to an acceptable value within a range. When I see a number in a BMW book I just add one unit on either side and shoot for that.

Beware when reading torque specifications that you understand what unit you are working with. U.S. assemblies are usually spec'd in lbs/ft or lb/inches of torque. (Yeah, I know; you see foot-pounds used as a method of expressing torque all the time. It has become the usual method of expression, but properly speaking, work is measured in ft/lbs and torque in lbs/ft, or lb/inches.)

A lb/ft of torque is the amount of leverage exerted by a one-pound weight hung at the end of a one-foot lever extending out at 90 degrees from the point of attachment. That is the same as two pounds hung on a six-inch lever (half a foot & twice the weight, you know?), or a half-pound hung on a two-foot lever. If the pull is exerted at less than 90 degrees the torque is correspondingly less.

An lb/in of torque is a one-pound weight hung on a 1-inch lever. Lb/inches are used to express small amounts of torque. You can convert directly by using a factor of twelve. 10 lbs/ft of torque is 120 lb/inches. 96 lb/inches is eight lbs/ft, etc.

Now what kind of torque wrench do you want?


Torque Wrenches

Before we get into what kind of wrench you want, you need to understand a few things about torque wrenches generally.

You should assume for safety's sake that your torque wrenches are not accurate within 20% of the ends of their scales. That's not always true, but unless you know to the contrary, you should assume that it is so. In other words, you should only rely on a reading in the middle 60% of the range of the wrench. If your TW reads from 20 to 150 lbs/ft (a range of 130 lbs/ft) you should only rely on readings from 46 to124 lbs/ft. Thus, you are better off buying two of them, one that reads up to, say, 75 lbs/ft for medium fasteners, and another that goes from 25 to 200 for large bolts. Then, for the small fasteners an inch/lb wrench is desirable.


There are three basic types of torque wrenches, and several other variants that you probably will never see and certainly never need. I'll deal here with just the two commonest types, the beam, and the clicker. They probably have more elegant "real" names, but those are good enough. The dial-type (and its first cousin, the digital readout) is expensive enough to be rare in the home shop.

The Clicker

The one that most guys instinctively lust after is the clicker. The long handle has a rotating section down at the grip. That section is marked to indicate units of torque, lbs/ft, or lb/inches, or newton/meters (the metric unit). You dial up the value you want, turn the locking knob on the end of the handle, and then go to work. When you turn the wrench and reach the value you set, the wrench gives a "click" and tiny slip that lets you know you are there.

This type of wrench has some down sides though, for the guy who doesn't use one a lot.

First, they are expensive, two to three times as much as the beam type for equivalent quality. From Sears, one will typically run somewhere from $50 to $80, depending on the torque range you buy. A Snap-On wrench can run you over $250.

The second drawback arises when you don't set the desired value properly. Typically, that comes about when you switch from one bolt to another and forget to reset the wrench. It can also happen if you misread the unit markings. In either case you can get a seriously mistightened bolt, with the problems that arise from that. Like a computer, the wrench will give you what you tell it, not what you want.

The third drawback is that they require periodic checking and calibration. They work by preloading a friction joint (that's what you are doing when you choose a setting on the handle) that slips when you reach the torque value set. With use, that friction joint wears and changes its responses, so the 25 lbs/ft setting on the handle may generate 25 lbs/feet this time, but what will it give you 100 uses down the road? Twenty lbs/ft? Thirty? The only way to be sure is to get it checked regularly. Now that is neither difficult nor expensive, but it is a nuisance. Most guys wind up not doing it, so their torque values are only a guess.

Furthermore, as use accumulates, they may lose repeatability if not given routine maintenance. Typically, that's just a shot of oil, but if you don't do it regularly within the period of use, then the response becomes erratic. Too little oil, or too much? Each can change your results. What that means is that you may set it for 10 lbs/ft for a series of fasteners, and it will give anywhere from 8 to 12, and you don't know which. As I said earlier, having them all the same is more important than an exact single value. Perfection, of course, is having an exact, single value on all the fasteners.

Assuming that they are functioning properly, they are especially good for two types of jobs. One is tightening a fastener that places the wrench in a position that you can't see the scale (on the beam type.) The other is production jobs, doing fast assembly of a series of bolts in an assembly that all call for the same value. You just won't find a lot of that on your bike at home.

The Beam Type

Although I own clickers, this is the one I use for most jobs now, having gotten over my infatuation with the magic click.

All wrenches are levers; right? As you pull on a lever it bends. If the lever is carefully made it will bend a specific amount in response to a specific load, and as long as its elastic limits are not exceeded, it will return to its original position when the load is released.

Look at this picture. You see two bars coming out of the "head" of the wrench at the left end? One of them, the one with a black knob on the end, is the lever that twists the bolt. Note also that is has a flat scale plate mounted on it. (FYI, that plate is typically marked in both English and metric values. It is a convenient place to make a quick conversion from one to the other if you need to.)

The other bar, the top one, is the beam with a pointer on the end over the scale plate.

When the wrench is turning a bolt and the bolt starts to resist, the handle, with its scale plate, begins to twist out from under the top beam, or pointer. When the pointer is over the desired torque value inscribed on the scale plate, you're there. No friction-induced clicks, just your Mark I Calibrated Eyeball telling you to quit. While you can apply the wrong torque, it won't be because you forgot to change the setting.

They sometimes need calibration too. The instructions typically tell you to do it like this. "Verify that the pointer is over ZERO when there is no load. If it is not, bend it until it is." Doesn't get much simpler than that.

They are much less expensive than the clicker. A decent one from Sears can be had for $35 or under.

Dial Type

I insert this picture just so you can see what they look like - this is just one type, but you get the idea. You just turn 'em until the dial indicates the torque you want. They are elegant, expensive, and need calibration on a machine. You can see a more precise figure on them than you can on a beam-type, but do you really need it? Probably not. Certainly not on your bike.

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