Proper Torque is Critical

Story by Pat Morrison//
March 1 2012

Over-tightening bolts can cause several problems. It can strip threads, which can cause massive headaches if it happens in an engine case. This is especially true if it’s in an out-of-reach area like the engine-oil drain plug beneath the bike.

While stripped threads are a major concern, they are by no means the only problem. Over-tightening can deform engine covers, causing leaks, or worse yet, it can crack them — and if you own an older, classic bike, sourcing a new engine case can be difficult and expensive. Over-tightening can also snap a bolt in its mounting hole, requiring special extractors and some skill to remove. In some cases, it can also be dangerous; over-tightening a wheel axle, for instance, can put unnecessary stress on wheel bearings, leading to premature wear or causing them to overheat and seize.

Despite the many problems that can arise from this all-too-common occurrence, it is one of the easiest scenarios to avoid when working on your motorcycle. All manufacturers, whether of motorcycles, automobiles or snow blowers, provide torque specifications for every nut and bolt that holds their products together. All of these specs are found in the service manual, and many user-serviceable ones, like axle nuts and drain plugs, are included in the owner’s manual.

Several factors determine a torque specification: the nut or bolt’s diameter, the pitch of its thread (fine or coarse), the material it is made of, its grade (tensile strength), as well as the material it is being threaded into, and what function it is performing. A bolt threading into a cast-iron cylinder head can withstand higher torque than one threading into an aluminum head. Also, if the bolt is holding a muffler onto the frame, chances are it will have a higher torque setting than the bolt that clamps the exhaust to the clutch cover or an engine case.

When a bolt is properly torqued, the threads are slightly stretched and it is the rebound effect of the metal wanting to return to its natural position that holds the nut or bolt tight. If a thread is over-tightened past its torque spec, it is considered to be over-stretched and therefore should not be used again. You can imagine the problem this causes with over-tightened threads in an engine case, as it is not something that can be easily replaced.

Torque specifications are usually given in Newton-metres or foot-pounds, while smaller bolts may be in inch-pounds. These measurements represent the amount of force needed at the end of a tool to tighten a bolt. In the case of foot-pounds, it is how many pounds of pressure must be applied one foot from a bolt’s axis. A torque specification of 25 ft-lb means you would need 25 pounds of pressure at the end of a 12-inch tool to achieve the proper torque. This is more difficult than it sounds, because not all tools are alike; a shorter tool would require more pressure, a longer one would require less.

There is only one tool that should be used to tighten bolts properly – especially critical ones – and that tool is a torque wrench. This doesn’t mean you need to carry a torque wrench in your toolkit, but you should double-check a wheel axle tightened by the side of the road as soon as you can.

Torque wrenches are calibrated to compensate for their length, and their sole purpose is to indicate when the proper torque is applied. There are two types. The simple beam-type torque wrench uses a flexible shaft that bends when torque is applied, indicating the amount of torque by means of a needle on a scale. There is also the click-type torque wrench – when using this one, you will actually feel and hear a click when the desired torque is reached.
The beam type is less expensive to buy, starting at about $25, but it is a bit more involved, because to achieve the proper torque setting when tightening a nut or bolt, you have to keep an eye on the scale, which isn’t always easy to do depending on which position you’re in. The click type is more expensive, starting at about $50 and going up to more than $400, but it can be adjusted to the desired torque setting using a twist handle, and because it clicks when the proper torque is reached, you don’t have to watch it.

When tightening nuts and bolts, bear in mind that torque specs are given for clean, dry threads. Grime on the threads will create resistance that can throw the settings off, and usually, less torque will be applied during tightening. Conversely, oil on the threads will lubricate them and increase the amount of torque applied, possibly leading to stripped threads even if you adhere to the proper torque setting. In special instances, like on certain cylinder-head bolts, manufacturers will specify that threads be clean and dry, but that oil be applied to the mating surface of the bolt head for proper torque. Remember that you should refer to the service manual for this information – if you’re skilled enough to be servicing the cylinder head, you already know that you need a manual handy.

So, before tackling that next project, invest in a torque wrench. It’ll save you both money and headaches later on. 

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.


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