The Nuts and Bolts of Torque: All about tightening fasteners.
November 15, 2005
Filed under Motorcycle Features: Bikes, Blokes, Culture and Beyond
Torque – the nuts-and-bolts kind – is important, but many of us don’t understand exactly why. We know things like fenders and spark plugs tend not to fall off or unscrew if we tighten them without the benefit of a torque wrench.
Since this is the case, why are torque specifications and torque wrenches so important? I can best answer this by examining how a bolt works and what happens when you tighten one. Bolts are like springs, and they hold our motorcycles together. The tighter they are (within limits) the greater the spring pressure they apply to the parts they’re clamping.
If the bolts are too loose, oil falls out or the head gaskets blow. If they’re too tight, parts become distorted or even break, and the oil falls out or the gaskets blow. A torque wrench allows you to apply a reasonably precise amount of spring tension to the bolts. Bolts and the parts they hold together resist distortion. For example, if you flex a fender to the side a bit, it resists the force and springs back to its original shape when you remove the pressure.
When you tighten a bolt, you flex it and make it slightly longer. The bolt resists this flexing and becomes a spring, just like the fender did. If you severely over-tighten the bolt or push too hard on the fender, you could bend them. A bolt that’s been bent (permanently stretched) by over-tightening doesn’t apply the correct amount of clamping pressure and is likely to eventually fail, either by loosening or breaking.
The trick is to apply a correct amount of tension to the bolt and have that tension remain when the bolt is in use. Let’s take a look at how these things are accomplished. Lubrication Threaded fasteners like nuts and bolts are high-friction devices. We depend on the friction of the threads to hold them in position. But if there’s too much friction, the critical stretch may not be correct.
The purpose of lubrication is to ensure that the applied torque deforms the bolt along its axis (stretch) instead of around its diameter (twist). If you’ve ever had a bolt back off a little when you released the torquing force of a wrench, then there was too much friction between the threads, and the bolt did not receive the intended amount of stretch (spring tension). Lubrication of the bolt threads is important only to ensure that this doesn’t occur.
The common standard for thread lubrication is a light coat of oil of about 10W viscosity. A thin coat of most engine oils does just fine. The idea is to allow the threads to move against one another and light oil does that reliably. Of course, this standard assumes that the threads of the bolt and nut, or case, or whatever, have reasonably smooth finishes. Exotic or extreme-pressure lubricants such as gear oils or moly pastes are mostly a waste of time and can actually be harmful if they reduce friction too much.
Friction, Locking & Loosening As I mentioned, there must be some friction between the bolt and the parts it’s clamping. If there were none, the pull of the tensioned bolt would cause it to unscrew. The trick is to control the friction so that there’s enough to prevent unscrewing, but not so much that torquing becomes inaccurate. Generally, a bolt remains tight if it’s not shaken or heated. Vibration can unscrew bolts.
So can thermal cycling. This loosening problem is as old as machinery. We’re all aware of some of the traditional ways manufacturers deal with this problem. There are split washers with sharp offset ends that allow the bolt to turn easily in the tightening direction but strongly resist loosening. Star washers, while meant for low-stress applications, really dig in and keep things tight. There are also Belleville washers, wobble washers and countless other gadgets designed to prevent loosening.
Then there are the chemicals. Most of us have used Loctite and similar products. Harley relies heavily on such chemicals to maintain the torque of a majority of its fasteners. Those little stripes of paint-like material on your genuine Harley bolt contains micro-balloons filled with a thread locker similar to Loctite (it even may be Loctite, for all I know). When you tighten such a bolt, some of those micro-balloons burst and release the locking compound.
As I understand it, this feature is good for four uses, after which you need to replace the bolt or start adding Loctite. Back in the ’50s and ’60s, we used 3M Weather Strip cement to keep all those poorly made bolts attached to our Triumph racers. We’d tighten or (mostly) over-tighten the bolts without benefit of a torque wrench, and then entomb the bolt head with the 3M “gorilla snot” (it was bright yellow).
Honda bolts didn’t need the help because of their superior thread design, more precise manufacture and harder aluminum that created and maintained greater friction between the threads of the bolts, nuts and threaded cases. Bolts can loosen for other reasons, too. Harley head gaskets continue to be of composite construction. These gaskets crush and become thinner due to repeated thermal expansion of the engine. The thinned gasket allows the cylinder studs to shorten, which reduces the clamping pressure.
I measured this torque loss back when I was working with the early Evo engines. An initial torque of 40 ft-lbs would drop to 25 after a couple of hours’ use. This loss of clamping pressure was the main reason those engines had a problem with head-gasket failures. I don’t know what Harley engineers did, but they haven’t had a significant problem with blown head gaskets for some years now. Most of the motorcycle industry has moved on to all-metal head gaskets to remove this particular concern. The Screamin’ Eagle gaskets are all metal (Cometic makes them) and are most reliable.
Clamping Force & Temperature The clamping forces that hold your Harley engine together change with temperature. This is because the bolts are steel and most engine parts are aluminum. Almost all metals expand when heated. Aluminum grows about 10 times more than steel for a given temperature rise. This difference in thermal expansion is why those old Evo head gaskets would blow at full throttle when the engine was cold, but not when it was hot. There was less clamping pressure when cold, and it often wasn’t enough to keep the heads from lifting at high cylinder pressures.
A freestanding Evo cylinder grows about 0.040 inch when raised from room to running temperature (70F to 250F). An accomplished engine builder measured total Evo cylinder assembly growth to be 0.060 inch from the base of the cylinder to the top of the valve cover. The same numbers, I’m sure, apply to the Twin Cam engine, as well. While the alloy cylinders and heads grow enough to measure with a ruler, the studs and bolts holding these things together are lengthening only a tenth as much.
This differential expansion guarantees that a hot head and cylinder will press together very firmly. This is another reason, by the way, to warm your engine before hard use. Distortion When it comes to cylinders, there shall be distortion. All that aluminum alloy has to go somewhere when warmed between a set of tight head bolts. Harley, I assure you, spends a great deal of time researching shapes, torque values and gaskets to minimize the distortion of engine parts due to temperature variations.
If you decide to increase the torque of your engine’s head bolts (they’re actually special nuts, but who’s counting), you’ll certainly alter the distortion pattern of the parts, with the cylinders being the most important. The Motor Company spent some considerable time and effort arriving at the 42 ft-lbs (more or less) it specifies for cylinder- bolt torque. If you want to use more torque, you should also hone the cylinders with the torque-plate bolts tightened to the same specification you intend to use.
About Harley’s 1/4-turn Head-Bolt Torque The whole point of using a torque wrench to tighten a bolt is to slightly and accurately stretch the bolt to create a particular amount of tension. Another way to do this is to tighten the bolt moderately to get most of the play out of the assembly and then rotate the bolt enough to stretch it the correct amount. Harley uses this latter technique to properly tension its head bolts.
The Harley head-bolt torque procedure calls for a torque wrench but doesn’t use that wrench for final tightening. The Harley procedure includes preloading the bolts by about 16 ft-lbs. The mechanic then makes a witness mark on the head bolt and cylinder head with a pencil, and finally turns the head bolt an additional quarter-turn to establish the final torque. This procedure ensures that the bolts are accurately tensioned and reduces possible adverse effects of friction.
Triumph used the same technique to tension its connecting rod cap nut/bolt assemblies, calling for a preload on a nut/bolt, then a careful measurement of the bolt’s overall length. The last step was to continue to tighten the bolt until it stretched .004- .005 inch. This technique ensured correct tension on the bolt. I never had one fail, or heard of one failing. Summary When you torque a bolt, you slightly stretch it and invoke its spring (elastic) properties, which applies a clamping force that holds the assembly together. Lubrication ensures that all the torque force becomes tension in the bolt.
Friction devices, chemicals and other locking gadgets then hold the tensioned bolt in position so that it doesn’t loosen. When you understand that torque specifications exist to make sure bolts get stretched just enough to apply proper clamping pressure, then the mystery goes away. The purpose of a torque specification is to ensure that we tension those bolts (springs) correctly. Need I point out that you ought to have a torque wrench and shop manual, and that you ought to use the one and refer to the other? But you knew I was gonna say that, didn’t you?