The Nuts and Bolts of Torque

All about tightening fasteners, covering bolts in general...

Here's a quick reference guide to torques for common bolts... HERE

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

 

i need to no the torque for the rocker arms its in the book but it saides 60 to65 that to much