Nuts and Bolts - Part 1

      Various parts of your car are held together by springs. You probably refer to them as bolts.  They’re actually springs. A bolt is a round piece of steel with a head on one end. When you turn the nut on the other end of this bolt you’re really tightening a steel cylindrical spring. Tightening the spring increases the clamping force between two parts. 

   You need to stretch these springs to a rather precise length. We generally do this by turning the nut to a given torque figure. If you tighten the nut too much you’ll actually break the spring. If you don’t stretch this spring to the correct length though hold the parts together 

Let’s use a Porsche connecting rod bolt as an example of a spring. This bolt requires a torque setting of 40 ft-lbs. When you reach that point on your torque wrench the bolt will have stretched between 0.0055 and 0.0065 of an inch. The 40 ft-lbs is an approximate measure. We’re pretty sure that when we reach 40 ft.-lbs we’ve achieved the necessary clamping force. At least we hope so.

The only truly accurate way to measure the clamping force of a bolt is to measure the stretch. Several companies make tools that can measure bolt stretch. Measuring bolt stretch is the most accurate measurement of clamping force you can make. The problem is it’s not always possible.

Torque settings: The most common way of tightening nuts and bolts is to tighten them to a specific amount of torque. We often use torque as a verb to mean we’re applying a twisting force to tighten a bolt. We use ft-lb (or Newton-Metre) as the unit of measurement. 

Torque settings are notoriously inaccurate. When you tighten a nut with a torque wrench you’re too often measuring the amount of friction being generated by the mating surfaces in the joint. As the clampload is increased so is the torque. There is a relationship between torque and clampload. This relationship is determined by the amount of friction in the joint as well as the clampload being developed. The really big problem is that you can’t measure the friction in the joint. If we could do this there would be no need for tightening procedures other than torque. 

Lubrication and Torque: Friction in the threads can give you a false reading. In extreme cases this friction can account for almost 30 per cent of the torque reading. The specified torque value generally assumes clean and dry parts. That means no dirt, no rust and no dried up gasket sealer. You don’t want anything except shiny metal on the bolt. Wire-brushing the threads will normally help remove rust or sealant.

We can minimize the effect of this friction by using lubricants on the threads. A huge number of lubricants have been tried with varying degrees of success. Among these has been grease, chicken fat, mayonnaise, graphite, Teflon, plastics and encapsulated formulas that smash and liquefy under pressure. However sophisticated they might be they’ve all fallen short of the goal of totally eliminating friction. 

If you're installing a fastener that has a dry torque specification and the threads and bolt face are oiled you'll need to reduce the torque by 15 to 25 percent, because the slipperier surfaces will decrease friction. A lightly oiled carbon steel bolt won’t require the same torque specification as a bolt that uses anti-seize or ATF as a lubricant. Teflon-bearing lubes or moly-sulfide engine assembly lubes can reduce friction enough to require a 50 percent reduction in tightening torque.

Just substituting a zinc or cadmium plated bolt or washer for an unplated one calls for a 15 or 25 percent reduction in applied torque. The plating acts as a lubricant. If you fail to heed this advice the fastener will be seriously overtightened. You'll either snap it or crush a gasket to the point where it leaks. 

Rust or burrs on the threads can increase friction enough that a fastener tightened to the specified value won't provide sufficient clamping force. The shop manual will specify whether the fastener is supposed to be dry or lubed. Engine fasteners, like head bolts or main cap bolts are often specified to be tightened with 30-weight engine oil on both the threads and the washer. Don't forget the residue from the parts washer or that can of kerosene you're using to clean parts has oil in it. Even a quick blast of compressed air to dry off a fastener leaves an oily film behind, affecting ultimate torque. If you're really fastidious, clean up all of your bolts with some aerosol brake cleaner followed by dry air. 

The greatest sources of friction comes from the condition of the threads. It’s always a good idea to use a thread chaser on new bolts before you install them. You can also use a chaser tap for the bolt holes. This insures that your torque reading is coming from the tension upon the bolt and not from friction within the threads. A good general rule is that if you can’t assemble a nut and bolt with your fingers you shouldn’t be using them. If you’re using Nylock nuts just the opposite is true. If you can install a Nylock nut on a bolt with just your fingers the bolts is worn out.

Everyone knows that you can take a spring and stretch it so far that it never returns to its original shape. The same thing can happen with a bolt. It’s called a yield point. What happens is that you tighten the bolt too much and it can never return to its original manufactured length. This problem most often happens with wheel studs and the use of air tools.

A bolt has to stretch to be utilized as a spring. Because a bolt is a very stiff spring it has to be tightened in such a way that it will give you a repeatable clampload and be fairly evenly distributed around the joint if it has multiple bolts.

You can check the condition of your bolts by using a stretch gauge. You should keep a record of all your critical bolts. When you rebuild a component measure all these bolts before you install them. Now check them against your records. If the bolt length is .001” longer than it’s original length it should be replaced.  

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