About this time last year, I wrote in this column about, for lack of a better description, securing your nuts. But from the number of pieces that have fallen off of cars over the past year, I thought it might be worth a little more discussion.
The most effective and reliable method of preventing any nut or bolt from loosening is to tighten the thing properly to start with. When a threaded fastener is properly tightened, the residual stress within the bolt will lock the assembly. Even the best mechanical locking devices can only supplement this action, they cannot replace it. The next most effective method, where the application allows its use, is the elastic stop nut. Regardless of the effectiveness of residual stress as a locking medium, for those applications where the fastener will be subjected to high levels of vibration, cyclic stress and/or turning torque, common sense, the survival instinct and the FAA require some sort of additional positive antirotation or locking device. In critical applications, these devices also act as insurance against human error.
Lock Washers
There are a great many locking devices on the market today. The most popular is also the least effective – the lock washer. There are three basic types of lock washers: the spring washer, the wave washer, and the serrated, or star, washer.
Neither the spring washer nor the wave washer will do anything worth talking about-other than to provide the user with a false sense of security. Think about it for a moment. From experience, you know that it takes very little load to compress a spring washer. For example, the spring washer will be completely closed long before we reach recommended torque when tightening a bolt. Once compressed, the spring washer is nothing but a flat washer. If, for whatever reason, a bolt should loosen to the point where the spring washer opens enough to become a spring, there was too little residual stress in the assembled bolt for any sort of safety. In other words, the thing wasn’t tightened sufficiently. Exactly the same is true of the wave washer which is, for some reason or other, popular in Germany. If you decide to use a spring lock washer, a flat washer should be placed between the lock washer and the work surface to prevent damage to the surface. This is not necessary with the wave washer.
I am willing to admit that there are installations where the serrated or star washer can be effective. These installations are limited to the smaller sizes and almost always have to do with machine screws bearing on a relatively soft surface – aluminum or plastic, for example. The teeth of the washer can and do bite into the surfaces of soft materials and offer reasonably positive protection against rotation. They are available with either internal or external teeth and also as coned washers for countersunk bolts.
I try not to use lock washers. I use prevailing torque-type self-locking nuts on all through holes, and check or jam nuts to lock rod end bearings and threaded adjusters. With blind holes, if I do not trust the thread tension of a properly tightened bolt, I use the appropriate grade of Loctite and/or safety wire. I do, however, carry a selection of aircraft spec (AN-935) spring lock washers around with me – just in case. I will not use industrial spring lock washers because they are liable to be too brittle for my taste.
Safety Wire
I use a lot of safety wire. I use it for two reasons. First, safety wire on a bolt head provides an easy and positive visual indication that someone has tightened it. We don’t even want to think about the mentality that would allow anyone to safety wire a bolt without first checking to make sure that it is tight.
Second, if the bolt does loosen, the safety wire will prevent it from falling out. Contrary to popular belief, even the best job of safety wiring will contribute virtually nothing to the task of preventing a bolt from loosening to the point where effective levels of residual stress disappear. All that the safety wire can do is limit the rotation of a bolt and prevent its physical departure. Even when the bolt breaks, a good job of safety wiring may save the day because the bolt head won’t fall out and get jammed in the works.
Loctite
Essentially, the Loctite compounds are anaerobic liquids. They fill the voids between mating parts and, in the absence of air and in the presence of metal will cure and bond the parts together. The mating parts are not only locked together, they are also sealed against leakage and/or corrosion. The following are some of the more important compounds that you should be familiar with.
Threadlocker #242 (blue). A general-purpose, medium-strength compound that, as the name suggests, does most jobs, as long as the thread in question is engaged for a minimum of 1 1/2 diameters.
Threadlocker #271 (red). A high-strength compound for small fasteners. Loctite’s definition of small is somewhat different from mine – to them, any fastener under 1” in diameter is small. This compound is especially effective in aluminum and magnesium castings. After it has cured, it is very resistant to fuels, oils, and solvents. Where once I Helicoiled all of my Hewland transaxle and bellhousing studs, I now use #271. Contrary to popular belief, it has the same temperature range as #242 (to 300 degrees Fahrenheit). Its shearing or breakaway torque is, however, some 2 1/2 times greater.
Threadlocker #262 (red). A hard-to-find, high-strength compound with “controlled torque tension”– whatever that may be. It is specifically compounded for high-strength fasteners that will be subjected to heavy shock/vibration loads and high levels of stress. I use it on ring gear bolts, universal and CV joint bolts, axle retaining nuts, flywheel bolts and the like. It has about 15 percent greater breakaway torque than 271, and the same 300 degree Fahrenheit temperature limitation. Most engine builders use #262 to install all of the studs in the block. The general feeling is that red Loctite converts a Class 3 thread fit into a Class 5.
Threadlocker #290 (green). Believe it or not, this is a penetrating compound that locks already assembled threaded fasteners. I first ran into it under the name “Loctite Super Wick-In” in Australia. The fluid is drawn into assembled and torqued threads by capillary action and cures to a medium strength similar to that of #242. Like all of the Loctite compounds, #290 does not like oil or grease, so I spray the assembly to be treated with Loctite Primer N, Brake Clean, or some similar aerosol cleaner before I apply the magic. Since I end up running a lot of cars that have been assembled by folks I do not know, I find Loctite #290 to be a great comfort.
