The Nuts and Bolts of Nuts and Bolts
by Curtis Von Fange
The nuts and bolts of nuts and bolts is an interesting and essential piece of knowledge that applies to our older tractors. An improperly torqued capscrew on an engine head or a shear bolt that is too hard on the driving shaft of a bushog can create havoc and make an expensive and uncalled for repair. Letís examine the purpose and design of these fasteners in order to ensure their proper use.
Fasteners are probably one of the aspects of mechanics that is given the least amount of thought. One figures that a bolt is a bolt and if it fits the hole and tightens down that it will hold. Unfortunately, this is often not the case. All manufacturers take great care to make sure that their product, whether it is a tractor, an engine, an implement, or a simple accessory has the fasteners engineered to meet the demands of that particular part. Contrary to popular thought, all bolts are not created equal.
Put mildly a bolt is a metal rod that has a head at one end and a screw thread at the other. The bolt is passed through the parts to be joined then the nut is installed and drawn up thus holding the parts together. A capscrew is, basically, a bolt without a nut. In other words it screws into a prethreaded mating part like an engine block or similar casting. Capscrews might hold an accessory like a water pump to the engine block while a bolt and nut can be used for holding an alternator to the adjusting plate. They are both fasteners.
Bolts and capscrews may be identified by type, length, major diameter, pitch (threads per inch), length of thread, class or fit, material, tensile strength, and the wrench size needed to tighten. All these variables match the fastener to the particular application for which it was designed. A bolt too long not only wastes material but also might interfere with other implement operations. A capscrew too soft might snap off before the proper torque is reached. The wrong thread size will strip out the nut or receiver hole and cause a new set of problems. Letís look briefly at these assorted criteria.
Bolts and capscrews are not all made of the same quality material nor is the tempering the same. Charts that indicate the hardware strength and recommended usage are available at auto parts stores or in service manuals. A simple way to determine the nature of a fastener is to look on the face of the hexagonal surface of the bolt end. Markings are stamped (or, as the case may be, not stamped) on this face that indicates the bolt type. Specifically this shows the tensile strength of the fastener. Tensile strength is the amount of pull an object will withstand before breaking. When a cap screw or bolt is tightened, it actually stretches (about .001 per 30,000 lbs.) due to the tension being applied. Generally, the higher the tensile strength of the fastener the more torque it will accept before breakage. No markings on the bolt head indicate a low carbon steel bolt with a tensile strength 64,000 psi or under. They are a soft bolt, commonly called a grade two or grade three. They have an indeterminate quality and are commonly used for simple use. A bolt head with three raised slots Ďstampedí on it indicate a grade five, medium carbon steel, tempered, and used as a minimum commercial quality fastener with a tensile strength of 105,000 psi. Four raised slots is a grade six, medium carbon steel, quenched/tempered, medium commercial quality, 133,000 psi rating. Six raised slots is a grade eight, medium carbon alloy steel, quenched, tempered, 150,000 psi, and the best commercial quality. A grade twelve has eight raised slots surrounding an oval, and is a special alloy steel, quenched and tempered, and is recommended for critical use and competition purposes.
Make sure that you are using the correct grade and type fastener for the application. A case hardened bolt in the propeller shaft of a bushog will not snap under overload conditions. Damage to the gearbox or other vital spots will result. The proper use of a soft bolt in this case will give the protection required, as the bolt will shear when overloading conditions apply. A capscrew that is too soft on an engine head will not take and hold the torque necessary to hold the part down under operating temperatures and conditions. Warpage of the head and/or capscrew breakage will result. Consult the specification tables for torque values when working with the particular application and make sure the proper fastener is used.
There are two basic types of bolts that are used for our purposes. They are the Unified National Fine thread and Unified National Coarse thread variety. Thread pitch, or the distance between the crests of a thread to the same spot on the crest of the next thread, helps determine the type. The smaller the pitch, the greater number of threads per inch. The threads per inch is best determined using a thread-pitch gauge. Coarse threaded hardware is best used in cast iron and aluminum because it wonít strip the mating hole as easily as fine threads. It is also used widely because it screws in and out more quickly and is less subject to stripping and galling (the threads ripping particles of metal from each other thereby damaging both threads). Fine threaded hardware tends to take more torque and, as a result, has a little better holding capability.
There are additional thread series when delving into industrial and commercial types of equipment. The threading standards are different and the class or fit of the bolt is more closely watched. The class or fit is the operating clearance between the nut internal threads and bolt external threads. The six classes range from a relatively loose fit to an extremely close tolerance where utmost accuracy is essential. If one purchases hardware from an industrial salvage yard one might notice that the fasteners may or may not fit standardized hardware. The dimensions might seem the same but the capscrews just doesnít seem to fit. The reason might be that the threads are a different type, based on a different thread per inch ratio, or the class/fit ratio might be of a higher tolerance than the hardware or receiving hole you are trying to match. In these cases make sure and mix apples with apples before trying to force fit a capscrew into a hole. A thread gauge will help identify what you might be working with.
Keep in mind that the Unified Standard does not include the metric sizes found in newer machinery made overseas. Some of these fasteners are color coded so they are recognizable. Once again, a thread gauge will help to determine the type of fastener to use before stripping or galling out the receiving hole or nut.
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