Fasteners,

 At First glance this would seem a subject that really does not need a technical article, but after seeing so many strange ways that people seem to fasten things to their cars I think perhaps we do need one.

 First lets get some simple terms cleared up;

 For non-metric fasteners The American Standard for Unified threads refer to coarse threads by the designation  UNC and fine threads by UNF. Many of us grew up using USS for coarse and SAE for fine. The USS, or United States Standard was used for many years on threaded products made in United States. This is actually an obsolete designation though most manufactures' still recognize it. SAE ( Society of Automotive Engineers) is incorrect for fine threads but many of us in the industry ( myself included) find it hard to break old habits. Unlike metric fasteners US fasteners make their determination of coarse or fine by the number of threads per inch. The more threads and it is fine the less and it is coarse. An example would be 1/4 -20 is a coarse 1/4 inch thread while a 1/4-28 is fine. Metric fasteners do not have a hard differentiation between coarse and fine. In fact they have multiple coarse and fine threads defined by the thread pitch. In this article we will be talking mostly about American fasteners.

 Coarse threads (UNC) is the most common thread, but not in the automotive industry. Its most common uses are for threading into lower tensile strength materials such as cast iron, mild steel, and softer material, Aluminum alloys, copper, brass etc. It is also used where quick install and disassembly is required or if corrosion is a problem. It is also widely used where a chance of slight thread damage may occur (think of all of the "needs some assembly" products)

 Fine Threads (UNF) have greater tensile strength than a comparable sized coarse thread. Fine threads are used in applications where the amount of thread engagement is limited or where load carrying capacities are high and the maximum holding power of both external threads and mating internal threads is required.

 So in most automotive applications, except British, suspension & internal engine fasteners are usually fine and things like manifold, floor panels and the like are coarse. The British are in love with fine threads and use fine threads in cast castings, alloys and just about everywhere else. One of the reasons so many British cars have stripped bolts.

 So far this is pretty straight forward and you do not have much control in what fastener you use unless you are re-tapping a hole. But how you attach the fastener and what washer to use and when is another story and experience has taught me that most people get it wrong, even people in the industry.

 First let's briefly talk about fastener strength. US bolts and fasteners come in different grades that can be easily recognized by the number of marks, or hashes on the bolt head. The number is always 2 less than the actual rating, grade 5 would have 3 marks. For anything that you plan on doing in the automotive world grade 5 is the minimum rating and for most applications all that you need. There are two types of strengths, Tensile and Proof Load. Tensile is the maximum tension applied load a fastener can sustain prior to or in coincident with it breaking. Proof load is the maximum amount of applied load that a fastener must support without evidence of permanent deformation.

 Here are a few numbers, grade 5 bolts between 1/4in and 1 inch diameter all lengths have a tensile strength of 120,000 psi. Grade 8 bolts between 1/4 inch and 1-1/2 in all lengths 150,000psi And while Stainless steel looks real pretty you may think about it for a bit a 3/8 diameter grade 8 bolt has a tensile strength of 150,000psi while that same bolt in Stainless has a tensile strength under 15,000psi

 The numbers get a bit more interesting in proof loads. your 1/4 - 1 inch coarse thread nut has a proof load of 120.000psi and the same sizes in fine are 90,000psi. Now if you use grade eight nuts the numbers are the same for coarse or fine at 150,00psi Again Stainless looks great but lags behind in strengths.

 The most common problem we see is the incorrect use of washers. Everyone uses them but no one seems to know where they go and in what order or why. We are going to discuss 5 type of washers and their use. First you have, what should be the most common washer for most of you, a fine or SAE washer (here we use SAE) These washer have a proportion that are related to the dimensions of the bolt head and nut and its principal use is to minimize embedding and to aid in proper torquing. Or in other words this washer needs to be between the head of the bolt or nut and the object you are clamping. If no lock washer is being used a flat washer is always used. Most often the washer is under the head of the bolt, not the nut. A flat washer is always used when you are clamping soft metals, Aluminum alloys, copper, brass, etc. It always the flat washer that is in first contact with whatever you are clamping.

 A Coarse washer is used the same as a fine washer but usual has a larger overall diameter to better spread the load over lower strength materials. (one problem with coarse washer in the automotive industry is even though you would like to use them to greater spread the load over aluminum mating surfaces you just do not have the room).

 Split lock washers provide greater bolt tension per applied torque, provide a hardened bearing surface to more uniformly apply a torque load and provide protection against looseness. The original height of an unused split washer is roughly twice the height of the washer in a compressed position. Since these washers are hardened they should return to about 2/3 of their original height after you have released them. If they do not toss them out. You can only use them over so many times and you people working on British cars they make those high collar washer the British love new. The flat washer goes under the split washer, not on top of it. That is the most common thing we see. Also if you are tightening against steel and the holes all are the same size you do not need a flat washer, the split will do fine. Though you may need one flat washer on each side of what you are fastening because of the materials or hole sizing you only need one split washer. Splits are generally under the nut but do not have to be.

 The last washers we will talk about are toothed washers. There are both internal and external toothed washers. In the automotive field you find internal toothed washers commonly found in places where low fastening torque is used but you worry about vibration loosening things up, oil pans are a great example. External toothed washers need to be used every where you are trying to make an electrical contact. They need to go against the metal part you are clamping to. If you use a flat washer against your sheet metal and then the toothed washer against it you made great contact with the flat washer but still have a lousy ground. I like to use a toothed washer, then a flat, then the wire and then one more toothed. bad grounds are the cause of most annoying electrical problems.

 Washers, except toothed, come in different hardness's and you should make sure you use they appropriate harness for your application.

 Last but not least let's briefly talk about nuts. There are many types but for our purposes we are only going to mention a few in this article. For your normal nut you need to check that the threads are undamaged before you re-use them and the nut is not deformed in any way. The bolt should protrude a thread or two at the minimum after you have torqued the bolt down. If you do not have this protrusion you have not fully engaged the nut and it will mostly likely loosen up and/or you will distort the threads in the nut. ( we see this all of the time) Many of you use lock nuts. The three most common type are a ny-lock which uses a plastic insert to lock the nut in place, an all metal one time use lock nut and a re-usable all metal lock nut..  When using any lock nut THE MINIMUM THREAD ENGAGEMENT IS FLUSH WITH THE HEAD OF THE NUT. Preferred is a thread are two beyond the nut. If you are not flush your lock nut is now worse than using a regular nut. We see this a lot in one of the scariest places, suspensions. Many suspension kits replace the original castle nut with a ny-lock nut. Usually these are thicker than the original but they send along the same thickness washers and the nut does not fit. Just because it came in the bag do not assume it is correct. YOU DO NOT RE-USE NY-LOCKS. ONE TIME AND DONE. IF YOU TIGHTEN IT DOWN AND TAKE IT OFF FOR ANY REASON AT ALL THROW IT AWAY AND GET ANOTHER.  The all metal one time use lock nut is used one time only too. It is probably the best nut to use if you never want to have it come apart. Unfortunately you will destroy the threads of the bolt if you do decide to take it off. This is a one time nut and a one time bolt usage. I only use these in real high vibration areas. Some spring U bolts and some drive shaft bolts and the like. There are now available from some specialty fastener manufactures a reusable all metal lock nut that can be used in places where temperatures are high or you need more strength than a ny-lock is capable of and you get to re-use this nut multiple times.