If you don’t want to make mistakes in auto repair, follow my advice and pay attention to the instructions that are in your service manual. There are 3 major mistakes made in mechanical work.
Incorrect Sequence of Assembly, Disassembly or Adjustment When taking something apart or putting it together, performing the steps in the wrong sequence can cost you time and money. Sometimes you can break something. Read the entire procedure before disassembling. Perform everything in the sequence that the instructions tell you to, even if you can not immediately see a reason for it. When you’re taking something apart that is very intricate, you might want to draw a picture of how it looks when assembled, in order to make sure you get everything back in its proper place. Most service manuals will give you an exploded view of each task. When making adjustments, perform them in proper sequence. Often one adjustment affects another, and you can’t expect satisfactory results unless each adjustment is made in sequence.
Over-torquing or Under-torquing You need a torque wrench that will indicate the amount of torque exerted in tightening a nut or bolt. While it is more common to over-torque and cause damage, under-torquing may allow a fastener to vibrate loose causing serious damage, especially when dealing with an aluminum part. Pay attention to torque specifications and utilize a torque wrench in assembly. If a torque figure (specifications) is not available, remember that if you are using the right tool to perform the job you will not have to strain yourself to get a fastener tight enough. The pitch of most threads is so slight that the tension you put on the wrench will be multiplied many times in actual force on what you are tightening. How crucial torque can be is seen in the case of a spark plug installation, especially where you are putting the plug in an aluminum cylinder head. Too little torque can fail to crush the gasket, causing a leak of combustion gases and consequently overheating the plug and engine parts. Too much torque can damage the threads or distort the plug, changing plug gap.
There are many commercial products available for ensuring that fasteners will not come loose, even if they are not torqued just right (a very common brand is Loctite). If you are worried about getting something together tight enough to hold, but loose enough to avoid mechanical damage during assembly, one of these products might offer substantial insurance. Before choosing a thread locking compound, read the label on the package and make sure the product is compatible with the material, fluid, etc. involved.
Cross Threading This occurs when a part such as a bolt is screwed into a nut or casting at the wrong angle and forced. Cross threading is more likely to occur if access is difficult. It helps to clean and lubricate fasteners, then start threading with the part to be installed positioned straight in. Then start the bolt, spark plug, etc, with your fingers. If you encounter resistance, unscrew the part, and start over again at a different angle until it can be inserted and turned several times without much force. Keep in mind that many parts, especially spark plugs, have tapered threads, so that gentle turning will automatically bring the part, you,re threading to the proper angle, but only if you do not force it or change the angle. Do not put a wrench on the part until it’s been tightened a couple of turns by hand. If you suddenly encounter resistance, and the part has not seated fully, do not force it. Pull it back out to make sure it’s clean and threading properly.
By Jesse Vibbert
General | May 22, 2010 | 
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