If you don’t want to make car repair mistakes, follow my advice and pay attention to the instructions in your service manual. There are 3 main mistakes that are made in mechanical work.
Incorrect sequence of assembly, disassembly or adjustment When disassembling or assembling something, performing the steps in the wrong sequence can cost you time and money. Sometimes you can break something. Read the entire procedure before disassembling. Do everything in the sequence the instructions tell you to, even if you can’t immediately see a reason for it. When you’re taking something apart that’s very complex, you might want to draw a picture of what it looks like when it’s put together, to make sure everything is back in place. Most service manuals will give you a blown up view of each task. When making adjustments, make them in the proper sequence. Often one adjustment affects another and you cannot expect satisfactory results unless each adjustment is done in sequence.
Excessive or insufficient torque You need a torque wrench that indicates the amount of torque exerted when tightening a nut or bolt. While it is more common to over-torque and cause damage, under-torque can allow a fastener to vibrate and loosen causing serious damage, especially when dealing with an aluminum part. Pay attention to the torque specifications and use a torque wrench in the assembly. If a torque figure (specifications) is not available, remember that if you are using the right tool for the job, you won’t have to work hard enough to tighten a fastener. The pitch of most threads is so small that the tension you put on the wrench will be multiplied many times by the actual force of what you are tightening. The importance of torque can be seen in the case of spark plug installation, especially when the spark plug is fitted to an aluminum cylinder head. Too little torque can fail to crush the gasket, causing a leak of combustion gases and consequently overheating the spark plug and engine parts. Too much torque can strip the threads or distort the plug, changing the plug clearance.
There are many commercial products available to ensure that fasteners will not loosen, even if they are not tightened correctly (a very common brand name is Loctite). If you’re concerned about making something tight enough to hold it, but loose enough to prevent mechanical damage during assembly, one of these products could offer substantial insurance. Before choosing a thread locking compound, read the package label and make sure the product is compatible with the material, fluid, etc. involved.
cross threading This occurs when it comes apart, like a bolt screwing into a nut or casting at the wrong angle and being forced. Thread crossovers are more likely to occur if access is difficult. Help clean and lubricate fasteners, then begin driving with part to be installed positioned square. Then light the bolt, spark plug, etc., with your fingers. If you encounter resistance, unscrew the piece and start again at a different angle until it can be inserted and turned several times without much force. Note that many parts, especially spark plugs, have tapered threads, so a gentle twist will automatically bring the part to the correct angle, but only if you don’t force it or change the angle. Do not put a wrench on the part until it has been hand-tightened a couple of turns. If you suddenly meet resistance and the piece has not fully seated, do not force it. Pull back to make sure it is clean and screwed on correctly.
When working on your car, always take your time, follow directions, and be patient. Once you have some experience, working on your vehicle can become an enjoyable hobby.