Torque spec comes into play after a fastener has been chosen. Bolt torquing is very important. When you thread and tighten a bolt the melt stretches a little, but the metal doesn't want to be stretched so it resists the stretches and this is where you get the holding power from. This is why when you over tighten it can't hold the load because it can't return to its normal shape. Further to this in time the bolt continues to stretch and will end up snapping.
Torque-to-Yield Bolts
A torque to yield fastener is a one-time use that is based on this torque principal we just talked about. The torque spec usually have a 25% safety feature giving you a 25 per cent over tightening leniency. The torque to yield bolts don't have this safety buffer. They are not meant to stretch the metal. This may sound like a disadvantage but it's not really. They are extremely strong as they afford the whole 100% of their strength whereas the basic bolt only gives you about 75%.The one disadvantage they do have though is they can only be used once. So if you remove them you have to replace them with a TTY (torque to yield) bold. You are going to end up with 25% less strength with the basic fastener.
The TTY are used in many cases so it's important for the technician to be aware of this during the estimation process of the procedure.
Tightening Sequence
A tightening sequence can also be specified in conjunction with torque specs. A crises cross pattern is use to install wheels and now some installation patterns are being suggested for body panel fasteners. There is a certain amount of expanding and contracting during temperature fluctuations with composite panels. This means the fasteners have to adjust to these changes and must be designed to allow for this.
Shop Tips
Whenever possible replace the fastener with the same type as the one your remove. Check the manual however to ensure it is the correct part and you are doing the proper procedure. Every part is specific because there is a reason for it.
Keeping track of what you are removing is important and time saving so tossing them in a pile is not the answer. Just use some small plastic bags, put them in and then mark what they are on the outside of the bag.
If you do not have a manual, you can ask a dealership to show you their service manual.
Use all the parts that come with your replacement parts.
If you are not sure about the label, compare height, thread pitch and diameter to the new piece and this will help you choose the right replacement fastener.
Make sure to use coated materials to prevent corrosion. A classic example of making this mistake is using a non-coated steel boat with an aluminum panel. If the coated items you require are not available then use a plastic or fiber gasket, a washer or even a layer of paint to produce a barrier between the materials.
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