Time and technology has changed the way many cars are put together these days, there have been many new introductions for example like plastic clips, adhesive and Velcro, replacing trim nails and Concrete Screw that normally held panels together in the past. There is still the use of bolts when assembling mechanical components, but even some of these has changed. Today, on major assemblies, torque to yield bolts are used in engines.
These bolts stretch to the yield point when tightened; the bolts are often used when exact clamping loads are needed on parts, they can be used on connecting rods, crankshaft main bearing caps, cylinder heads, front engine dampers and even flywheels. Torque to yield bolts will no doubt be used if the shop manual indicates a bolt needs to be discarded and replaced with a new one.
This can become expensive and a total waste of time if you have to buy new bolts all the time. Too many these old bolts may still look usable but beware looks can deceive, if using old bolts it may cause engine failure an expense nobody wants. Through the process of tightening bolts it pulls them to their elastic limit due to stretching, that’s why new ones are needed to give maximum force on today’s engines.
It is easy to understand if you tighten a bolt to much then the harder it is for it to become accurate on a clamping load. Friction occurs when tightening bolts these are like a ramp, when turning this has to slide against another thread or ramp in the bolthole. The tighter it goes the harder it becomes to move or push up the threads ramp.
When stationary the term used is stiction as against friction, as the bolt becomes tighter it is the station that will affect a torque reading. People may use lubrication to help threads slide easier but the quality between one lubricant and another can vary. It is far easier to follow manufacturer’s guidelines for the best type of lubricant that is needed for the bolt when tightening. This is important because if you use the wrong lubricant it may become too tight or not tight enough.
There are several stages to follow when tightening torque to yield bolts this is done using both a torque and turning angle. The first step is to tighten the bolt to a low torque specification, this is done to ensure even clamping load is in place when parts are assembled. The second stage is to tighten again slightly to the higher torque stage. This will still allow enough friction and stiction on the bolt threads as the torque is tightened very little. The normal procedure after this is too turn each bolt a specified number of degrees normally two to three steps. As an example, the bolt might be turned 90 degrees, and then another 90 degrees until a further 70 degrees has tightened the bolt sufficiently. This will ensure the clamping force that has been exerted by the bolts is both accurate and even.