Time and technology have changed the way many cars are assembled these days, there have been many firsts such as plastic clips, adhesives and Velcro, replacing the nails and screws that normally held panels together in the past. . There is still the use of bolts when assembling mechanical components, but even some of these have changed. Torque bolts are used today in major assemblies to yield in engines.
These bolts stretch to the elastic limit when tightened; 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, engine front shock absorbers and even flywheels. Torque will certainly be used to yield bolts if the shop manual indicates that a bolt needs to be scrapped and replaced with a new one.
This can get expensive and a total waste of time if you have to buy new bolts all the time. Too many of these old bolts may still look usable, but beware, appearances can be deceiving, if using old bolts you can cause engine failure, an expense no one wants. Through the process of tightening the bolts, it brings them to their elastic limit due to stretching, which is why new ones are needed to give maximum strength to today’s engines.
It’s easy to figure out if you overtighten a bolt then it’s harder to be accurate in a clamping load. Friction occurs when tightening the bolts, these are like a ramp, when turning it has to slide against another thread or ramp in the bolt hole. The tighter it goes, the more difficult it becomes to move or push up the thread chute.
When stationary the term used is static friction rather than friction, as the bolt gets tighter it is the station that will affect the torque reading. People can use lubrication to help threads slide more easily, but the quality from one lubricant to another can vary. It’s much easier to follow the manufacturer’s guidelines for the best type of lubricant needed for the bolt when tightening it. This is important because if you use the wrong lubricant it can become too tight or not tight enough.
There are several stages to go through when torque tightening to yield bolts, this is done using both a torque and an angle of twist. The first step is to tighten the bolt to a low torque specification, this is done to ensure that there is an even clamping load when the parts are assembled. The second stage consists of slightly retightening to the highest torque stage. This will still allow enough friction and tack on the bolt threads as the torque tightens very little. The normal procedure after this is to turn each bolt a specific number of degrees, usually two or three steps. As an example, the bolt could be turned 90 degrees and then another 90 degrees until another 70 degrees have sufficiently tightened the bolt. This will ensure that the clamping force exerted by the bolts is precise and consistent.
Special tools are used that can measure the degrees a key or bolt has been turned, these are low cost protractors that have a moving pointer, while some tools use electronics when measuring an angle of turn and can cost several hundred Dollars. Both of these types work well, and although electronic is more expensive, it can actually get the job done faster.
Some may ask the question why is the torque now needed to yield the bolts? When maybe they weren’t needed in the 60’s or 70’s. The answer to that question is pretty simple as the engine materials are different now. For example, aluminum cylinder heads expand differently than cast iron blocks. When an engine heats up, all the bolted parts must be able to slide on their joints or move against each other. Due to the elasticity in torque to yield bolts, it allows movement between parts but still maintains uniform sealing and clamping loads.
Today’s engine parts are much lighter than decades ago, heavy cast iron parts could withstand variations in torque without failing. Aluminum alloys and thin-walled castings in use today need precise tightening torque, if it is not correct, leakage or distortion can occur.
Today engine design is changing due to the need for uniform clamping forces, the automaker Ford has prototype engines where the bolts holding the cylinder head can go through the entire engine block and thread into the cylinder head. lower casting that holds the crankshaft. It’s hard to imagine that just a few bolts can hold an entire engine together. This would definitely change the way engines were assembled and disassembled in the future.