Torque!
Torque is fun. It’s the rotational force that your engine produces that ultimately turns the tires and makes the fun machine go.
It’s also the rotational force that we apply to the fasteners in our cars with the use of tools. You are applying torque when you channel those uggas and duggas into a wheel bolt, for example. But the tool that we use to usually use in the garage to measure it is the torque wrench. I’ll write more about choosing and using a torque wrench in an upcoming post, but for now, just know you probably should be putting at least one in your tool budget.
Today, though, I wanted to write about the importance of understanding how torque is applied practically and why it is important. It’s easy to do wrong, even with the right tools.
Why?
Fasteners almost always have a torque specification. The is usually expressed in units of pound-feet (lb-ft), or Newton Meter (N m). Some may be more critical than others depending on the type of fastener and how it is being used. Chances are though, that manufacturer of the vehicle or part has specified the torque to be applied to the fastener. These are often found in the factory service manual, or even in third-party service manuals. Getting the torque right is important to ensure correct and even clamping force, prevent loosening, and to prevent failure of the fastener. Too little and the fastener may back out, too much and you’ll discover the hell of broken fastener extraction. So take that extra few minutes and get that spec!
How?
As I said, the tool here is the torque wrench, but there are some techniques that are important to understand, too. Sometimes fasteners may need to be tightened in stages. First stage of all the fasteners in a part for example, maybe to a spec halfway to the final torque. Then, all of the fasteners may be tightened to the final specification. The number of stages and their values will be specified by the manufacturer. When tightening multiple fasteners in stages, there is often a specific order to follow, too. Both of these procedures are intended to equalize force and minimize warping of the work, so do follow them!
Sometimes, there is an additional step beyond the torque spec instructing to rotate the fastener an additional degree spec. This is a torque-to-yield or “stretch” bolt. Like the name implies, this fastener stretches when installed to spec. That’s right, one time use. Do not re-use these bolts. Throw them out and get new ones. Don’t put them in the bolt bucket, don’t be tempted because they “still look good”. They’re done.
You may also find that the manufacturer advises applying some kind of lube, anti-sieze compound, or other treatment to the fastener. They account for this in the torque spec, so follow that too.
What if you can’t find a spec? It’s out there somewhere, I’m sure. But if you’re really in a pinch and can’t find the data, here’s what I’d do. First, if it’s an aftermarket part, go back to the factory spec. Usually the part is engineered to the same spec, so us that. If it’s of a different design, and still no spec, still go back to the factory. Hey, it’s better than guessing. If you’re still lost on the spec, find a similar model, or even a similar application to use. A flywheel bolt is probably similar from model to model and better than nothing between makes. If you’re really in a pinch and just gotta get it done, then use some sense. Larger fasteners can probably handle more torque. Under-tightening may make for a bolt that backs itself out, but over-tightening can break the fastener, so proceed with caution.
I hope this helps you understand the importance of torquing fasteners to spec, and saves you some money and aggravation!
