How bolts work

(andy@nobugs.org, June 2001)

The basics

I was recently removing the fork leg from a CG125 when I broke on of the pinch bolts. This got me thinking about why it happened, and what I could do to avoid this happening again. In the end, I couldn’t get the broken part of the bolt out, so I had to replace the bottom yoke entirely.

First things first. Bolts are used to hold things together. When you tighten a bolt, there are various stages. At first, you are taking up the slack and you can do this just by turning the bolt with your fingers. At some point, all the slack has been taken up and it gets much harder to turn the bolt. At this point, the bolt is pressing against whatever it is holding together.

It gets harder to turn the bolt at this point because from now on you’re actually starting to stretch the bolt. It’s easy to think about elastic bands stretching, because a small pull makes a big change in it’s length. Metal streches too, only you require a much bigger pull, and the resulting change in length is much smaller.

So, you get out your socket set and carry on tightening the bolt. If you’ve been working on bikes for a while, you’ll start to develop a ‘feel’ for when the bolt is tight enough. If you don’t have that awareness, you’re quite likely to overtighten the bolt and strip the threads. That can be expensive. As you tighten then bolt more, you’re actually pulling the atoms of the metal further apart. There are attractive forces between the metal atoms which fight against your efforts to pull them apart, which is why it gets harder to tighten a bolt. However, there is a stage where you’ve pulled the atoms so far apart that a fracture can occur, and the bolt breaks. The best way to tighten a bolt correctly is to use a torque wrench and follow the torque settings in the bike manual.

But in reality

Reality is never straightforward though. In theory, there should be nothing to stop you finger-tightening the bolt until it’s snug. In practise, it’s often quite hard to turn some bolts, even when they are slack. The reason for this is that dirt gets stuck in the bolt threads, and it gets trapped when you start tightening the bolt making it harder to turn. This is a warning sign — if you can’t finger tighten a bolt until it’s snug then there is something wrong. Take it out and clean the threads. Put a little grease on the threads before you try again. It’s a bad, bad idea to get out spanners early to carry on trying to tighten a bolt that isn’t turning smoothly. Even if you get it tightened, that dirt is still in the threads, and it’s quite likely that it’ll jam the threads when you try to next remove the bolt.

Dirt-jammed threads are a pain when you’re putting a bolt in. They’re much worse when you’re trying to remove a bolt. Imagine what happens if the thread is jammed at the far end of a bolt which you’re trying to remove. You turn the bolt head, so the top of the bolt is getting pulled out. However, the bottom of the bolt is jammed in place. In this situation, it’s quite likely that the two halves of the bolt will seek a divorce. Suddenly, the bolt becomes very easy to turn, because the top half has broken off and isn’t under tension any more. The bottom half of the thread is stuck inside some bit of your bike. Good luck getting it out!

Remember: bolts should turn smoothly. If they don’t, there is something wrong.

Get a brand new bolt and nut, and tighten them. They tighten smoothly, without any grinding feeling and with hardly any resistance until the bolt is snug.

If you’re stuck

If you’re trying to remove a bolt, and you discover it’s stuck then the absolute worst thing you can do is to try to turn the bolt head harder by using an extension bar on the spanner, or hitting the spanner with a hammer. If the bolt isn’t loosing smoothly then there is something wrong. If you loose sight of this now, you’re running the risk of breaking the bolt and causing yourself a lot more hassle.

At this point, the right thing to do is to clean up the exposed threads at the back of the bolt with a wire brush, apply lots of penetrating oil to the bolt and take a break. Penetrating oil takes a while to do it’s job, and trying to rush the job when you’re already in a potentially delicate situation is the wrong mindset.

After the penetrating oil has had a chance to work, get an impact driver and fit the correct attachment. Give it a good whack with a hammer, and hopefully the bolt should loosen. Impact drivers have many advantages over spanners. Firstly, the hammer blow presses them into whatever they’re turning. This is good if you’re removing a screw, since you won’t chew the head. Secondly, the jarring impact is good at unjamming stuck fasteners. Thirdly, it gives a quick, firm turning motion to start the fastener turning.

At this point, I have to recommend the book Zen and the Art of Motorcycle Maintenance. It’s a bit about motorbiking, a bit about philosophy and talks a lot about your mindset when fixing stuff. A classic book.

New bikes

If you’ve just bought the bike, you’re at the mercy of whoever last assembled the parts you’re working on. If they cleaned up the bolts before tightening them, greased the threads, and tightened to the correct torque value, you’ll have an easy time. If they reused rusty bolts with dirt stuck in the threads and didn’t use grease, then you’re faced with the prospect of totally stuck fasteners. There’s a reason why bolts become seized.

Even if a bolt was assembled carefully, corrosion will eventually set in. This especially affects bits of the bike which are rarely worked on. For this reason, it’s probably worthwhile to go over each part of the bike at some point, looking out for stuck bolts, dirty and corroding threads. Do something about it before it gets beyond hope. You can get big tubs of assorted nuts, bolts and washers from various bike shops. Get one – since if you have a new bolt to hand, you’re less likely to try to resuse an old dirty bolt which would be better thrown away.

Washers

Washers have various uses. They spread the forces of a tightened nut or bolt head over a larger area, reducing the stress in the surrounding area. Also, they help stop bolts loosing through vibration. This is especially important on motorbikes, where there is a lot of vibration. Finally, they can stop the annoying situation where you’re tightening a bolt, but the difficult-to-get-to nut on the other side is just rotating round as you rotate the bolt head.