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Glue-in bolts, expansion plugs, adhesive bolts, bolts. What now?


Bohrhaken als Fixpunkt und Zwischensicherung beim Bergsport
Bolts as an anchor point and intermediate protection point for mountain sports

Sport climbing and bolts. These days, they belong together like bouldering and low-cut trousers. Perhaps the reassuringly shiny silver metal found on crags has already helped you achieve undreamt-of performances. Or maybe you’ve cursed because you haven’t been able to sport a bolt.

Set correctly, bolts significantly increase safety whilst rock climbing. But what is correctly? And what is incorrectly? And how can you tell the difference? What is a mechanical bolt and why don’t adhesive bolts stick? Here are the answers…

Standard requirements for bolts

As in almost all areas of mountain sports, there is the European Standard (EN) as well as the UIAA standard. For bolts, these standards include:

  • Tensile strength to the outside (axial) must be at least 15 KN
  • Tensile strength downwards (radial) must be at least 25 KN
  • All parts of the bolt must be made of one material
  • Only UIAA: all materials must be made of stainless steel, so they are corrosion-resistant (must not rust or otherwise decay)

What kinds are there?

The first two types of bolts include glue-in bolts and mechanical bolts. You’ve probably heard of glue-in bolts as “adhesive bolts”. This is only partly correct. So, it’s better to say glue-in bolt. Mechanical bolts are then divided into “expansion systems” and “drop-in systems”. I’ll go into more detail in a moment.

The bolt, resin and rock

AustriAlpin Klebehaken
Frequently found in our shop: the classic Bühler bolt.

Glue-in bolts are called that because they bond together with a resin glue and the rock. Makes sense, right? Strictly speaking, glue-in bolts do not stick to the wall, but form a tight fit with the surrounding rock. So, you drill a hole, clean it, fill it with resin, put the bolt in, let it harden and off you go. Of course, it’s not quite as simple as that, but I’ll go into more detail in another blog post.

The interesting thing for you is, what do these things look like? In general, you’ll often see classic Bühler bolt. You can usually recognise glue-in bolts by the fact that they are in one piece and are surrounded by resin.

The Mechanic

Petzl - Coeur Goujon - Bohrhaken bestehend aus Expansionsdübel, Mutter und Lasche
Petzl – Coeur Goujon bolts consist of an expansion plug, nut and lug

Mechanical systems form the counterpart of the glue-in bolt, so to speak. These always consist of at least two components: the hook itself and the hook lug (plate, hanger). The expansion systems function in a similar way to conventional bolts. When the bolt expands, pressure is exerted on the surrounding rock and the bolt is anchored into the rock.

Form-fitting mechanical systems can be thought of in a similar way to standard bolts, but just for rock instead of wood, and they’re extremely stable. A thread is turned into a narrower drill hole so that the bolt forms a tight fit in the rock.

Confident? Yes. No. Maybe.

But how can you tell which bolt is great and which one you should approach with caution? Unfortunately, this is sometimes very difficult to determine from the outside. However, there are a few ways…

  • Corrosion = rust
  • For mechanical systems: is the thread visible beyond the nut?
  • For glue-in bolts: twist test with a carabiner
  • Are they unusual homemade bolts?
  • Are the bolts at a sufficient distance from cracks, edges, etc.?

It should be clear that if you find a completely rusty bolt, you treat it with caution. Even if there are just traces of rust, you should still be careful. In expansion systems for example, there is a hollow space that provides space for moisture, which is the key factor for rust. Rusted bolts should also be used as an indicator that an anchor point is not totally secure. Why do bolts rust when they are all standardised? The problem is with the alloyed bolts. These can meet the requirements of the European Standard, but the thin, corrosion-resistant layer of the bolt can get damaged during set-up, which offers rust the opportunity to attack.

A thread protruding far beyond the nut can be an indicator of a setting failure in expansion systems. There’s another invisible problem here: if the drill hole or cavity is too large, the bolt cannot expand (enough) when the nut is tightened. This results in the nut not being able to be turned to the end of the thread and appears to be tight. However, the axial tensile strength may not be guaranteed under certain circumstances.

Verbundanker kannst du mithilfe eines Karabiners testen
Glue-in bolt test

You can test glue-in bolts with a simple trick: take a carabiner and try to turn the glue-in bolt. Can you turn the glue-in bolt? Very badly! You might curse loudly or under your breath and put up your own belay device, climb down to the last one or escape upwards and hope for improvement. But, if a glue-in bolt can be turned, this is clear indication that the resin has not set properly and that the bolt is not safe enough.

Homemade bolts always pose a risk because they have not been subjected to any standard testing. Unclean welding seams, hairline cracks or even lightweight aluminium constructions may make the bolts look nice on the outside, but always involve an incalculable risk. Unfortunately, these are sometimes hard to detect.

This is really a matter for the setter, but it’s good to know: expansion bolts should always be set at a sufficient distance (at least 15 centimetres) from cracks, edges, holes, etc.

The location of the wall also plays a significant role in the load on the bolts. Temperature, moisture, exhaust gases and chloride, such as salt water, can greatly accelerate the corrosion of the bolts. If the rock is very exposed and located next to the sea, a busy motorway or in southern climates, you should be particularly careful and look at the bolts carefully. The same applies for less frequented and/or older climbing areas. There is a higher risk here that the bolts will not be maintained regularly.


This post is not an instructional guide for setting bolts. Rather, it’s intended to provide guidance on what is important when it comes to bolts and the things you should consider to feel safe and exercise appropriate caution. In this case, you shouldn’t ‘learn by doing’.

If you’re a climber, then you need to remember this: bolts are generally an extremely reliable part of the safety chain, but you should always approach walls with your eyes open and your wits about you.

Teile den Artikel mit anderen Bergfreunden

Bergfreund Florian K.

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