Bolt Systems

Bolts Explained

Unlike traditional climbing where a knowledge of placing safe protection is essential, sport climbing uses pre-placed fixed protection that remains ‘in-situ’. Unsurprisingly climbers often lack knowledge in determining whether a bolt is safe to use. Understanding the basic principles in installation and what a certain bolt should look like if installed correctly is explained below.

Bolt Type

Bolts used by rock climbers exist in two types; expansion bolts and glue-in, or as more commonly known; resin bolts.

Broadly, a climb can be equipped faster using expansion bolts and they require less equipment to install. Resin bolts do require additional items of equipment, cannot be loaded immediately however once placed, this type of bolt lasts longer and has many more advantages.

Expansion Bolts


A mechanical fixing that works by expanding and compressing against the wall of the hole drilled into the rock. The typical climbing expansion bolt features a hanger attached to the expansion bolt and secured by way of a nut and washer. Some expansion bolts have a continuous sleeve, rather than just a shorter sleeve towards the end of the bolt. Other varieties have double wedge and sleeve features however in all cases the overriding principle remains the same.

An exception to the above is the Long Life expansion bolt but no longer produced by Petzl. This complete unit achieves the expansion effect by hammering a centrally located pin to produce the wedging effect. New requirements under EN959 certification require deeper embedment depth and this fixing is non compliant.


Other varieties of expansion bolt that maybe found on rock climbs but are generally meant for caving, consist of a self driving anchor into which a hanger is screwed after the sleeve has been drilled sufficiently deep. The threaded sleeve has teeth cut at one end, is screwed onto a handled driver then hit and rotated to ‘self drill’ the bolt hole. A spreader cone of metal is placed in the rear of the anchor and the assembly hit back into the hole causing an expansion cone to form. The hanger is generally made of aluminium (this system was developed for expedition caving) therefore not expected to be left permanently attached due to issues of dissimilar metal corrosion with the anchor that is made of steel.


A 10mm version designed for climbing was produced by Petzl, driven using an adaptor fitted to the hand driver. Due to the significant time in hand drilling a single placement these never proved popular.


Expansion bolts are installed by drilling a hole at right angles to the rock face, removing any dust using a piece of flexible tube or a hand pump then hammering the assembly into the bolt hole. The nut is kept just below the end of the bolt to prevent the threads being damaged by hammer blows. The hanger is aligned in the direction of any load and the nut tightened thereby driving the sliding sleeve over the wedge shape end of the bolt.


Battery powered drilling of typical expansion bolts and Petzl Long Life bolt

hand drill

Hand drilling 8mm caving use expansion bolts (same for the climbing 10mm version)


Battery powered drilling of the above bolt types


  • Minimal amount of equipment required to install (drill, hammer and spanner)
  • A load can be applied immediately
  • Easy to place on severely over hanging rock.


  • Hangers often work loose making clipping awkward or become lost as a result
  • Hangers cause karabiner damage due to the sharp edges
  • Hangers can be stolen
  • Not easily replaced using the same sized hole if rendered unusable by corrosion
  • Continually stress the rock – this can accelerate corrosion due stresses exerted into the metal
  • Expansion bolt within the rock is not protected (see resin bolts)
  • Hanger and bolt metal grades must be matched to prevent accelerated corrosion due to dissimilar metals
  • Cannot be retreated from without leaving a karabiner or quick link
  • Dangerously unsuitable if installed in certain rock types (soft sandstone)
  • Metal corrosion can still occur even if metals are matched; work hardening / machining returns one component back to it’s active state

Resin (Glue-in) Bolts

Resin bolts are secured by the adhesion (bonding) of a suitable resin to the rock surface inside the hole and a mechanical keying effect where the resin contacts the bolt. A popular misconception is that the resin sticks to the bolt and that provides the holding power. In reality the holding power between resin and metal is achieved by grooves stamped into the bolt or by twists in the metal creating a mechanical advantage. Resin bolts are a single piece unit formed either by a forging process or more commonly through the use of metal bar bent into shape, with or without welding.


Petzl Collinox forged resin ring

bolt prod

Bolt Products twist leg resin bolts


Fixe welded resin ring

Most producers of resin bolts offer longer bolts that can be fixed deeper into the rock to increase the strength of the placement or where poor rock exists as a surface layer and the hole needs to be deeper to gain an acceptable depth of placement.


  • Significantly stronger bolts than a comparable depth expansion bolt
  • Cannot be stolen
  • No parts that can rotate loose
  • No karabiner damage
  • Can be abseiled from by directly threading the rope through the eye
  • Typically have greater corrosion resistance
  • Single piece unit – dissimilar metal corrosion is not an issue
  • Can improve holding power by installing a longer bolt
  • Best overall system for long life
  • The only sustainable fixed protection system (titanium resin bolts) for SCC affected regions
  • The only type of fixing suitable for all rock types


  • More equipment required (hole cleaning brush, blower, resin gun + drill and hammer)
  • Hole preparation (removal of dust) is crucial to installing a safe bolt
  • The majority of resin bolts do not (Bolt Products and Titan Climbing excepted) feature a means of retention whilst the resin is hardening if placed upside down (this depends on the resin however – some are fast acting eg glass ampoules)
  • Can be messy using bulk resin dispensing systems depending on how well the user has been trained
  • Resin does have a shelf life
  • Cannot immediately load the bolt. Resin cure times vary from a couple of hours to a day.


As for expansion bolts, the hole for a resin bolt is generally drilled at or slightly off perpendicular to the rock surface depending on whether the bolt requires counter sinking into the face. Whilst certification tests for rock anchors do not require the bolt under certification (EN959) to be counter sunk, there are advantages in real world deployment for doing so and these include minimising torsion around the eye and a degree of concealment. In notably soft rock e.g. sandstone, notching or countersinking the bolt is considered essential (e.g. Australia).

Resin bolts that use a twist leg design (Bolt Products, Wave Bolt) do not expect the eye to be recessed (notched) into the rock surface. The opposing metal legs provide a very high resistance to torque loading whereas a single shaft resin bolt is more like to twist. The loads however to achieve sufficient torsion have to be high and in many cases the attachment link eg karabiner, would fail beforehand. The EN 959 and UIAA-123 Rock Anchor test does not require bolts to be recessed.


A blow pump and small brush are used in conjunction for the removal of loose dust and to clean the inside of the hole.

The adhesive comes in the form of single use glass ampoules or a bulk dispensing system (eg Hilti, Ramset, Sika, Exchem).


Glass ampoules are ideal for placing a single bolt and have the added advantage that the resin gels (hardens) very quickly so for inverted placements on steep rock the bolt can be held until the resin has hardened sufficiently to prevent slumping of the bolt. Glass ampoules do not have sufficient resin volume for use with bolts that are expected to be counter sunk. Typically ampoules are used with Petzl Collinox and Petzl Batinox rings.

Bulk dispensing systems are far more common and consist of a dispenser into which a cartridge or collapsible foil pack is loaded. In the case of collapsible foil packs (eg Hilti RE 500) the pack is placed inside a rigid foil pack holder that can be reused. The resin comprises two separate components that are discharged via trigger pulls into a mixer nozzle previously screwed onto the front of the dispenser. A prescribed number of trigger pulls are initially required to ensure sufficient mixing and this resin is typically discarded into the plastic packaging. A color change is typically associated with this process and an extension tube for deep holes may also be attached to the tip of the nozzle.


The mixer nozzle is placed at the back of the hole and retracted slowly as the resin is injected; the tip of the nozzle must be kept inside the resin to ensure air pockets are not trapped inside the hole. The bolt is then rotated as it is pushed into the resin to work the resin over the shaft of the bolt. As the bolt is pushed into the resin, any excess discharged is used to fill any gaps around the eye of the bolt. This process can be messy unless thin gloves and rags are used to shape and wipe away excess resin. Most dispensers also require the catch located on the rear to be depressed to alleviate the pressure on the pack and stop resin dribbling from the nozzle when not in use.

Resin type can vary significantly in appearance, permeability, dispensing method, reaction times and strength once fully cured. Of the various resin types, hybrid urethane / methyacrylate (Hilti HY 150), hybrid / epoxy urethane (Hilti RE 500) and glass ampoules containing vinyl ester (Fischer / Upat / Fixe) are of predominant use by climbers.


Image reference Bolt Products

All resin systems feature a gelling time and a curing time, both times are affected by the base material temperature. The gel time is the shorter time in which the resin remains viscous i.e free flowing. The cure time specifies when the bolt can be fully loaded and for climbers is particularly important in ensuring that the bolts are not used beforehand.

 Equipping New Routes

The conception of a new route maybe from the obvious gap in-between existing climbs or the development of an entirely new climbing area. Quality routes link obvious natural rock features on sound rock and preserve the character and independence of neighboring climbs. Adding a new climb that shares holds or crosses over existing routes not only leaves a potentially contrived experience but can cause problems if both routes are climbed simultaneously e.g. one leader falls higher on an adjacent route.

Once the line is chosen, an abseil inspection is typically the next stage to ascertain that  the rock is suitably sound for climbing upon and installing safe bolts. The choice of bolt type will take into account the environment, number of bolts to install and rock type. Fatal accidents have been caused (eg Blue Mountains, Australia) by failing to match appropriate bolt and rock types.

Prior to bolting the line will be cleaned of loose rock and vegetation from the top down to progressively clean off any dirt that has fallen onto handholds / ledges below. Once cleaned, the bolt locations need to be determined and their positions marked for drilling.


  • Bolts are closer spaced nearer the ground to minimize the likelihood of ground falls
  • May be spaced further apart higher up
  • Cruxes typically feature a bolt to protect the harder moves and allow the climber to aid past onto easier ground if the climb proves too difficult.
  • Placements need to sync with the rhythm of climbing
  • Be placed next to a solid hold to clip from
  • Can be clipped by shorter climbers
  • Allow the rope to run with the minimum  of friction
  • Pull the rope away from sharp edges
  • Be placed so as not to obscure holds once the rope has been clipped into a quick draw

Crucially the route should be bolted for the grade and not in reflection of the person equipping the line. Ethics aside, sport climbing is (should be) a well protected style of climbing and bolting routes with long run outs or protection far from cruxes is contrary to what this style of climbing is supposed to offer.

Depending on the individuals’ experience and how involved it is to equip the line (eg multi pitch) will determine whether a top rope / self belay is required to ensure the bolts are appropriately placed.

Belays and lower offs are located to enable an easy recovery of quick draws but allows the rope to hang to the side of a climber when top roping. A minimum of 2 bolts must be used and spaced such that the spacing reflects 2 times the depth of the bolt placement. This ensures that in the event of one bolt pulling due to rock / structural failure, the other will provide the necessary back up.

For expansion bolts, the route will be drilled and equipped bolt by bolt. For resin bolts it is generally the most economic use of the resin pack to prepare all bolts then glue up the entire route in one go as this minimizes the wastage of glue. Resins packs do not need to be fully used in one time however partly used packs require the initial trigger pulls of resin to be discarded as if it were a new pack. The volume ‘wasted’ is significant in that it will typically glue in several bolts.

Ultimately equipping new routes requires a high degree of organisation if the output is to be worthwhile. Equipment is best linked to the harness using cord lanyards to prevent loss and ensure everything is easily accessible.

Climbers who regularly equip new routes tend to prefer the use of static rope as it is more efficient to ascend upon and less prone to being cut by edges. Single rope technique (SRT) and rigging static lines with re-belays and deviations is proven to be more efficient than other forms of rigging such as self top roping. Consideration must also be given to the prolonged length of time spent hanging on the rope therefore many opt for commercial rigging harnesses that provide the comfort and attachments capable of suspending heavy weights eg drills.

Bolt Life

Many factors affect bolt life, predominantly these are climate and the bolt type. With the development of bolt systems and decrease in relative costs, sport climbing areas are typically equipped using stainless steel bolts rather than plated bolts and hangers.

With significant failures of expansion bolts in coastal limestone sport climbing regions (Thailand, Cayman Islands, Malta) a greater awareness of the benefits from using resin bolts have seen an increase in the use of this type of fixing.

Awareness of local conditions and knowledge is critical in ensuring the appropriate specification of bolt is used for routes to be equipped safely.

Metal Types & Grade

Expansion bolts are produced in galvanized steel and stainless steel, the common variety being stainless steel at A2 or AISI 304 grade as this generally represents the a good balance between cost and corrosion resistance. For environments where corrosion is an issue; eg marine environments, a marine grade stainless steel is used and this is designated A4 or AISI 316. Concerns regarding the suitability of marine grade stainless steel have prompted Fixe and Petzl to produce a more resistant range of expansion bolts that use a duplex 2304 grade of stainless steel or AISI 904L grade.

AISI Grade EN Grade Description Example
304L 1.4307 Approximately 18% chromium and 8% nickel by weight Fixe SS Glue-In Bolt
316 1.4401 2-3% Molybdenum included Fixe SS Glue-In Bolt
316L 1.4404 Variant of 316 – less carbon Climb tech Wave bolt
2304 Duplex 1.4362 Duplex Fixe PLX HCR Anchors
2205 Duplex 1.4462 Duplex stainless has a microstructure which is split roughly 50:50 between austenite and ferrite and balancing of these phases provides the following benefits:

•Higher strength which is around twice that of type 304 austenitic stainless steel.

•Good weldability.

•Good low temperature toughness.

•Improved resistance to stress corrosion cracking.

Bolt Products Saltwater
904L 1.4539 4 – 5% Molybdenum Not technically HCR! Petzl Coeur bolt HCR
926 1.4529 6 – 7% Molybdenum included True HCR Common in bar form – no products sold
254 SMO 1.4547 6 – 7% Molybdenum included True HCR Common in sheet form – no products sold

Resin bolts have a distinct advantage compared to expansion bolts as they can be manufactured in standard steel, marine grade stainless steel and critically titanium which is currently considered to provide the best resistance to all forms of corrosion.


The rate of oxidation (rusting) of the metal typically determines the lifespan of a bolt and when saltwater is present in the atmosphere this accelerates the corrosion process and marine grade steel is a mandatory requirement.

Even in non-marine environments, matching the grade of all metal used is critical in preventing another form of corrosion; dissimilar metal corrosion. The electrochemical difference in metal type creates a sacrificial process where metal ions move from the more anodic metal to the more cathodic metal.

The image above provides an example of dissimilar metal corrosion caused by the combination of aluminium hangers and normal steel chain.

Rock type and proximity to the sea can rapidly accelerate corrosion. The combination of magnesium ions in limestone reacting with chlorine ions in saltwater to form magnesium chloride has rendered marine grade bolts in Thailand and the Cayman Islands unsafe in a year. The surface appearance would suggest the bolt is safe to use however body weight in most cases is sufficient to break the bolt. This is known as Stress Corrosion Cracking (SCC) and is the result of a combination of three factors – a susceptible material (stainless steel), exposure to a corrosive environment (chlorides in this case) and tensile stresses above a threshold (expansion bolts).

In South Thailand, where SCC is a serious issue, Petzl Collinox marine grade resin bolts formed by hot forging have resisted SCC no doubt in part to the absence of material cracks.

Ultimately the only solution in those areas is to switch bolt and metal type to a titanium resin bolt in combination with an epoxy resin.


Resin bolts typically last longer than a comparable expansion bolt because the majority of the metal is protected by resin (no air gap) and no stress is present until the bolt is loaded by a climber. The hanger on an expansion bolt rarely sits perfectly flat and the complex loading exerted through the metal hanger from the expansion bolt acts as a catalyst once corrosion has developed in the metal.

Tests conducted by Deutscher Alpine Verien (DAV) on 15 year old resin bolts found no loss in strength and the general consensus seems to be that glued-in bolts have an almost indefinite life span.

Bolt Testing

Testing of bolts is either performed at dedicated testing facilities that load test materials or at climbing areas using purpose built, man portable testers.

There are three failure modes possible; bolt failure, rock failure and a combination of both. Regardless of the load direction, modern bolts fail in tension, the rock crushes and the bolt slides out even when radially loaded.

The EN 959 test requires a fall resistance of 25kN and an extraction resistance of 15kN, the UIAA 123 requirements is 25kN fall and 20kN extraction. This test is performed in a concrete block of 50N/mm2 which in practical terms is a medium to low strength rock type.

The use of auto lock devices in sport climbing can generate very high impact loads on fixed protection and in combination with friction, the requirement for bolts to hold 15kN is not a particularly high target, especially in combination with weaker rock types and variability in the rock itself.


Testing is typically conducted axially (worst case) and under increasing, static load to failure. Since bolts are designed for use with an energy absorbing element; a climbing rope, the load in a fall therefore becomes a force on the bolt. The bolt itself cannot absorb force, it transmits it into the surrounding rock mass.


For testing outdoors, portable testers consist of a hydraulic cylinder mounted on a tripod, linking arm and attachment eye which is linked to the bolt using a high strength steel quick link or karabiner. A hand pump is linked via a length of flexible hose and a pressure gauge used to record the pressure which is converted into kilo Newton.


Pull testing of a Bolt Products stainless steel marine grade 6mm * 80mm resin bolt in granite. The linking karabiner failed at 55kN.


Example of SCC found in an expansion bolt hanger. The bolt hanger failed at 5kN, well below the EN 959 test requirement.


4 thoughts on “Bolt Systems

  1. Did you build the testing rig yourself or is it available commercially?
    I have used hilti rig before but the gauge was limited to 25kn so unable to test many bolts to complete failure.

    • Hi Neil,

      The rig was privately built to my specification; a) practically portable for in-situ anchor testing on a cliff face b) capable of exerting sufficient force to fully extract all known anchors used by climbers c) able to be transported overseas.

      The issue with portable testers is producing sufficient pressure from a hand pump that is needed to extract some of the larger bolts that have higher tensile resistance without the whole rig being too big to abseil down a cliff with. Or reasonably transport more than 5m from a car.

      The ram travel is an inch or so but this is again a compromise on size; if you want a cylinder that can cover full extraction of most anchors in a single action then the rig won’t be small or then generating pump pressure becomes an issue. Technically my legs should be further apart to prevent ‘interference’ with anchor behaviour under load but I’m not certifying bolts and for general purposes e.g. extracting old bolts / behaviour of adhesives etc it works well.

      Hilti rig is great but force range limited as you note.



      • The other option is Hydrajaws, they have a 20kN and 50kN tester with some nice options e.g. digital gauge.

        Pricey gear though.

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