The Book of Mechanical Bolts

“Know what thy is shoving in thou hole.”

The Bolting Bible

This is free resource by HowNOT2 and our way of contributing to the community. It's nice to understand what you are clipping and trusting with your life, even if you never plan on installing or removing bolts. We also believe that if you are going to spend time and money to bolt something, you should do it well.

This book is in a blog format. This page points to all 17 chapters, and the end of each chapter points to the next.

A downloadable PDF is available HERE.

How They Work

Mechancial bolts are simple machines. A wedge gets sucked into something that expands. If it is small clip expanding at the bottom it's a wedge bolt and if it is sleeve over the entire bolt with 2 flanges expanding at the bottom, then it is a sleeve bolt.

Girth Matters

Back in the day, skinny 6mm or ¼ bolts were used and yikes. Now they are all getting replaced. It is very common to have a 10mm or ⅜” bolt for climbing and 12mm or ½” bolts are the standard for highlining since they can potentially see a lot more force than the ones used for climbing. If bolting in softer rock a 16mm or ⅝” bolt might be better, not to benefit from the strength of the bolt, but because a bigger bolt can hold the rock better. And if the rock is soft enough, you will want those fat bolts to be glued in.

Drill bit diameters are important to get right. Although ½” = 12.7mm, you CANNOT interchange 12mm for a ½” drill bit, especially for the Fixe triplex removables because they have a tight tolerance. Also, the Petzl Coeur Pulse requires a 4 point 12mm drill bit so the hole is perfectly round. Know your bolt and make sure your drill bit matches.

Length Matters

Mechanical bolt lengths describe the entire bolt so keep in mind how much will be below and above the surface. You can easily lose a 1/2" or 13mm with the hanger + washer + tightening of a wedge bolt. It doesn’t matter how long you think it is but how deep it actually penetrates!

Your length depends on how hard your rock is. 2.25” or 55mm is fine for hard rock. The softer the rock, the deeper you will want the bolt. 6” or 150mm is common for softer rock but if it is really soft or layered rock then you will want a (long) glue in bolt.

Wedge vs Sleeve

Wedge Bolt

Wedge bolts are generally simpler with fewer parts than a sleeve bolt and therefore cheaper. The Powers 5 Piece is a sleeve bolts that has 7 pieces and is finicky to install and is getting quite expensive,

Wedge and sleeve bolts are around the same strength. They are all super good enough. Sleeve bolts however will engage better with softer rock. If you have solid hard rock, wedge bolts are totally good enough.

Wedge bolts pull the entire rod up while the clip stays at the bottom to expand. Because of this, it sticks up when you are done. It's not a problem unless it is in the way of clipping carabiners to it. Sleeve bolts look nice having their heads flush to the hanger since it is just a threaded rod at the end sucking up a wedge-shaped cone. If someone is going to do a bad installation job, a wedge bolt sticking way out is an obvious red flag there isn't much bolt left in the rock but a sleeve bolt keeps all its secrets hidden.

Wedge Bolts in Detail

These bolts have a small expansion clip with bumps on the side located near the base of a bolt shaft. Those bumps don’t allow it to move since it is slightly bigger than the hole diameter. The very end of the bolt is cone shaped, so when the nut is tightened, it pulls the TAPERED END of the shaft up, expanding the clip. This kind of bolt is recommended only in hard to medium rock as the contact point is very minimal. Sometimes, this bolt gets extracted quite a bit if the clip slips, that the threaded rod sticks out so much it gets in the way of carabiners and leaves significantly less bolt in the rock.

If the threaded rod is protruding enough, it could also depress the gate of a carabiner open if the quickdraw was rotated upwards, possibly unclipping it or just reducing strength if loaded in that position. Never use these in sandstone or other soft rock as it can wear down the rock at the contact points under cyclic loads and become loose from the now oversized hole.

The real difference is the size of the expansion clip at the base of the rod. There is no real reason to use wedge bolts over sleeve bolts other than it’s easier to find 316SS, as most sleeve bolts are 304SS. So if you have an area that is prone to corrosion and have hard rock (as it’s not a good idea to install wedge bolts in soft rock), then these might be the right choice. But if the area is high risk of corrosion, you might as well put in titanium glue in bolts to make sure that it lasts.

Sleeve Bolts in Detail

These bolts are threaded rods with a coned nut on the end. These are called sleeve anchors because the sleeve part covers the entire bolt shaft. The hex head and the shaft are one piece, rather than threads at the top with a nut. The “nut” is instead at the bottom and is coned shape so the tighter it is, the more it expands the sleeve. Therefore the hex head stays flush on the hanger rather than the rod sticking out. The sleeve also allows for more contact area and is ok to use for all types of rock although the softer the rock is, the more glue in bolts are preferred. These bolts especially need to be tightened at a specific torque, so if you don’t take a torque wrench with you, practice at home to get the right feel for it before doing your project. If these become loose after placement, they could be prone to unscrewing themselves as the hanger is torqued back and forth by rope tension, and pulling out under body weight. See the buying guide for all your options at the end of the “Mechanical Bolts” section.

Sleeve bolts are better for softer rock because they have a larger surface area and can open the split sleeve wider than just a wedge bolt with a small clip at the end of it. The softer the rock, the deeper and bigger you will want your bolt. There is no downside to using a sleeve bolt in hard rock so it is a good idea to use them unless you want marine grade 316 stainless, which is hard to come by in a sleeve bolt.

Removable Bolts

Removable bolts are great where you don’t want to leave permanent bolts. You might want to remove them because it is a high traffic area. It could be a project that rarely will be repeated. Maybe you are unsure where you want the final bolts during development. It’s also great if you don’t want to wait for glue in bolts to cure, because these allow you to install the glue ins AFTER you use the removables. These also could be a great option for maintenance like in wet canyons where bolts can get damaged by seasonal flooding.

The concepts are the same as wedge and sleeve bolts, however the harder you pull on those bolts, the more they grab the rock. Contrarily, removables are designed so the sleeves can be pulled up separately after untightening, allowing you to avoid the wedging action that keeps the bolt in the rock. These should not be used as a long term anchor because if they ever were to loosen, they will not be safe to use. Just like all bolts, there are some downsides. They need to be drilled perfectly because if it is too big, it just spins in the hole and if it is too small then it’s a real bitch trying to remove them. If a hole is repeatedly used for a removable, mostly in softer rock, it can wear out the hole, and no one likes a hole that is worn out! If someone tries to repeat a highline, they may not know if it was a 12mm or ½” hole and that’s important because they require different bolts. If on top of a cliff, a hole can get filled in with debris and need extensive cleaning. Also, in my experience, removables can look pretty mangled after a few “removings” so that’s why they aren’t called “reusable bolts” but “removable bolts”. They can be reused but not indefinitely. Fixe’s Triplex (12mm) has a threaded rod with a tapered cone and Climbtech Legacy bolt (½”) is a flush hex bolt with a coned nut on the end but is unfortunately no longer available, as I have found Climbtech to be easier to remove than the Triplex.. I like using Bolt Products’ welded hangers on fixe triplex bolts so I can thread my rope directly into the hanger.

If you are real experimental and rich, you can try Climbtech's fancy removable anchor. They are designed similar to cams and it is just a round version of ball nutz. If you bottom them out (put them in too deep), they will be almost impossible to remove. These can be great if you need a temporary bolt for establishing a route but if you highline on them it could kink the flexible wire and the ½” ones are only rated for 11kn. If you use 8 of these for a highline, it should only cost over $600!!! They have ¾” and 1” sizes but we don’t need to be drilling holes in our rocks that big for temporary anchors. You can drill the hole at an angle to minimize the wire kink but if you plan on using that hole for glue, then they need to be drilled properly. The inventor of these does human testing on them in this video which I always appreciate when someone falls behind their products.

Petzl now has the Coeur-Pulse a 12mm removable that doesn’t require tools (assuming you already have a clean hole waiting for you). Those also are expensive but they can be used for highlining and are pretty fancy. They have a thin sleeve layer that gets pulled out of the way when you pull the trigger… aka… tooless. They require a perfect hole and so you need to use fresh 4 point bits. The fat heads on them limit how much you can clip to them but I do recommend them if you can afford them. Some previous versions have the hanger fixed on there and not free spinning, so if it is not installed in the right direction and it turns, it would turn the entire bolt in the hole compromising it. A video on installing them can be found here.

Washers

Washers distribute the pressure over more of the hanger (serious rocket science material here!). They might not be fancy but they are important. Some bolts, like Powers 5 piece rawl, comes with the washer. They are also very important if chain links are used instead of hangers (which is not an ideal method). In BoltBusters, we peeled this 1/2" hanger off a 3/8" Powers Bolt in this VIDEO! Use the right size hanger with the right size bolt, but also use washers when applicable. The most common mistake when using washers is to buy the shiny cheap ones at the store. Don’t use zinc washers! Stainless and stainless need to be together or that washer will rust quickly.

The Weird Stuff

Compression Bolts

Old school button heads have no moving parts. They are cut and shaped to be a little bigger than the hole and have such a tight fit that they stay in. The bolts are called split shaft, the concept is called compression bolts or positive locking.

3/8" Split Shaft Tested

The little tiny ¼” button heads are found more in blank sections of big walls where bolt ladders needed to be installed, rather than for anchors.
The next size up is ⅜” and has a threaded top with a nut. We found while installing them that it takes so much work to pound them in that the nut and hanger have to be preinstalled or the threads get too damaged to put on the nut. We tested these in shear and tension on BoltBuster and found the top of the bolt snaps off before

Spike Bolts

coming out, at least for a new bolt, which means they are super duper tight in that hole!. They rarely come in stainless and is the kind of bolt that is being replaced today. They existed, so we share them here, but please don’t use these. Spike bolts are similar in the fact they are bent but they are the same idea.

Nail drives

“Nail drives” or “Hammer set” or “Hammer Drive” or “Strike anchor” or whatever the hell you want to call it, it is a bolt that expands the sides as you smash a nail through the center. If that nail is flush, it isn’t coming out. Petzl used to sell one called the “Petzl Long Life”, clever name for a bolt, but apparently it wasn’t popular enough and was expensive.
Some are flush with the hanger and prevent hanger thieves, others have nuts that hold down the hangers. Since none available today are designed for life support climbing applications, they are not certified and can be a risk. ASCA broke an off brand (AALL American) ½” in tension at only 10kn, substantially below its MBS. ¼” strike anchors are a popular size online (not for climbing but general use) and those can break below 2kn.

Spits / Self Drives / Drop In

"Spits" is the general name name of self drives or drop in anchors. Drop ins require a much shallower hole and self drive bolts have their own teeth to be pounded in directly without a drill bit. There is a very narrow use case for these today and you can read about them in the Bolting for Caving section.

This VIDEO shows how a drop in works. Short story, just don’t use them, there are much better options these days.

What's Next?

Chapter #8 - The Book of Installing Mechanical Bolts