Many years ago I used to go on Sunday rides with the local North Devon CTC (Cycle Touring Club). One of our number could fix any problem that occurred on any of our bikes. In his capacious saddlebag he had a huge kit of tools and spares including, believe it or not, a small vice! A few years later I found myself in a similar position as I led groups of paying customers, and I had to be able to fix just about anything at the side of the road. My heaviest piece of equipment was a 12" adjustable spanner, drilled and modified to break down into two pieces by a friend of mine. This monster weighed about a kilogram and was needed in order to unscrew freewheels for spoke changes or failed freewheels - yes I carried a spare one of those to! In all my toolbox took up a whole front pannier and weighed 4 kgs...
Nowadays I still lead groups of cyclists, but now my "fix anything" toolkit weighs in at under 300 grams, is more versatile than the old one, and is pretty much the same as I need - and you need, for an extended solo tour.
What goes into a toolkit takes a great deal of thought and planning, often forgetting a simple tool can leave you stranded. A good toolkit also contains a few "special" items that will get you out of trouble when more conventional remedies fail. Lastly the number of tools you carry can be greatly reduced by careful selection of components on your bike.
The first thing to decide is what may go wrong with your bike, but which cannot or doesn't need to be fixed at the side of the road. For example frame failures need special tools and equipment. Bottom brackets require heavy tools to fix or adjust. I fit a sealed annular bearing type. This needs no adjustment and when it fails it generally gives plenty of warning, creaking noises, free play etc. It will go on for days like this, giving you chance to get a replacement, which are a standard fit anyway. The same generally goes for headsets, choose one that can be adjusted with an allen key. Hubs too can be had in annular bearing models, if you use cup-and-cone types or annular bearing models that are loaded by a nut you will need to carry cone and other spanners to fit, I use DT Hugi hubs which can be dismantled without tools and take a standard easily obtained annular bearing.
Next you need to look at what is likely to go wrong and prepare for it. Failures and their remedies will help you decide what to take.
1. Punctures - Sounds obvious, but they're the most common breakdown. The best thing is to try to avoid them in the first place. Pump your tyres to the maximum rated pressure (and more, most good tyres are very conservatively rated), and avoid riding over curbs, potholes etc. I carry a spare innertube at all times, innertube failures can be simple punctures, but major splits, punctures at the base of the valve stem, and torrential rain can make fixing a puncture a nightmare. Swap the innertube and fix the old innertube at the end of the day. Before you do this find what caused the puncture in the first place, it may be something poking through the tyre, badly fitted rim tape, a burr on the inside of the rim. To do this pump the innertube up and find the hole, then by using the valve/valve hole as reference find where the hole occurred on the wheel. If you can't find the problem look again, nothing is more depressing than pumping up your new innertube and finding it go flat immediately...
Before you set out make sure you are well stocked with patches and FRESH glue (it goes off quickly once opened). I've found the performance of glueless patches erratic at best. I use Michelin yellow tyre levers, have never broken one and would use nothing else.
2. Tyres - Tyre failure is a biggie, the only answer is to keep an eye open for problems, use good quality tyres (some cheap ones are a joke) and if possible carry a folding spare tyre, it may save your holiday. Emergency repairs may be made to a split tyre using cut up inner tube and superglue, but such a repair won't last long.
3. Spokes - In my opinion if you are going to cycletour you need to be able to repair broken spokes. People who say they've never broken a spoke don't do extended loaded cycletouring. There are no warnings, just a ping and a wobbly wheel. This will often happen when you least expect it, I've had one let go while pushing the bike out of my bike shed, it is fatigue that causes the break, not a sudden load. Ideally you should build the wheels you ride yourself, it's not difficult and will make repairs a doddle. To repair a broken spoke you first need a Hypercracker to remove the freehub (those using freewheels should just limp to the nearest garage...). Then a spare spoke, I carry five, and a spoke key. The "Spokey" is the best key for roadside use as it is so light, but I modify it by cutting the top half of the plastic circle off. This makes it lighter and smaller, but also much easier to pull off the nipple you are adjusting, this is a design fault that is problematic if you use any wheel where the spokes enter the nipple at an angle, such as four cross wheels, large flange or tandem hubs.
4. Cables - You're going to break cables so you need spares and possibly cutters. You can get away with four cables cut to the correct length and pregreased. I carry one long brake cable and one long gear cable. To cut them to length I carry a tiny pair of folding pliers called Sebertool M2's. These are the only folding tool that I've found that will cleanly cut cables, and the pliers can be handy. My only reservation is that an averagely strong man can snap the handles off, so a little care is needed. Now the way you've set up your bike becomes important, it should be possible to replace all the nuts with allen bolts, so your multi tool (more on this later) will cope with this, otherwise you need more spanners... Old style cantilever brake hangers are a pain as they need two spanners to adjust, replace them with one of those natty alloy blocks such as made by Tektro which only needs one allen key.
5. Chains - You must carry a chain link extractor and know how to use it. It is a "get you home" tool par excellence. It will repair a broken chain, or if your derailleur breaks it enables you to shorten the chain so you can ride home in one gear - essential. I also carry spare link so the chain will be the correct length if a link is damaged.
6. Pannier hooks - Carry a spare, it's light and may save a lot of bother.
7. Loose bits! - A bike vibrates and will shake any number of bolts loose. favourite are rack, mudguards and bottle mount bolts. Just keep an eye on them and make sure your multitool will fit them all. I carry a couple of allen bolts as spares.
This will be the heart of your toolkit. There are many available, and you need to choose one which will fit all the allen bolts on your bike has a gear adjusting screwdriver and a chain link extractor. The "Cooltool" has an adjustable spanner on it and is widely recommended, but I find the spanner is of such low quality it will round off any small nuts on your bike. It's one advantage is that it will take a headset adjusting spanner attachment, but it's worth changing your headset locknut for one that is gripped by an allen bolt instead.
The difference between a normal toolkit and a really good one are the odd items that fix the unfixable... I carry the following:- Superglue - Choose stuff for porous substances and liquid rather than gel. I've used it to fix split tyres, stop cables fraying, gluing cycling shoes, tent flysheets, panniers, pannier hooks, glasses, cameras, dental caps! - the list is endless and I couldn't do without it. It can also be used to stop bolts rattling loose but use with care, it can be a bit permanent...
Zip Ties - These little plastic fasteners are immensely strong and can hold all sorts of things together, they also weigh next to nothing. Dental Floss - This is almost unbreakable string in a convenient container. Again a great thing for holding broken racks, panniers etc together.
So, here's the total list of what I carry in my toolkit...
Tranz x.pert II multitool containing chainlink extractor, all allen keys
on my bike and a fine crosshead screwdriver.
Sebertool M2 folding pliers/wirecutters.
Cut down "Spokey".
Five spokes and two nipples.
Pair Michelin tyre levers.
Puncture repair kit and patches.
Long brake and gear cables.
Two spare allen bolts.
Spare chain link.
Spare pannier hook.
Six Large Zip Ties
Pack of Dental Floss