Deciding what tools and spares to take depends on where you are going and for how long. The quantity of tools and spares required for a day trip is vastly different to that required for an extended expedition into the remote areas of the mid-west or the deep woods of Canada or the Moab. A common problem with a long distance 4x4 trip is overloading of vehicles.
Some 4x4's are so overloaded that when the suspension breaks, the driver congratulates himself for having a spare spring, along with a spare axle, shock absorber, clutch plate etc etc etc. The overloading of the vehicle is in many cases the reason why various components break. By the same token, venturing into the Moab with just the vehicle’s original toolkit is inviting disaster.
What is needed is a common sense approach. If your vehicle is getting on in years, then chances are more components are near the end of their working life. This will necessitate carrying more spares, however it is not a license to overload the vehicle.
For example, all vehicles should always carry some method of repairing a blown radiator hose. It doesn't mean you have to carry every hose the vehicle has, but perhaps a multi-fit hose and a roll of rubber-weld tape would be sufficient,
A spare fan belt is critical to keep the engine running, but a spare air-conditioner belt will not stop you. However, the amount of room a spare a/c belt takes up is minimal and some may say an air conditioner can be considered vital in the hot outback!
Another common sense area is the amount of spare oil you carry. An 80 series diesel Landcruiser needs nearly 10 litres of engine oil, along with diff, gearbox and transfer case oils. Each of these oils is a different grade, even the front and rear diff oil is different, as the rear is a limited slip diff. To carry enough oil would mean around 20 litres in five different containers. In a break down situation engine oil can be used in all of these areas to allow you to get to civilization. It doesn't mean you can continue your trip indefinitely without obtaining the right oil as soon as practical. An exception to this is an automatic transmission. It needs the correct grade of oil to function.
Carrying sufficient tools is another contentious issue. You don’t need to carry enough tools or spares to perform an engine rebuild in the middle of the Moab, however some specialty items you should carry include the right socket to fit your diff drain plug, along with a small sump pump to fill it.
Carrying a torque wrench and a complete set of ring, combination and open ended wrenches in metric and standard sizes is a case of being over prepared. A set of wrenches in the correct size for your vehicle, along with a few sizes of crescents should be all that's required in the way of wrenches. One item you should ALWAYS carry is a Repair Manual for your vehicle. Even if you cannot fix your vehicle, chances are a passing good Samaritan 4WDriver can, if he has the manual.
Repairing a vehicle in the remotest of areas is about improvisation. If a spring does break, take a spare tire off its rim and place the tire between the axle and body. It may not be fancy, but it will get you mobile. Carrying a can of 'Bars Leak' or a roll of solder will allow you to fix most radiator holes or burst seams, using a screwdriver or similar heated in a fire to act as a soldering iron. A roll of wire is worth more than the largest selection of nuts and bolts in most cases. A roll of duct tape can be used for busted hoses. Also don't under-estimate the uses for the common household coat hanger.
What is to be considered are components that will stop a vehicle if they fail. For example, a blocked fuel filter has probably caused more problems, due to impure fuel, dust and vibration caused by rough roads and trails.
The most important thing is to let people know your intensions so you know help is on its way once you become overdue. When in the very remote areas, ALWAYS stay with your vehicle in a breakdown situation and wait for help. This may take days and is why you should always carry extra food and water.
In most populated areas it would be rare to go even a few hours without seeing another vehicle, so the consequences are not as dramatic. However, this does not mean you should venture out alone and unprepared and rely on a passing 4WDriver for help.
The most important thing is to be adequately prepared for the remoteness of your destination.
Member of SONOTA (South African National Offroad Trainers Association)