From our insanely detailed guide:
Timing tools are used to set or preserve the positions of the camshaft and crankshaft in an engine while the timing gear is being serviced. They are commonly used during replacement of a timing belt or timing chain.
The camshaft and crankshaft need to be kept synchronised for the engine to run, and to prevent engine damage. When an engine is built, their positions are set relative to each other and then this setup is maintained by linking them together with a timing belt or chain. If the timing belt needs to be removed - for example to be replaced at its service limit, or for a another component to be accessed - then the timing gears will be able to rotate relative to each other and lose their correct positions.
A timing tool is used to lock the gears so that they cannot rotate while the belt or chain is removed. During a full engine rebuild, a timing tool cannot be used because the camshaft and crankshaft will be removed - so the timing will need to be set up manually by setting the positions of the shafts using marks on the timing gears. This is a labour intensive job, and so garages prefer to avoid it if possible.
A timing tool therefore saves labour by negating the need to manually realign the timing gear.
Types of timing tools
Each engine has a different design and so there are thousands of different timing tools specifically designed for each engine. Specialist garages will possess the timing tools for engines that they frequently service.
Universal timing tools
Kits are available that fit a wide range of engines - they will come with instructions for each engine type that allow the tool to be set up in different configurations.
Continue reading: The Oil System