When an engine has all it's components in good working order
A Cummins L10 came in for injector replacement.
Injector replacements are, generally speaking, an easy repair. You remove all parts in the way of pulling and replacing the injector and re-install all good working parts as they were.
Not on a L10. When you are reinstalling parts the push tubes don't stay put on the rocker arms like other engines and have to be constantly reset to complete the injector install.
Once the injectors are all installed, after much sweat and swearing, it's time to run the overhead.
This is one of those steps that are made easier by the engine components being in good order to begin with. The overhead is set by matching timing marks up on an accessory pulley and the engine front cover.