Hello all users!
The whole sentence reads as follows:
This tool must always be used, having been engineered as an approved method of assembly rotation, by engaging the manually driven pinion with the engine flywheel starter ring-gear.
In my opinion, the whole sentence means the same as:
This tool, which had already been engineered as an approved method of assembly rotation, must always be used by bringing the manually driven pinion in mesh with the engine flywheel starter ring-gear.
By bringing ..... in mesh, I mean engaging.
It is an excerpt from the instruction on how to match the engine to the generator.