In order for these parts to be assembled correctly and fulfill their function of positioning and guiding the motor and pinion shafts, certain dimensions must be common, hence the importance of using one (or more) master sketches.
These master sketches were made after inserting and scaling flat images of the crankcase, using the Sketcher workbench directly without going into PartDesign, to make them independent of the different crankcase components.
They contain circles and segments whose centers or ends give all the strategic points of the constructions to come (the dimensions or other geometric constraints do not appear on the image because the sketch shown is not in edit mode) .
The master sketch 1 which will drive the entire crankcase parts representation project is in the XZ plane.
Its position has been chosen so that its origin is at the center of the bore receiving the gearbox shaft.
Importing external geometry from this sketch will give better stability to the project and the sources will always be the same.