Start FreeCAD, open Part Design workbench. You need section sketch which will be revolved around Y axis. Start a new sketch, and try create sketch like in the screenshot. The height is 137 and inner radius is 37.5. Use poliline and horizontal, vertical, coincident constraints. If any constraint is not necessary, click on it's icon and press [del].
Note: grid is useful, set grid to 10 mm (without snapping) in TaskView. FreeCAD shows you actual position near cursor.
Use lock constraint to fix sketch certain position (inner liner radius is 37,5).
Wall thickness is variable:
|Distance from top||Wall thickness|
|7||jump from 8 to 6|
|105||jump from 8 to 3|
Close the sketch and make a revolution around Y axis.
To achieve minimal engine length we need cut some material from liner. Select a top face of liner and start sketching.
Draw two big rectangles symmetrical to Z axis. You can use a construction line (like in the "Pin ring tutorial") as centreline. Distance between rectangles is 88.
Finish editing the sketch and use a pocket tool.
Set (in Property View-Data) Pocket length to 137.
The final effect:
Download RAW Video.
FreeCAD 0.12 r4907 SVN.