The attachment of the engine to the Model A and AA Ford flywheel housing, clutch housing, and transmission is not very robust for bending, and an inherent design weakness exists between the attachment of the rigid engine and the rigid clutch and transmission assembly.
Shown below are typical examples of cracks found on Model A flywheel housings.
The Model A flywheel housing is relatively easy to crack at the locations shown as the result of careless handling of the engine and transmission assembly during removal, storage, or installation in a car.
Another likely common source of so many cracked housings is due to careless driving habits, and hitting curbs and objects while driving.
A sharp impact blow is transmitted through the front tires, wheels, and axle, and is directed along the wishbone (radius rods) directly to the base of the clutch housing. This applied torque load can also easily crack a flywheel housing at its inherent weak spots.
This design flaw in the Model A Ford system was resolved in the Model B Ford design, which used an upper half flywheel housing together with a stamped steel structural oil pan making up the lower half of the flywheel housing.
In addition, the Model B integrated the clutch housing and transmission case together into a single casting for added strength and rigidity, and also moved the wishbone attachment from the clutch housing to the frame cross member.
Overall, the Model B Ford engine, flywheel housing, clutch housing, and transmission assembly was a very strong and rigid unit, and resolved the related design weaknesses found on the Model A Ford.