Isn't '75 about when the locally built Corona was starting to
be fitted with a local four cylinder crap donk?
IIRC it was between '75 and '78.

But most cars aren't monocoque by design.

With a monocoque construction, the outer skin is a structural member and
that certainly isn't true of most car bodies. To be specific, not completely
anyway. Aircraft bodies are true monocoque in that the entire outer skin
forms part of the structural assembly, but with car bodies things are a
little different.

The only *real* structural "outer skin" on your typical unitary car body is
the floor, as it's form is dedicated to the function of providing strength
in that it's pressed into a particular shape to provide a fair degree of
rigidity for the metal being used, and has frame rails and other associated
crap fastened to it to assist with that. The rest of the outer skin, namely
the "shiny side" offers little in the way of anything other than covering
the internal framework and making it look pretty.

Take turret or roof panels for example.

A *lot* of people think the roof skin of a car is a structural member, but
it's not. The internal frame that runs around the periphery of the roof at
the gutter level (or where the gutter would otherwise be on modern cars that
don't have them) and attaches to the pillars is the structural part, with
the outer panel existing purely for decoration and to stop you getting wet
when it rains. Quarter panels, guards, doors bonnets and boots are all the
same in that their purpose is to be formed in a particular way as to make
the car look pretty (or, at least what it's designer thinks looks pretty)
while also providing a weatherproof outer layer.

Strength in metal is more to do with shape rather than thickness, and if you
look at some cars the shape of their outsides would make it impossible for
the outer skin to provide much more strength than covering the internal
frame with gladwrap would.

--
Regards,
Noddy.

Just walk around a carpark after a big hail storm and look at the cars with
no dents compared to those that are near write offs :)

The change for most, if not all of the locally built ones came in
around the late 60's.

Valiant went to lighter panels somewhere between the VC and the VH but
I don't remember what model.  It was also the transition between being
able to metal finish the panels (for those who still knew how) and
either bogging or replacing them.

Modern panels are high tensile to boot, which makes them stronger but
near impossible to straighten for other than shallow dents.

Which is why newer cars are a pain in the arse to panel-beat. When you hit a
bent panel from an older car with a hammer, the metal bends and stays bent.
The newer sh.t steel flexes rather than bends, and takes a  lot more work
(or bog.....) to get into shape.