Thread reinstatement imminent…
So the parts don’t fit back together the the way they came apart. Now why might that be?
The bonnet and the bonnet frames can be covered separately, even though the same basic principle is at work - we are dealing with a steel spring.
People generally don’t look at stuff whilst it is working and think “How does that work?”, but they come here after it is broken or dismantled and want to go back to where they came from, which in this case is somewhere they didn’t know where they’d come from.
Both items act like a spring. In the case of the frames, they have been supporting a heavy weight for the last 45 years or so and have been subject to periodic vibration, twisting and g forces in excess of the static force of gravity when at rest. Unsurprisingly, the metal has to move to accommodate every bump in the road or it has to transmit the force through the structure into the bulkhead. The metal takes on a set which is inevitably different from day one and come restoration time, this 3D spring is unloaded and naturally tries to unwind. This is now a bit of a defined term, as it won’t unwind to where it came from because some of the metal is work hardened a bit and some of it is weakened by corrosion. It has been periodically heated up to the ambient temperature in the engine bay, bit by bit, but now it creeps back when cold. If you now shot blast it, apply a rust treatment inside and repaint it, it’ll be good as new, but it’ll be a different “new” and it’ll flex slightly differently to before and probably point its bonnet support hinges to a different place in three dimensions. Ally to that that people will maybe want to fit “upgrades”, like polybush suspension mounts which may transmit force differently or use “uprated” torsion bars which transmit a different proportion of forces into different parts of the structure when going over the same bumps as before.
So what about the bonnet. This starts life bolted to the frames in two points only and held pressed onto a strip of rubber at the bulkhead. Everything else is free to flex and move subject to being screwed together plus a little bit of lead loading at the front of the car after we lost the covered headlights. Just like the frames that it hangs from, it has been subject to heat cycling, twisting, corrosion and vibration for years and now it is going to be “restored”.
Mistake number one is that the hapless restorer dismantles the bonnet first into its sub components. The spring which was held together through thick and thin is now free to unwind without the constraints to movement that the other panels gave it. This isn’t a disaster so long as it never went past its Youngs modulus point, but if there is any crash damage, no matter how light, then this will unwound differently to how it was put in. How do you know that? Well the hapless restorer is about to shot blast the bonnet back to bare metal, piece by piece using probably a very aggressive medium like sand. If you were to say to someone, “Panel beat the paint off of this panel!”, they’d tell you you were insane, but this is exactly what countless people do. Sand will not only take the paint off, it will heat up the metal unevenly and you’ll end up with a subtly different curvature all over as the “spring” is being held differently and the warmest parts are now thinner and more flexible than the cold, as yet unblasted portions. It may look the same but it isn’t in three dimensions. Any latent crash damage will now pop out as all of that bondo which was keeping it rigid is removed. If it is corroded then the even curvature is now taken up by a kink in the metal around the new hole or holes.
In short, if you are going to shot blast metalwork, use a medium like garnet or plastic pellets as these are both far too soft to hurt the metal but impart enough energy to remove paint and filler and do this whilst the bonnet is still in one piece so it retains its current 3D geometry. (You can then dismantle it and do just the seams afterwards.) People don’t do this usually as it is more expensive. Also, it’ll flash rust in seconds, so follow the blaster with a hot zinc phosphate spray just behind the blaster.
If you adopt this approach, you just might have fewer and less serious alignment problems going forwards.
This is particularly important if you are going to replace some of the panels, e.g. with a brand new underpan plus second hand wings from another (maybe equally crash damaged) car.
Put bluntly, even reusing all of the original panels, if poorly treated from a physics standpoint during restoration, you can easily make it not fit. Heaven help those who cobble together something from multiple bonnets of dubious origin.
It can all be made to fit, but you’ll have to do some work before moving onto the paint stage. Sadly, many professionals are under pressure to work down to a price rather than up to a standard and that’s a subtly different ball game to be in.
kind regards
Marek
) that seems different to common or garden MS. I have no evidence except eyes and feel but it wouldn’t surprise me at all if the picture frame was some kind of medium tensile steel, just not Reynolds 531.