I spent roughly 6 years working in a body test lab for an OEM. We ran durability tests on components like doors, hoods, hinges, latches, etc, So, I may be able to shed a little light on this.
To start with, one has to think about why a threaded fastener like a bolt is even torqued in the first place. Seemingly a dumb question with the obvious answer being: "So that it won't loosen up and fall out." While this is obviously true, another reason is to provide a specific clamp load for the joint that the fastener is holding together.
For a given fastener torque, a fine thread fastener will provide more clamp load than a coarse thread fastener. If you think about the threads on a bolt as an inclined plane, you can picture why this is the case. This same characteristic also makes them less likely to back out.
Also, as previously stated, fine thread fasteners are somewhat stronger, as their minor diameter (thread not rolled as deeply into the fastener) is larger than their coarse thread counterparts.
Joints are often classified as "hard" and "soft". A hard joint would typically be something like a rod bolt or main bearing cap, where there was little or no compliance in the components that are being joined. In contrast, soft joints are typically those found in automotive sheet metal like hinges, strikers, reinforcements, or mounts. They often involve multiple layers of sheet metal that must be squeezed together. Applying and retaining a specific clamp load on joints like these can be a challenge, since the sheet metal itself can be compliant, or there still might be small gaps between the various layers that might shrink over time resulting in a loss of clamp load. Sometimes the fastener might have to be torqued beyond what might normally be considered adequate, in order to mitigate these effects, and keep the component from slipping.
Something like a door hinge is a classic example of a soft joint. If the original engineers specified a fine threaded fastener for that joint, it is very possible that they were concerned about the above in order to keep the hinges from moving around. Previously, it was stated that there is no stress on the hinge bolts when the door is closed. While it is true, that the hinge bolts are not under as much tensile stress when the door is closed as when it is open, those hinge joints still have to support the weight of the door as the vehicle is driving down the road. Not sure what the weight of a '46 PU door is, but it will effectively increase many times over when the vehicle hits a bump or pot hole. I could easily imagine 10 Gs (and I'm probably being conservative here) going into that area of the body so a 50 or 60 pound door now effectively weighs 500 - 600 pounds. Once again, the concern is not that the load will exceed the strength of the fasteners, it is whether or not the clamp load is sufficient to keep the hinge from moving.
That's not to say that sometimes there might be other reasons for an OEM to use a fine thread bolt. Perhaps using one meant that a common fastener set could be used in multiple locations in order to reduce inventory in the assembly plant. And, I'm also not totally ruling out the theory that some engineers do, in fact, harbor a secret grudge toward service technicians.
On the way to my office, I often used to walk by the area where full vehicles were tested. Basically, they took data from a vehicle on a test course, and played it back into a test vehicle with its suspension connected to large servo hydraulic actuators. These rigs attached to reinforced concrete blocks in the floor containing maybe 100 cubic yards of concrete, yet still, on some of the severe events, you could feel the floor of the building vibrate - there was that much force going into the test rig (and vehicle). Sometimes, you would swear that the car or truck was going to jump right off of the rig. I guess what I'm trying to say is that there is much more force and flexing going on in a car or truck body than you would think.
The two and a half times the diameter is a rule of thumb that I have heard before. Bolt stretch is critical for maintaining clamp load. Did you ever wonder what the indentations on the heads of some flywheel bolts are for? The fastener guru told us that since the bolt is too short to provide sufficient stretch along the length of the bolt, the indentations provide for additional stretch in the head area to help compensate.
Lastly, I would be cautious about implying that grade 5 bolts will be plenty strong for anything on our trucks. If, in fact, the system or component was built with grade 5, then obviously no problemo. However, if there was a grade 8 fastener put in a location by the factory, there was probably a very good reason for it. Critical interfaces like motor and suspension mounts, or the steering gearbox come to mind. I personally would never, ever, install a weaker fastener than what was originally specified.
Best Regards...
