Ford and the Birth of the Model T

A friend of mine ordered a set of dimensioned drawings for the P-51 Mustang. He was investigating the possibility of going into business making P-51s.

But when talking to people who owned P-51s, he was told that the drawings were made after-the-fact. The true design was encapsulated in the jigs and machinery developed for the factory floor, and they'd all been scrapped after the war.

The drawings were useless.

I'm pretty sure that P-51s are maintained these days by making replacement parts by hand and custom-fitted, a very expensive proposition.

As long as parts were mechanically produced in series, they are standard. It doesn’t really matter then if the drawings were made ex post from existing parts. If you produce from a description with a precision equivalent to the original machinery tolerance, you should end up with virtually identical parts.

> I'm pretty sure that P-51s are maintained these days by making replacement parts by hand

This is fairly common for old cars too when spare parts are not mass produced anymore if they were at all. It used to be really common for all repairs not that long ago to be honest.

My grandfather was a mechanics in the 50s and he liked to explain that machining parts was a common occurrence then because getting parts shipped would take considerably longer than just making them and everyone involved knew how to machine parts anyway.

You don't. Tolerances are almost never bespoke except in the rarest occasions of cutting edge green field development.

You can literally buy an old book (the info is all on the internet too but not in one place) and skim through thousands and thousands of pages that tell you what different classes of tolerance for different applications are. You don't need to know what they spec'd. You just need to know what the part was and how it was used.

The thought process goes like "This worn the f out coupling is .9443 inches major diameter, so it was a 1" (nominal) coupling, and it must slip under load, so I will use a class-whatever fit, and the tolerances for that are +/-.0gfy"

An imprecise drawing is also a sufficient starting point for the process.

And like 30% of the time then you get half way through figuring out how to set it up and realize that you can literally just buy whatever you're looking for at tractor supply or Alibaba or whatever and modify it slightly.