So you're standing at the parts counter.....

jos51700 said

And I also have a notebook that lists all my mods, and has wiring diagrams (including stuff I added or modded), in the car at all times. I even put part numbers in there for wear items. Brake shoes? Oh yeah, that's part number XXXX from a '78 Volare that still had 3 different options.

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This is probably one of the most important things to do for your car!
I keep a "build book" as the car is going together, when it's done, I rewrite it into a new "owner's manual" and it stays in the car.
As time goes on and other projects come and go, it's tough to remember sometimes what exactly went into the car. Heck, I have a hard enough time remembering what I had for breakfast, let alone what year transmission is in this particular car...