Here's a simple "I'm an Idiot" question:

I can fix, build or make just about anything but there's one area that I just cant seem to grasp; Nuts and Bolts.

The sizes, threads and hardness for each application. I end up buying a mix of everything on the shelf. What's the simple way of knowing every bolt size on a 1969 GM car. I know it's possible because I've watched many mechanics go grab everything they need and always seem to get it right.

I know it's not Quantum Physics, just seems like it's my weakness.

 

Practice, my young grasshopper...

I've found that after 35 years, you get to learn what size is used where.

 

Thanks Joe, but I've been in this mental block for more than 40 years now....Young grasshopper....LMBO

 

I find experience along with calibrated and sealed eyeballs comes in handy.
Just close your eyes and become one with the bolt and it will tell you.

Weedhopper

 

If I find my car at a yard I'll yank nuts and bolts. Pay attention as there are many styles. Sheet metal joining some with attached washers, bracket to engine etc. There are stamped heads that are unique to factory bolts that you can't find at a store.

I restored a '72 Cougar and it had been restored previously and every nut, bolt and washer had been replaced with hardware store fare. This was in '87 and the yards just don't have the donor cars to supply factory stuff anymore.

Start a collection of nuts and bolts now.

 

Body part large bolts with the sheet metal threads are the hardest to find. Any time I am in a wrecking yard I pick up as many as I can, sort them and kept them in plastic containers with the sizes noted. I have several coffee cans with assorted nuts, bolts and washers. I used to have my kids sort them for me, now they are raised and gone and I can't get my wife to do the job so some day I will have to sit down with a case of beer and get er done.