This came from the 1915 Machineries Handbook. These are the sizes that railroads used. The SAE sizes that we buy in the store today have slightly smaller heads on the bolts and smaller nuts. Today these sizes are some times known as heavy nuts as opposed to the SAE size. When modeling if your prototype needs a 5/8 in. bolt for example the head would be 1-1/16 in. wide. Divide this by 48 ( O Scale is 1/48th size) and you would get .022 in. You can measure the heads of the bolt castings you are using to get a good close size.

Nut and Bolt Sizes
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