Scratch building requires dealing with a variety of dimensional systems. Not just English and metric, but also systems for wire, drills and hardware. The following table links the dimensions that are the most likely to be used when modeling in 1:8 scale. Each row represents a common diameter.
Tap & Die information has now been added. Ken Krausfeldt pulled that information together and I'm grateful for all of his help.
Click on the image below for a downloadable version (1MB PDF) ...