Mary,

1. Close the piping input editor.
2. Go to the "utilities tab."
3. Create/Review Units -> copy a units system you're using, but change the setting to specify manual input of OD. Save it as something meaningful to you.

On a new file, use this new file.

On existing files, use the "change model units" on the same tab on the ribbon interface to change this setting.

You CAN model them all in the same file. Unfortunately, this setting is stored in an awkward location "under the hood."

Consequently, once you change it, any changes to sizes of piping requires you to key in the OD manually... unless you change the "units" back.