On Tue, Nov 30, 2021 at 05:27 PM, Chad Gray wrote:
A Google search for the phrase: "standard spacing for the mounting holes on PCB"
yielded this: https://www.google.com/search?q=standard+spacing+for+the+mounting+holes+on+PCB&sourceid=chrome&ie=UTF-8 (which of course you probably already did.) But I did get some useful info: https://www.onsemi.cn/pub/collateral/and9867-d.pdf which is a manual for mounting Onsemi's power modules.
My bet is there's no absolute standard. I did try: "ieee standard spacing mounting holes on PCBs" and got some references, but nothing like a standard.
It seems a lot of electronic devices use metric measurements in their physical mounting, so I would stick with the metric system for conformance.