whenever I open a new part my custom mass/sections properties change back into mm instead of staying in inches im using solid works professional 2021
When you start a new part, change the units, then save it as a part template (*.prtdot) then use that template when creating new parts.
See this blog post. It will answer your question and point you in the right direction: Why aren't the changes I made at Tools > Options > Document Properties saved when I start a new Part?
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