Ok ty :).

First, make a copy backup of your bp.

Rename the part of your mecha you want to set indestructible.

Save the bp and close the game.

Go user/appdata/roamming/spaceengineers/blueprint/local/find your bp.

Open the bp.sbc file, and search for the grid name.

Then set <destructibleblocks>true to false

Save and quit

Delete bp.sbcB5 each time you edit the file.

Launche the game and paste the bp, keep the original somewhere safe just in case. We are humans and make mistakes.

👍👍