I think I have found a solution to this problem: With the 0.90 update, Proc Fairings removed single parts and added size changes with a right-click menu. The new tech requirements for that menu can be found under ...\GameData\ProceduralFairings\common.cfg: "PROCFAIRINGS_MINDIAMETER" and "PROCFAIRINGS_MAXDIAMETER" This is how I modified the file for now: PROCFAIRINGS_MINDIAMETER { start = 1.00 precisionEngineering = 0.500 sandbox = 0.1 } PROCFAIRINGS_MAXDIAMETER { start = 1.50 largeFairings = 3.0 veryLargeFairings = 6 veryLargeFairings = 10 sandbox = 50 } This also means most old parts can be removed from the tech tree and if I understand the code properly, we can now add new middle steps for the size (by hand only?) as we see fit (not sure on it tho) or as we unlock new tank sizes.