The cover definitely needs to slip on easily if measuring shim thickness with solder.
I would think that the cover needs to be re-heated after torquing the cover screws so as to settle the dust, so to speak.
Re-heating the cover may transfer heat to the solder and make it yield easily.
Heating/re-heating the cover might also negate any measurements obtained.
My kingdom for a known good shim plate!
That's not a bad deal actually.