Does anyone have any suggestions on how to undo the seized parts?
(Penetrating oil and freezing the parts didn't make any difference.)

Fred
K4DII

nope, they are probably 'galled'. a condition that happens when you put
together two unlubricated stainless steel parts. essentially the pressure
of rubbing has spot welded the pieces together. this condition does not
respond to penetrating oils and other normal methods since the parts are
not rusted or dirty.


Unfortunately I would also say this is what has happened - I had the same
experience with new replacement stainless bolts
on my bike - great, I thought, fit and forget, except when I was halfway
through installing the first bolt I had to take it out and it seemed
unusually stiff. I HAD to get it out so slow extreme force was eventually
applied even if it stripped things, but it came out - a call to the supplier
confirmed it must always be used with copperslip grease or similar to stop
this very problem occurring.

I think the only solution may be force but very slowly to minimise heat
causing more spot welds, else it will be a spark eroder or similar drastic
action

Nick