Seized-up S/42

[sheepherder]

If you are dead set on soaking it, my $.02 is PB Blaster...I soaked six 11-year old spark plugs in aluminum heads with it and they all worked loose/out, with a lot of tightening/untightening...And no damage to the threads...

...But I also know from experience that if the PO re-assembled the cannon assembly and got the coupling link *behind* the recoil lever (instead of in front), then it will jam before you can pull the toggle assembly back...My 1900AE came this way, and it took a bit of wiggling, upside down poking, and blind luck to get it to where I could disassemble it...

...But surely yours is just simple rust and will slide out like an eel when it's been soaked for a while...

Edit: The area below shows a correctly assembled coupling link...If it was behind the recoil lever, it would jam...

(I can see a lot of uses for those x-ray pics...)