I have used carburetor cleaner, also with great success. I had a bolt on an Underwood I took in trade that was totally seized up: firing pin, extractor plunger and spring. I soaked it for a couple days, disassembled, and soaked it some more. The firing pin channel was so gunked up, even after the second soaking, that the firing pin would barley protrude. I soaked it a third time and used a little pipe cleaner, and finally got it squared away.
