Reply to bboaz:
CAT 60
Serial Number is 5R3408SP I think it is a 1945 model.
The Auto Patrol I own and just freed up had been sitting for 20-30 years. No direct water to the cylinders that I could see but plenty of opportunity for our high humidity air to rust the cylinders.
For four months, used Kroll, PB Blaster, Penetrant and heat on the outside of the cylinders to loosen them up. 3 of them loosened up right away. The worst refused to be pursuaded.
Pulled the cylinder head and it was about 2 inches down the cylinder. I filled the cylinder up with the above slurry and set it on fire. It burned for 10 minutes, I tapped the outside of the cylinder with a hammer, then burned for another 10, repeat once more. Once the cylinder had all the fluid burned out I found bugs, rust, grime in between the piston and ring land. It was very unlikely the pentrant was able to get down to the rings (now, or ever).
I took a razor blade and carefully got the grime out and trimmed off the rust. I then used a small torch to heat the crevice between the piston and wall and then sprayed PB Blaster in to "shock it".
Did this for an hour. Then grabbed the fan and wiggled back and forth. Little by little coming free. Shocked her again. Full rotation with no apparent liner damage within another hour.
Hope it works for you!