If you have been running an all castor or high percentage castor you may just be gummed up like you suspect. You could give the piston and liner a warm anti-freeze bath and that will clean off all the varnish/gummed/castor. I have seen this before on older engines that were run with an all castor fuel. With the anti-freeze bath no scrubbing will be required but I have had to leave parts in the mix for over 24 hrs a few times to get them clean. If you have questions just e-mail me.
Fred