ORIGINAL: downunder

But on the off chance it had a stuck ring (or rings in this case ) I pour enough acetone into the cylinder, with the engine inverted of course, to get a hydraulic lock then fairly gently force the piston into the lock so the pressure forces acetone into the ring grooves. By rocking the prop so it goes in and out of the hydraulic lock it'll work the acetone through and dissolve the castor.