RE: OS 55AX
Paint thinner is definitely not the desirable solvent to clean an engine with. It literally leaches out any oil it is in contact with including the pores of the brass bushing.It also causes damage to any plastic it is in contact and dissolves ,redeposits and hardens gunk in the wrong places.
Your engine was literally starved of oil as castor even though a very good lubricant is rather viscous and would need some time to work in while metal to metal contact was progressively destroying the conrod especially running at high rpms. I suspect the lubricating hole and micro pores may have been inadvertently plugged up by a mixture of varnish and gunk dissolved and hardened by thinner.
Use kerosene to clean the internals of your engine after separating the seals and O rings .Wipe dry, apply light oil and reassemble as recommended by OS.