Soak them in fuel for about 30 minutes. This should free them up. Then, go run them right away so they don't sit with raw fuel in them. Heat will loosen them most of the time, but that doesn't really get the "gum" out of the engine.

Sure do appreciate the quick responses.  Won't be flying until this fall, due to heat-related instructions from my mean old cardiologist, but will sure follow your advice.  BTW, that "mean old cardiologist" has kept me alive and active for many years, for which I am very appreciative.
 
Glen

The Heat Gun beats Boiling Water , Temperature Wise .

A old trick was to use on Tight ABC engines , for initial runs / start up .

Here were its a ' bit chilly ' at the moment , carefull working over / around - to bring up to temperature
over the whole case ( bare ) at least initially , might save a cracked / warped block .

Definately , for dropping bearings in & out , the Temp Exceededs boiling water ( Aldi had Temp lazers last week , didnt get one . )
and got them dropping out with no bother .

For a stuck engine , That or the oven . A Engine Castored for decades can take some time to de solidify .

Best to pull backplate to ensure your turning INTO con rod , puling / stretching , the rod , can lead to tears .
Particularly on  little cheap diesels etc , or stretch / oval the rod ends / bearings .

Really Stuck , a good squirt of CRC or suchlike Ea Heat Cycle , will help the penetration , for grossly neglected cast offs . 

A few fingers turning a prop or spanners maybe ok , wrestling it full force with your palms , may lead to trouble .
Heat Mitts / Gloves are gunnabe required .