I was trying to take 5 minutes to pull the plugs out of my 239 and fill the cyliders with PB blaster to un-stuck the motor, but I couldn't find a socket that fits. My 3/4 spark plug socket was a not go, and it sure looks 3/4 to me.

Any suggestions?

 

I'd have guessed a 13/16s would work. I hope you don't have so much rust on the plugs that the hex flats changed sizes. If so, oh-oh. You are using a hex (NOT 12 POINT!!!) socket, right? If not, that's the first thing to do. Good luck.