Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I just received a DK Products steam engine thru Live Auctioneers.

It is built like a tank, but unfortunately the screw in the top of the
upper sightglass fitting is frozen.  Right now I'm using PB Blaster
penetrating fluid to loosen it but I'll need to wait a while before
giving it a try.

Does anyone else have tried-n-true methods for loosening the screw?
Modest heating?  The fitting, silver colored, is non-magnetic so it
probably nickel-plated brass as is the boiler. 
   
I hope it isn't cross-threaded.

Thanks,
Wayne