'Tight' General Motors Compressors

General Motors compressors, including R4, HR6, HD6, HT6, and V5, are normally very difficult to turn by hand right out of the box.  Each piston end has a Teflon piston ring that is pressed onto the piston.  These rings 'flex' and created a positive seal in the cylinder.  Similarly would be that new jar of grape jelly.  Just because you can't get the lid off doesn't mean it's locked up and the jelly is bad; you just need the right tool.  In the case of GM compressors, that tool is called a spanner wrench.  Most auto parts stores and technicians have these.