OT: What is "Galling?"

Alloys, and most any application of like metals in high pressure applications.

The condition is so bad that Class #1 or very close fitting threads and clearances on moving parts is "Not Recommended" fot these alloys.

Even Class #2 thread fits are recomended only with the use of plenty lubrication and under very clean conditions. As an example of the tolerance

Class #2 is the fit used for Aircraft quality a Class #3, and #4 are the fit of Farm grade bolts which are exposed to the elements and dirt without other protection and this is for commonly used steels.

ViggoG