Well, about half right.
"Galling, according to ASTM standard G40 (2006), is: “a form of surface damage arising between sliding solids, distinguished by microscopic, usually localized, roughening and creation of protrusions (i.e., lumps) above the original surface”. In other words galling is material transfer from one metallic surface to another caused by movement and plastic deformation. Galling usually refers to adhesive wear and transfer of material between metallic surfaces during sheet metal forming and other industrial applications."
http://en.wikipedia.org/wiki/GallingGalling is the process of micro-welding then micro-tears of the welded bits of metal as they slide together under pressure. Yes, it will ruin threads. Yes, aluminum is prone to galling. No, it has nothing to do with dissimilar metals.
Dissimilar metals plus a conductive media result in "Galvanic corrosion, a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water
[or engine coolant with depleted corrosion inhibitors], forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material
[such as an aluminum cylinder head]."
[Comments mine]http://en.wikipedia.org/wiki/Galvanic_cell#Galvanic_corrosionI've seen a lot of galling of uncoated steel threads against uncoated steel threads. Coating of bolts, even the black oxide coating often seen, helps prevent galling. Identical alloys of stainless threaded together dry may gall terribly.
Keep in mind that lubricating the threads when it isn't called for in the spec changes the torque spec to an unknown value. The torque value is based on the friction of the threads, the stretch of the bolt, the strength of the bolt, and the compression of the base metal and gasket. If one of these is changed, the torque spec must be changed. If the friction is reduced and the same torque (twist) applied, the bolt may stretch to failure or the base metal or gasket may be compressed too much.
I always use a dab of anti seize on sparkplug threads in an iron or aluminum head. Never Seez is a brand name of one popular anti seize product, and I've used cases of pints of it, along with others like Never Seez Pure Nickel for high temperatures and Marine Never Seez for added corrosion resistance, Fel-Pro C5-A, and MolyKote G-N.
"Galling is prevented by the presence of grease or surface coatings, even if the surface coatings increase friction. It usually does not occur when joining dissimilar materials (e.g., threading 18-8 stainless steel into 17-4 stainless steel) even though both of those materials are susceptible to galling."
Wikipedia link above.Gaul is the ancient Roman name for the parts of Western Europe we presently call France and Belgium.
That clerk at the NAPA auto parts store is a total fool. Threads are specified by the diameter and the pitch...the pitch is the number of threads per inch or per millimeter. Fractional (English) threads will be something like 1/4-20 which is 1/4" diameter and 20 threads per inch, also called National Coarse or USS thread. 1/4-28 is National Fine or SAE thread. Metric threads will be something like M6x1...6mm diameter and 1 thread per millimeter. There are choices of fine or coarse metric thread pitch--M6x.75, for example. Another dimension will show the length of the shank, not including the head. The strength of the bolt will be in the fractional system labeled as Grade 1, Grade 3, Grade 5, Grade 8, and more detailed grades. In the metric system the higher strength grades will be labeled as 8.8 or 10.9.
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