|Originally posted by David Hambleton:|
Should steel screws into aluminum have anti-sieze or thread locker or something else applied?
Excellent questions. I highly recommend using anti-seize
on all threaded fasteners unless something else is specified by the manufacturer, but especially where dissimilar metals are involved.
How do coatings affect the torque specs which I understand are applicable to dry threads?
Most published torque values for threaded fastener assemblies assume clean and dry threads, but some specify clean and lightly oiled threads, while some require a specific thread locker or sealant.
Anything you apply to threads ... anti-seize compound, oil, thread locker, or sealant ... affects the torque to use for a given fastener. (Actually, torque is a pretty poor way to determine optimum fastening tension, but it's relatively easy to measure and has a long body of historical data to support it.) Most high-quality manufacturers of such thread compounds conduct tests and publish data on how each product affects torque requirements. The thing to look for in the specs is the "torque coefficient" ... usually called the "K Factor" or sometimes the "Nut Factor." If you can't find a published K Factor, I would call the manufacturer before using that product.
For reference, the K Factor for clean and dry threads varies somewhat with fastener material (steel alloy, plating, etc.) and cleanliness, but it's a number usually in the range of 0.20 to 0.22. In comparison, when applied to clean threads the rated K Factor for the anti-seize I usually use (standard Never-Seez™
) is 0.13. This means that when using Never-Seez the torque should be reduced to about 65% (i.e. 0.13 / 0.20) of the value specified for clean and dry threads. For me, "2/3" is easier to remember than 65% and it's still close enough.
In actual practice I tend to reduce specified torque to about 75-80% when using Never-Seez on an all-steel threaded joint (steel bolt and steel nut, or a steel screw in an iron or steel casting). On the other hand, aluminum castings have less strength margin and are easier to pull the threads out of, so with bare aluminum (with no thread inserts) I will usually dial in no more than the 65% figure ... and even then I will approach it cautiously.
Be careful. If the manufacturer specifies the use of anti-seize, oil, thread locker, or sealant on a fastener before assembly (e.g. the head bolts on a Fiero V6), then the published tightening torque value already takes the K Factor of the thread treatment into account.
Incidentally, anti-seize compound is not a lubricant and it should never be used where oil or grease is specified. On the other hand, threaded fasteners lubricated with oil or grease before assembly, even when properly torqued, are more likely to loosen in service than those assembled dry or treated with anti-seize only.
Bottom line: Use anti-seize, look for the K Factor, and always use a torque wrench. It's much easier to overtorque small fasteners when tightening "by feel" ... with or without anti-seize ... than is it to undertorque them.
[This message has been edited by Marvin McInnis (edited 08-02-2013).]