By: ThaiDesign (offline) Thursday, 24 November 2016 @ 11:26 AM ICT
Yes they do, but all threads stretch when properly fastened, that is the fundamental of most fastening designs and exactly what a torque wrench measures. When you buy springs in the US, they generally rate them as pounds per inch, in Europe and most other countries it is Newton meters or kilograms per millimeter.
Torque wrenches give the same values when torquing fasteners. Screws and bolts are rolled or cut into a helix shape (spring) and work just like a spring. The more stretch, the more resistance to backing out. Unless, of course, you go past the tensile strength of the fastener and t breaks. Almost every fastening system I have used is based on tension. You can confirm this information on NGK’s website.
The problem is, anti-seize is a metal powder mixed with a petroleum grease base to assist with application. Plugs dissipate a large percent of the heat from a cylinder and reach temperatures that burn petroleum lubricants. When the grease burns away, the metal powder essentially acts as a dry lubricant and protects the plug from corrosion and binding. During installation, the reduction of friction will cause the threads to stretch more than normal. If the nickel coating has worn off the plug, NGK recommends replacement.
At one point, NGK recommended anti-seize on areas where the nickel coating wore off, along with a 20 percent reduction in torque. The company has since rescinded that recommendation, possibly for liability reasons. Peo9ple don’t always follow complicated instructions well, or maybe plugs have backed out. So, NGK defaulted to the recommendation of replacing the plug. Always follow the OEM recommendations. If they recommend anti-seize, use it. The OEM’s are pretty good about including the torque difference already. If not, they will notice a rise in head failures and warranty claims.