Originally Posted by CapriRacer
A couple of thoughts:
The purpose of torqueing lug nuts (or any bolt/nut/fastener) is to stretch the threaded portion to create clamping force. It's this force that holds the parts together.
When you lubricate the threads of a fastener, it stretches the fastener more than with dry threads. Torque limits are published to allow for enough stretch to prevent vibrations from loosening the parts, but not enough to cause the fastener to fail.
Lubricant Torque Reduction (%)
No lube 0
Graphite 50 - 55
White Grease 35 - 45
SAE 30 oil 35 - 45
SAE 40 oil 30 - 40
Source:
Engineering Toolbox: Lubricated Bolts and Reduced Torque
As you can see, it might be possible to permanently stretch a bolt if you don't reduce the amount of torque when using a lube.
And that doesn't include lubricating the contact surfaces.
The good news is that car manufacturers use SAE grade 8 lug nuts and bolts (or their equivalent.), but specify torque limits below the max. That means that most over torqueing won't result in damage.
Thank you. This topic of AZ on threads is confusing. My can of Permatex AZ says to apply it to clean threads and torque the bolt/nut to manufacturer spec. No mention is made of reducing torque to accommodate the AZ. I called them once and their guy said the same thing. Yet, everywhere else ppl say to reduce torque.