Lubricating Bolts On Vehicles With Anti-Seize Paste

A lubricated bolt will be tighter with a higher preload at a given torque. The lubricating effect of the liquid will not cause a properly torqued bolt to loosen but it can cause the over-tightening of the bolt and subsequent failure.

An anti-seize is a paste is made with grease, dry lubricants like molybdenum disulfide or graphite, and soft metal powder such as copper, aluminum or nickel alloy. As the temperature rises to a high value, the grease evaporates, leaving a coating of semi solid lubricant and soft metal. During the installation the soft metal particles deform, smear over and fill microscopic voids in the mating surface of the threads. This prevents a contact between iron atoms under extreme pressure. This effectively provides a reversible cold weld between the fastener components, causing the nut and bolt to become as one for the duration of the bond. As a side benefit, anti-seize will also protect against rust, oxidation, and galvanic reaction so aiding the subsequent dismantling.

You can use anti-seize on a bolt but must take into consideration the reduced torque required to achieve the same preload (tightness). With anti-seize applied to the bolt, the torque required to achieve the same clamping force is 2/3 to 3/4 that of the dry bolt. With a sensible amount of anti-seize the 3/4 rule can be applied so a 50 ft-lb torque specification becomes 37.5 ft-lbs. Avoid coating the head seating area of the bolt or the conical seat of a wheel lug nut with anti-seize paste.

For critical applications the effect of lubricated bolts must be considered. There is enough safety factor in automotive applications so they are fairly forgiving.

Chemical thread locking compounds such as Loctite also protect against corrosion.  These compounds are anaerobic, and only cure in the absence of the oxygen in the air. After application with surrounding air the Loctite will not cure until the fastener is assembled the air is excluded, solidifying in the space between close-fitting mating parts. Keep this period brief to be on the safe side. Thread locking compounds do not have as much of a lubricating effect as anti-seize so torque settings do not change as significantly. For successful locking, parts must be scrupulously clean. Most brake cleaning solvents do not leave a residue so are ideal.

One popular locking method for bolts is nylon insert locking (Nylok) nuts. These  provide ease of assembly, and good functionality. The additional torque required for Nylok nuts is insignificant in terms of torque for proper preload and would come within manufacturer tolerances. Contrary to some expert’s advice, the nuts are acceptable for limited re-assembly, unless the nylon locking ring is noticeably worn.

 

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