A nyloc nut has a nylon component that locks the nut in place on the bolt, preventing it from loosening under vibration. It can be reused many times and is suitable for internal engine applications, but should be replaced in critical fastening connections.
A nyloc nut is a fastener that incorporates a nylon component to effectively lock the nut in place on the bolt. The nylon component of the nyloc nut has a smaller inner diameter than the actual nut, thus acting to lock the nut in place by tightly squeezing the nylon around the bolt when tightened. Unlike a standard lock washer, the nyloc nut prevents the nut from loosening under vibration as the nylon is tightly engaged in the bolt threads and provides resistance to rotation when tightened.
A common problem with standard nut and bolt fasteners is that they can loosen or even come free over time and with vibration. Chemical thread locking compounds work well to reduce the occurrence of loose nuts, however, this can be time consuming and, in hard to reach areas, nearly impossible. The nylon nut allows you to tighten a bolt without fear of loosening under almost any condition, and there is typically no more difficulty installing the nylon locknut than with a conventional locknut. A disadvantage of using a nyloc nut is that the nut cannot be tightened using only your fingers. The nylon insert requires the use of a wrench on the nut once the threads contact the nylon inside the fastener.
There is generally a slight price increase for a nyloc nut over a standard nut, and this was once a problem since the lock nut was a disposable item. This means that once a lock nut has been used on a bolt, the nut must be discarded and a new nut must be used each time the fastener is removed. Since then, advances in the nylon formula have made this requirement obsolete. The modern nylon locknut can be reused many times without fear of failure, making the locknut a viable option in the most critical fastening areas. Testing to reuse a locknut is to twist the nut onto a bolt, and if the nylon provides resistance to turning using only your fingers, the nut is safe to reuse.
While the nylon locknut can be reused in many applications, it remains a recommendation to use a new locknut on any critical fastening connection. Minimal risk of failure ensures no fastener reuse in all critical applications, such as cylinder head bolts, connecting rod bolts and main bearing cap bolts. Modern nylon formulas allow the nyloc nut to be used in many internal engine applications without issue.
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