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Builder’s Tips: Three Threads
From Bits and Pieces Newsletter, October 2014
By Ian Brown, Editor – Bits and Pieces, EAA 657159
Checking bolt tightness is important. There is a myth that three threads showing through a bolt/nut assembly is a good target. It’s the maximum, not the ideal. The standard we all follow, the FAA’s Advisory Circular (AC) 43-13B, states that there must be a minimum of one thread showing through a nut and a maximum of three threads.
If you take a look at a the typical bolt and measure up the space taken by a standard Nyloc nut, you’ll find that if you get more than three threads showing you’re quite close to bottoming out the nut on the grip part of the bolt. A sure sign is that the wrench stops progressively getting tighter and suddenly gets very tight.
Another indicator is that when you turn the wrench on the nut, the bolt will turn as well, indicating that it is not in tension. In this photo, you can see that the nut is bottomed out on the thread. Counting peaks of whole threads, you can just see four complete ones; that’s why three is the maximum. Even if you feel the bolt is in tension, it may not be enough to provide the strength it was designed for.
Incidentally, you should not get more than one thread down inside what you’re attaching; so bolt length and grip length are really important, and the use of washers not only protects what you’re attaching but also keeps the threads out of the gripped area.
It’s not specifically spelled out in AC 43-13B, but you can imagine the rationale for all of the above. Less than one thread showing puts you close to not having all the threads in the nut working for you. More than three threads gets you close to bottoming out on the end of the thread of the bolt. More than one thread inside the gripped part risks damaging it and also running out of threads for the nut.
We are allowed to use up to three washers to make up the difference of a slightly mismatched bolt grip length, but logically picking the right bolt length makes sense. Of course, washers don’t help the rule about the number of threads showing through a nut.