What's the difference between an AN bolt and a Grade 5

If you work on aircraft long enough someone is going to ask you that question or something similar such as "Can I use a Grade 8 bolt to attach the propeller?" The quick answer is the aircraft bolt has 40% greater fatigue strength when compared to a similar size and alloy commercial bolt.
Aircraft hardware requires strength AND endurance (fatigue strength). A bolt used on a propeller must endure millions of tensile stress cycles, whereas a bolt used on a building is stressed only once when it's tightened. In general, aircraft threaded fasteners have:
1. 40% greater fatigue strength
Aircraft structural threaded fasteners commonly use the UNJ threadform while commercial threaded fasteners use a UNF or UNC threadform. UNJF is called the "J" thread. It looks the same and is compatible with a UNF thread but the root radius is larger on the UNJ.
• The J thread increases fatigue strength by 40%.
• The J thread increases thread shear strength by 10% (thread tensile stress area of 110.765 compared to 103.20 mm2 for 1/2-20 thread).
2. Greater tensile strength for the same nominal size and thread
• J threaded fasteners have a slightly larger tensile stress area so are slightly stronger in tension for the same nominal size fastener and material.
3. Greater thread shear strength
Most aircraft hardware has a class 3 thread tolerance (UNJF-3B) while most commercial hardware has a class 2 thread tolerance (UNF-2B). A tighter tolerance often results in more thread engagement between the male and female thread. Go to the hardware store and screw a bolt and nut together and notice how you can rock the nut on the threads. Now try this with the same size aircraft bolt and nut - no nut wobble because there is less clearance between the male and female thread. It's theoretically possible to
get an aircraft bolt and nut that will not thread together if both happen to be at their maximum material limit.
4. Better corrosion performance
Most aircraft hardware is cadmium plated whereas most commercial hardware is zinc plated. Cadmium offers greater corrosion protection and is less apt to create hydrogen embrittlement in high strength fasteners.
5. Different torque values
Cadmium plating has excellent lubricity (low K factor) so that for a given amount of torque from your torque wrench, more clamping force is produced than would be if the plating were zinc. If you were to substitute a zinc plated bolt you would need to increase the torque wrench setting to achieve the same amount of clamping force. Existing torque charts based on aircraft cadmium hardware would not be accurate.
6. More stringent inspection standards
Here's one example: The common tapered pipe thread used on fittings is called "NPT." For aircraft fittings these threads are technically "ANPT" or Aeronautical NPT. The only difference is the requirement for two additional gage inspections. Nominally, its the
same thread but with greater assurance that the thread is correct.
The question "how long will it last?" (fatigue strength) rather than "how strong is it?" is the most important question in keeping aging aircraft flying. Has a repair that lasted for 12,317 flights proven itself? Not if you're on of the passengers on Japan Flight 123 (the deadliest single aircraft accident) during the 12,318'th takeoff. Aircraft structural threaded fasteners, along with proper repair techniques, are designed to maximize fatigue strength. Most commercial products are not.
For more on aircraft threaded fasteners see my web site at http://www.mechanicsupport.com/Articles.html

No votes yet


The real quick answer is that it does not matter what the difference is because only approved hardware can be installed on an aircraft. I guess some homebuilt and experimental have different rules but for commercial use there is no swapping of parts allowed. I remember that the Gazelle helicopter used the same door handle as a Renault 5 (I think) but cost 10X as much, same with the windshield wiper system but we could not go to the store and buy parts.
Many electrical parts are also made by automotive companies and are cheaper but if no certified tag, i.e. approved then they can't be used.
Good answer though.

By Stephen Peszel
(comment recieved my email and posted by AskBob)

Yes I totally agreed with you. It depends upon the importance that where we use these components. As per safety is concern we must use a certified products so that the accident factor minimized and we make our work more safely.sta kon terminals