AN Bolts and Designations

One of the safety features built into the Challenger series of aircraft is the use of AN type bolts throughout. Avoid the temptation to replace any of these bolts with a common hardware store bolt, even stainless steel.

AN bolts are designed and manufactured according to military specs, and everything about them is important. By sticking strictly with the correct AN bolts, you can rest assured that your airplane will not come apart in flight due to a bolt failure.

The tensile strength and resistance to shearing forces is critical. The shank of the bolt (called the “grip”) is unthreaded except for the final end section.

Since the shank is acting like a pin in many applications, we don't want any of the threaded part of the bolt to rest against the sides of the hole it passes through. Bolt threads that are sitting within the hole will create undesirable wear and will reduce the useful life of that part due to an increase in rubbing action caused by the threads.

When a bolt of the correct length is used, it will have all the threaded section just outside the hole in the metal. One or two AN washers must then be used to avoid having the nut bottom out on the unthreaded shank prior to seating on the metal.

Correct use of Washers

AN washers are exactly 1/16" thick, and the use of one or two will allow us to space the nut just right. One washer is always required, and two are considered the limit to use. If three washers are needed, it's best to go to the next size shorter bolt.

AN bolts are made in 1/8-inch increments in length so that in all cases, either one or two washers can position the nut correctly.

After the nut is secured on the bolt, not less than one nor more than two threads should be exposed at the end of the bolt. The thread exposure can be adjusted either by using an extra washer or by going to the next size bolt length, either 1/8" longer or shorter.

Nyloc Nuts

A “Nyloc” nut has a ring of plastic (perhaps nylon) crimped into the the top of the nut so that perhaps two threads of the bolt engage this plastic ring. The plastic material prevents the nut from loosening under vibration.

A Nyloc nut can be reused many times. There is no specified limit to the number of times a Nyloc nut can be put on and taken off as long as the nylon grip section at the outer end of the nut remains intact and effective.

If the nut will start to go on with your fingers, but will not go past the nylon locking section, the nut is still serviceable. But if you can turn the nut all the way onto the bolt with only your fingers, then the nylon section is worn out and the nut should be replaced. Throw away Nyloc nuts that can be finger tightened all the way down to the washer.

AN Bolt Designations

Let's look at two typical bolt designations, namely, AN3-14A and AN4-25A.

The first two letters, AN, stand for Air Force/Navy, which is the military spec code.

The next number after the AN refer to the diameter of the bolt in 1/16" increments. So a 3 would be 3/16", and a 4 would be 4/16", which is 1/4".

The numbers after the dash specify the length. The first number is whole inches and the second is the number of 1/8 inches added.

If there is only one number, such as on short bolts, that number designates 1/8" increments only.

Therefore, 14 stands for 1 whole inch plus 4 increments of 1/8", which gives a total length of 1-1/2" .

The 25 would be 2 inches plus 5 increments of 1/8", which gives 2-5/8" total.

You can see how bolt length can be adjusted by just going to one higher or lower number in that second designation. Each number represents 1/8".

The last letter, in this case A, designates an undrilled bolt. If there is no letter "A" after the last number, the bolt will be drilled in order to accommodate a castle nut with a cotter pin, hair pin, or other type safety pin that holds the nut in place.

Drilled bolts with castle nuts and safety pins are used where bolt tension is not required beyond just being snug, such as the bolts that hold the wing roots, or the strut ends in the Rony brackets. Those bolts are stressed entirely against shearing forces and there is no tension force on them.

Also, some bolts that may be designed to be taken on and off from time to time, and which are stressed only in the shear direction, will have castle nuts and safety pins used on them.

It is not recommended that Nyloc nuts be used on drilled bolts since the nylon section of the nut can be slightly ripped by any sharp edges where the hole is drilled through the end of the bolt, and that might allow the nut to back off over time.


    AN4 - 35A         Diameter = 4/16 = 1/4”         Length = 3-5/8 inches         Undrilled

    AN5 - 26         Diameter = 5/16”         Length = 2-6/8” = 2-3/4 inches         Drilled

    AN5 - 30A         Diameter = 5/16”         Length = 3 inches         Undrilled

So, wherever an AN bolt supplied by Challenger must be replaced, use only another AN bolt of the same dimensions or, if necessary, the next size larger AN bolt that will do the job.

Author:   Bud Connolly

For more information on fasteners see Experimental Aircraft's article:
by: Ron Alexander