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AN Bolt

Installing AN Bolts

A large part of the activities when building aircraft is drilling holes and handling bolts, which usually go into the drilled holes. Aircraft kits are nowadays pre-punched, this means that all (well, almost all) holes are already in the skins or ribs.

These holes were sometimes undersized meaning that cleco's can be installed but rivets will not. You will have to enlarge the holes during assembly and alignment. Some kit manufacturers are supplying their kits pre-drilled to the correct size these days.

For an aircraft structure to be able to withstand the designed loads the bolts need to be torqued to the correct value. We will provide you with some tips for using proper procedures for installing and torquing AN bolts.

Throughout the building process there will be numerous instances where bolts are used to fasten parts or materials together. In some cases it may be for the builder to determine the correct length of the bolt to be used.

But whatever you do: ONLY use aviation grade bolts in your project and torque them to the correct values!

Bolts & Torque

Aircraft Hardware

The "Rule of Thumb" for determining bolt length is that the bolt must be long enough to pass through the parts or material being fastened together so that:

  • The threaded part of the bolt is never in shear (no threads are allowed inside hole)
  • No more than three and no less than one thread must be showing when the nut is attached and tightened to the correct torque value
  • At least one flat washer must be used under the nut and no more than three are allowed
  • If you need more than three washers use a smaller bolt

More precise determinations of grip length are found in a number of books including the Standard Aircraft Maintenance Handbook (its a must have for builders, published by Jeppesen Sanderson Inc.).

Bolting practices

The bolts used in aircraft are about .001 to .003 inch smaller than their nominal diameter. This permits them to be installed in a properly drilled hole. The bolt should lie solidly against the surface of the material. Any burr must be removed before it is inserted and the shank (grip) should extend entirely through the hole, no threads should be in contact with the bearing surfaces, see above.
Most applications with bolts require the use of a washer, use the specified type and torque to the specifications of the manufacturer.

Bolt Torquing

The following information is provided for reference when using AN grade hardware. Most of the time torque values are done to feel. But this table does provide a good outline. Occasionally bolts, other than a standard bolt will be called out for use in the builders manual. Please ensure that these special bolts are used where needed. The aircraft designer selected these as they provide the strength for the connection where a standard bolt can not provide this.

Standard torque table (in inch/lbs):

Fine thread seriesCoarse thread series
Bolt SizeStandard NutsShear NutsBolt SizeStandard NutsShear Nuts
10-3220 - 2512 - 158-3212 - 157 - 9
1/4-2850 - 7030 - 4010-2420 - 2512 - 15
5/16-24100 - 14060 - 851/4-2040 - 5025 - 30
3/8-24160 - 19095 - 1105/16-1880 - 9048- 55
7/16-20450 - 500270 - 3003/18-16160 - 18595 - 110
1/2-20480 - 690290 - 4107/16-14235 - 255144 - 155
9/16-18800 - 1000480 - 6001/2-13400 - 480240 - 290
5/8-181100 - 1300660 - 7409/16-12500 - 700300 - 420
---5/8-11700 - 900420 - 540

Reference: Standard Aircraft Maintenance Handbook.

Bolt Installation Recommendations

Certain standards are to be followed when installing bolts in aircraft, some of these recommendations are here:

  • Keep bolt and nuts free from grease and oil when installing and torquing
  • Always use a torque wrench to determine the torque values
  • Torque with smooth even pulls
  • Verify torque value a second time
  • To determine the bolt length, a minimum of one thread must be visible outside the nut and no more than one thread can be inside the bolt hole
  • Whenever possible tighten the nut with the correct torque value
  • Typically you would use a bolt and a nut with a washer under the nut as bearing
  • When the bolt is too long you may use no more than three washers, but this does increase weight
  • Fiber self lock nuts should be used with un-drilled bolts, when using a drilled bolt make sure that no burrs exist
  • These fiber nuts may not be used with high temperature applications (exhaust)
  • Install bolts pointing aft and to the center of an aircraft

Standard nuts (amongst others) are: MS20365, AN310, AN315. Shear nuts are: MS20364, AN320, AN316, AN23-31

Written by EAI.

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