Bolting Guide

For full guidance on recommended practices purchase or download a copy of FAA AC43-13-1B/2A 

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 shorter bolt

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. Burrs 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.  

Standard torque table (in inch/lbs): Remember when installing the additional torque to drive the nylock [friction lock] must be determined and added to the recommend torque listed below

Fine thread series			Coarse thread series
Bolt Size	Standard Nuts	Shear Nuts	Bolt Size	Standard Nuts	Shear Nuts
10-32	20 - 25	12 - 15	8-32	12 - 15	7 - 9
1/4-28	50 - 70	30 - 40	10-24	20 - 25	12 - 15
5/16-24	100 - 140	60 - 85	1/4-20	40 - 50	25 - 30
3/8-24	160 - 190	95 - 110	5/16-18	80 - 90	48- 55
7/16-20	450 - 500	270 - 300	3/18-16	160 - 185	95 - 110
1/2-20	480 - 690	290 - 410	7/16-14	235 - 255	144 - 155
9/16-18	800 - 1000	480 - 600	1/2-13	400 - 480	240 - 290
5/8-18	1100 - 1300	660 - 740	9/16-12	500 - 700	300 - 420
-	-	-	5/8-11	700 - 900	420 - 540

Reference: Standard Aircraft Maintenance Handbook. 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
• 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
• 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 increases weight
• Nyloc self lock nuts should be used with un-drilled bolts, when using a drilled bolt make sure that no burrs exist. Do not use with drilled bolts smaller than AN4 [1/4'']
• Install bolts pointing aft and to the center of an aircraft where practical

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

When bolting is completed and checked mark each bolt using a splash of paint - red or yellow automotive touch up pens are ideal for this task as is nail polish.