Our series start with a primer on drilling holes and using 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 are undersized meaning that cleco's can be installed but rivets not. You will have to enlarge the holes during assembly.

Drilling and Bolts

Drilling holes
It is good practice to use a #40 (2.5mm - 3/32") drill when doing any assembly, if possible never drill to final hole size until the parts are completed for primer preparation and final assembly. This allows for any movement between parts or adjustment you may want to make that may cause misalignment of the holes during assembly.

Always use a sharp drill bit as:

• A sharp bit will not wander as easily as a dull one
• You can drill faster with a sharp bit
• You don't have to push as hard, risking bending flanges etc

And remember to:

• Always be careful to drill squarely to the work surface (a drill press works great)
• Always make sure when drilling through multiple parts or layers that they are clamped together tightly so as not to move around while drilling and that drill shavings do not get in between the parts
• Always debur holes on both sides of each part before final assembly
• Work in a clean area as not to scratch the parts (and the Alclad layer)
• Leave the protective plastic film on as long as possible
• When drilling large holes to use a step drill when possible
• Drill slowly as you do not want the drill to 'bite' the material and create square holes

Drill hole location
Holes must be drilled more or less on a exact location. You will notice every deviation from the true line especially on a wing. To help avoid these minor errors use these guide lines:

• Use a template like a hole spacer or hole finder, check your tool supplier for these items
• The use of pilot holes punched or drilled prior to final drilling is recommended

Drilling speeds depends on the material you are going to drill. Aluminum conducts the heat (from drilling) very good and can be drilled at high speeds (Between 1200 and 4000 rpm, use lower rpm's with larger drills. 5/32 and smaller: drill with 2500 - 4000; larger than 5/32" use 1200 - 2500 rpm).

Cleco's
After drilling a hole, a cleco is temporarily inserted to attach/hold the parts together. Picture on the left shows what they look like.

Cleco's are installed with a special tool: a cleco plier. Clicking picture to the right will show to handle them.

Stainless and normal steel will harden if you use a too high rpm and drill pressure (= more heat). Be sure to use low rpm and use a cutting coolant to cool the drill. Basic guidelines: Normal sheet metal work use 135gr drills, plexiglass use 90gr drills and cobalt (140gr) drills for high heat treat steels.

Four common drills (and spares) are usually needed for the majority of holes to be drilled: 3/32" -> #40, 1/8" -> #30, 5/32 #21 and for 3/16" -> #11 drills. I also recommend to get two overlapping step drills (to 1" hole size), they come in handy at times too.

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

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

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

Bolting practices
Aircraft bolts 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 the bolt is inserted and the shank (grip) should extend entirely through the hole, no threads should be in contact with the bearing surfaces.
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 bolts are used where called out. The aircraft designer selected these as they provide the strength for the connection where a standard bolt can not provide.

Standard torque table (in inch/lbs):

Reference: Standard Aircraft Maintenance Handbook.

Aircraft 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 torqueing
• 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 visibile 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
• Fiber self lock nuts should be used with undrilled bolts, when using a drilled bolt make sure that no burrs exist
• 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