(1) Flush head. The flush-type Jo-Bolt, as shown in figure 2-10, normally takes the same size countersink or
dimple required for the same corresponding size standard screw head. The nut and bolt are made of alloy steel. The
sleeve is annealed corrosion-resistant steel. All components are cadmium-plated. There is one flush-type Jo-Bolt
designed to fit a countersunk or dimpled hole prepared for a precision rivet. Nut shank size and material specifications
are the same
(2) Hex head. Hex-type Jo-Bolts, as shown in figure 2-11, have an alloy steel bolt and an annealed corrosion-
resistant sleeve. The bolt and sleeve are cadmium-plated.
(3) Millable hex head. The millable hex-type Jo-Bolt, as shown in figure 2-12, normally takes the same size
countersink or dimple required for the corresponding size rivet. The bolt is alloy steel and the sleeve is corrosion-
resistant steel, both cadmium-plated. The nut is an aluminum alloy. After installation, the nut is milled flush
(4) Oversize. The oversize-type Jo-Bolt is used in special applications where the hole size has been exceeded
and standard Jo-Bolts cannot be used.
Huck Bolts (Lockbolt). The Huckbolt is a form of bolt, combining the features of both a high strength bolt and a
rivet, with advantages over each. The Huck bolt is generally used in wing splice fittings, landing gear fittings, fuel cell
fittings, longerons, beams, skin splice plates, and other major structural attachments. It is easier and more quickly
installed than the conventional rivet or bolt and eliminates the use of lockwashers, cotter pins, and special nuts. Like the
rivet, the Huck bolt requires a pneumatic hammer or pull gun for installation. When installed, the Huck bolt is rigidly and
permanently locked in place
Fitting Structural Bolts. There are two basic methods for fitting structural bolts. These methods are presented in
the following paragraphs.
(1) Bolt fit. All bolt holes should present a good mechanical fit. Generally, it is permissible to use the first
lettered drill size larger than nominal diameter, except where standard hexagon bolts are used in light-drive fit
applications and where close tolerance bolts or clevis bolts are used. All drills above 1/2 inch are required by the inch-
diameter size of the bolt used. Special fits will be discussed under the respective bolts. Bolt holes must not be oversized
or elongated as the bolt in such a hole will carry none of its shear load until the parts have yielded or deformed enough to
allow the beanng surface of the oversized hole to come into contact with the bolt. A bolted joint with an oversized hole is
shown in figure 2-13. It must be remembered that bolts are not swaged to fill up the hole as rivets are. Loose bolts may
cause failure of other bolts since they are forced to carry a greater load than originally intended. Oversize or elongated
holes may be drilled or reamed to take the next larger bolt provided the greater hole size does not weaken the part.
Consult the aircraft maintenance manual or maintenance officer before using this procedure. All bolt holes must be
normal to the surface involved to provide full beanng surface for bolt head and nut.
(2) Drive fit. A tight drive or light drive fit may be used. Tight drive fit is obtained as bolt moves into place when
struck sharply with a hammer. Light drive fit may be considered an interference fit of 0 0006 inch for 5/8 inch diameter.
(Other sizes are proportioned) Obtain a light fit as follows:
(a) Measure several bolts of correct nominal size with a micrometer, and separate them into three groups
large, medium and small.
(b) Drill initial hole approximately 1/32 inch undersize (1/8 inch undersize for holes 3/4 inch or larger), redrill
to 1/64 inch undersize.
(c) Select reamer to cut hole that will give proper interference when using bolts of small group. Ream one
or two holes and try fit of small bolts.
(d) When hole is too small, use a reamer of same nominal size, but with a slightly larger cut.
(e) When either of first holes are too large for light-drive fit conditions with small bolts, use medium or large
group to satisfy light-drive fit conditions. Tap lightly into position.
Take particular care to avoid elliptical, eccentric, or otherwise untrue holes. These holes may allow the bolt to
be driven according to requirements yet prohibit the necessary hole contact along the entire grip length.
Substitutions for Huck., Hi-Lok. and NAS Bolts. These bolts can be replaced by the bolts listed in table 2-6.
Diameter dash numbers for all bolts are the same. The hole sizes shown in table 2-6 are applicable to all