Technical data

13773-001
20-70
Page 730 Nov 2000
D. Fastener Flushness
(1) Flush Head Screws
(a) The gap under the head of screws and bolts, and under nuts, shall be.002” maximum for
no more than 40% of the circumference. The heads of flush rivets, screws and Hi-Loks on
the exterior of the airplane shall be flush with the local skin contour to within the tolerances
specified. For fasteners installed in the exterior skin, head protrusion for flush head screws
shall meet the following requirements. Heads shall not be shaved. Check gaps at the
outer periphery of the intended bearing surfaces as indicated by the darkened areas.
Gaps of any thickness shall not be continuous for more than 40% of the peripheral dis-
tance of the head or nut.
(2) Driven Rivets
(a) The aforementioned rivets should be flush with the exterior finish. Driven rivets must never
protrude the surface more than 0.005”.
(3) Blind Rivets, Screws and Hi-Loks
(a) The aforementioned fasteners may be countersunk in the exterior finish no more than
0.005”. These fasteners must never protrude above the finish by more than 0.005”.
(4) Camlocks
(a) The aforementioned fasteners may be countersunk in the exterior finish no more than
0.020”. These fasteners must never protrude above the finish by more than 0.010”.
E. Self Locking Nuts
When torquing a self-locking nut, the nut should be run down on the threads of the bolt until the nut
almost contacts the mating surface. The amount of torque required to run the nut down (friction drag
torque) should be measured and added to the amount of torque specified for the fastener. To assure
an accurate torque measurement; a smooth and even motion must be applied.
Note: When checking friction drag torque, use a torque wrench which allows the friction drag
torque to fall in the middle of the overall range of the torque wrench.
Impact-type wrenches should never be used on any fastener whose torque is crucial. If a nut is slightly
over torqued, it must be loosened and then re-torqued to the correct value. Never back off a nut or a
bolt and leave it un-torqued. When installing a castle nut, start alignment with the cotter pin hole at the
minimum recommended torque, plus friction drag. If the hole and nut castellation do not align, change
washers and try again. Exceeding the maximum recommend torque is not permissible. Never use a
tap in a self locking nut or nutplate. Discard these parts, if this has occurred. Never install a self locking
nut or nutplate backwards on a bolt. Discard these parts if this has occurred.
CAUTION: Whenever a nut is over torqued the nut and bolt must be inspected for damage. If dam-
age is suspected, replace the nut and bolt. Over-torquing of fasteners can result in failed
fasteners and/or components. Under torquing a fastener can result in premature wear of
the fastener and/or the fastening material which can result in failure of the component
and/or fastener. Due to the diminished friction torque, self locking nuts should be
replaced with new ones after removal. If you choose to reuse a self-locking nut, ensure
nut has the minimum prevailing drag torque. Nuts or bolts with self-locking devices must
be discarded if the fastener can be fully hand tightened.