TM55-1500-345-23
c. Attaching Parts. Parts, such as nuts, bush-
Application, Dissimilar Metals. Each sur-
b.
ings, spacers, washers, rivets, high-shear rivets,
face to be joined will receive a minimum of two
sleeves for
screws,
screws, s e l f - t a p p i n g
coats of the appropriate primer, and, in addition,
"shakeproof" fastener studs, self-locking nuts,
the following precautions will be taken.
"speed nuts", clamps, and the like, do not need to
(1) Where magnesium is one of the
be painted in detail except when dissimilar metals
metals of dissimilar metal faying surfaces, the
or wood contacts are involved in the materials be-
ing joined. Such parts will receive a coat of epoxy
metals will be separated by use of an approved
barrier tape. The tape will extend not less than
primer, MIL-P-23377 (wet or dry, at installation).
Rivets (5066) in magnesium alloy need not be in-
inch beyond the joint edges to prevent moisture
from bridging between the dissimilar metals.
stalled with epoxy primer. For magnesium dissimi-
Sealing compounds conforming to Military Speci-
lar combinations follow the general requirements
of paragraph 3-5. Close tolerance bolts passing
fication
MIL-S-81733
may
be
used
as
through dissimilar metals will be coated before in-
moisture barriers in lieu of tape and when us-
stallation with a zinc chromate compound con-
ed, the sealing compound will be applied bet-
forming to Federal Specification TT-P-1757, or a
ween the surfaces and squeezed out of all
boundaries and the excess removed in a man-
wet epoxy primer in accordance with Military
ner that will fillet all edges. The fillet width
Specification, MI L-P-23377. Aluminum alloy
will be not less than 1/4 inch. Joint areas
washers (5356) of suitable design will be used
which would retain water will be filled with a
under machine screws, countersunk fastners,
to Military
sealing compound
conforming
boltheads, and nuts that would otherwise contact
Specification
MIL-S-81733
or
MIL-S-7124.
magnesium.
When the use of a barrier tape or sealing com-
pound is impractical because of mechanical or
other factors, a primed 5356 aluminum alloy
required in painting operations on large
shim extending 1/4 inch will be used in lieu of
assemblies or structures either for protective
the tape or sealing compound.
reasons, as in the precautionary note below, or for
(2) Butt joints consisting of a riveted
purposes of delineation.
magnesium sheet and a riveted dissimilar metal
sheet will be protected by grooving the seam to a
Mask areas such as windows, canopies
a.
width of not less than 1/8 inch and filling with
and large openings with combinations of barrier
sealing compound conforming with Military Spec-
materials.
ification
MIL-S-81733.
The
depth
of
the
groove will be sufficient to retain the sealing
(1) Federal Specification MI L-B-121,
compound c o n f o r m i n g t o M i l i t a r y S p e c i f i c a -
usually a treated paper which is oil and moisture
tion MIL-S-81733, which will be subsequently
resistant.
applied and smoothed flush with the surfaces
of adjacent dissimilar metals.
(2) Federal Specification MI L-B-131,
usually a laminated foil and cloth or foil and paper
(3) Butt joints consisting of a magne-
with good water vapor resistance.
sium sheet and a dissimilar metal sheet, either of
which is secured by mechanical means other than
NOTE
riveting, will be protected by use of an approved
adhesive barrier tape or a primed 5052 aluminum
Both types above are heat sealable.
alloy foil which covers the cut ends of either or
both dissimilar metal sheets and extends not less
(3) Crepe-backed paper masking tape,
than inch beyond the joint edge.
Military Specification MIL-T-21595, Type I.
Cover small or regularly shaped parts
b.
(4) Organic adhesive barrier tapes or
with tape alone.
metal foils will not be used on exterior surfaces.
