a. The M998 body
of aluminum alloys that have been heat-treated
to obtain high strength.
Welding cannot be used to make body repairs. Heat generated in welding will reverse the heat treatment
process and cause a great reduction in strength of material.
b. The M998 hood and engine access cover are made
of fiberglass (sheet molding compound). Cracks,
splits, or holes may be repaired with a glass reinforced plastic laminate repair kit, MILIR-19907C. (Refer to
c. Solid 3/16-inch diameter aluminum rivets are the primary method of joining body components. The
rivet is inserted into a hole through two pieces of metal, and a second head is formed by manual or pneumatic
impacting or by squeezing the rivet. A bucking bar is used to backup the rivet to form the rivet head. When
making repairs, use blind rivets of the same size, or oversize, diameter with the appropriate grip length. Rivets
3/16-inch in diameter are identified in appendix F, table F-1. For other rivets that may be needed, the NSN for
the rivet can be determined by cross-referencing the rivet part number to an NSN. To determine the proper
rivet part number, the following part number breakdown is provided:
MS20600 - basic MS number
AD - indicates aluminum
6 - indicates rivet sleeve diameter in 1/32-inch increments (6 x 1/32 = 6/32 = 3/16-inch)
W - indicates serrated stem rivet
2 - indicates maximum grip length in 1/16-inch increments (2 x 1/16 = 2/16 = /8-inch)
d. Blind structural aluminum rivets of 3/1l6-inch diameter are used in applications where there is access
from only one side of the part. Blind rivets are installed using a tool that pulls on the rivet stem causing a
bulbed head to form on the back side of the part. Fastening is complete when stem breaks off. High strength
is obtained in blind structural rivets by mechanically locking the remaining stem inside rivet body.
e. Steel pull-type lockbolt fasteners of 3/16-inch and 1/4-inch diameter are used where tension or high
shear loads exist. Lockbolts are two-piece unthreaded fasteners. One part is a high-strength, steel-headed,
bolt-like part with serrations on its shank. The mating part is a collar which is swaged over the serrations
causing the fastener to be locked in place.
f. To facilitate repairs to the body, it is acceptable to replace lockbolts and rivets with 3/16-inch AN3
series and 1/4-inch AN4 series bolts. Do not replace lockbolts with rivets. Standard threaded fasteners should
not be used as these will quickly wear the aluminum structure. Bolt lengths should be chosen so that the
cylindrical portion of the bolt is bearing on all members being joined. AN3 and AN4 series bolts are identified in
g. Fatigue strength of riveted joints and seams is increased by applying one part epoxy adhesive. This
adhesive requires special material storage and metal preparation along with a low temperature heat cycle for
curing. Because of its impracticality in field repairs, epoxy adhesive will not be used. Where possible, extra
rivets and thicker metal gages should be used instead of adhesives. When making repairs, note epoxy
applications. Parts may be difficult to separate, even after rivets are removed.
a. General. The damaged area should be thoroughly cleaned and inspected to determine cause and
extent of damage. Body parts should be inspected for holes, cracks, dents, distortion, or breaks. Fasteners
should be inspected for breaks, stretching, looseness, cocked heads, or hole elongation. Seams, flanges, and
joints should be inspected for straightness or local deformation as an indication that fasteners may have been
stretched or holes elongated. It is possible for this to happen and fasteners still appear to be tight in their holes.
In addition, make thorough inspection of adjacent areas to determine if high loads have been transmitted from
the damaged area to other areas. This can result in secondary damage in the form of distorted panels or
seams, loosened or sheared fasteners, elongated fastener holes, and cracks.
b. Classification. After extent of damage has been determined, affected parts should be classified into
one If the following categories:
Damage repairable by patching
Damage repairable by insertion
Damage necessitating replacement of parts