TM 5-3895-359-14&P
CRANKSHAFT MAIN BEARINGS
The crankshaft main bearing shells (Fig. I) are precision
made and are replaceable without machining. They
consist of an upper bearing shell seated in each cylinder
block main bearing support and a lower bearing shell
seated in each main bearing cap. The bearing shells
are prevented from endwise or radial movement by a
tang at the parting line at one end of each bearing shell.
The tangs on the lower bearing shells are off-center and
the tangs on the upper bearing shells are centered to aid
correct installation.
Various types of bearings have been used. Currently,
multiple layer copper-lead coplated or aluminum
triplated bearings are in use. These bearings have an
inner surface, called the matrix, of copper-lead or
aluminum. A thin deposit of babbitt is then plated onto
the matrix. This babbitt overlay has excellent resistance
Fig. 2. Removing Main Bearing Cap
to friction, corrosion and scoring tendencies which,
The lower main bearing shells have no oil grooves,
combined with the material of the matrix, provides
therefore, the upper and lower bearing shells must not
improved load carrying characteristics. These bearings
be interchanged.
are identified by the satin silver sheen of the babbitt
when new and a dull gray after being in service. The
Thrust washers (Fig. I), on each side of the rear main
former copper-lead bearings had a copper color when
bearing, absorb the crankshaft thrust. The lower halves
new and turned very dark during engine operation.
of the two-piece washers are doweled to the bearing
cap; the upper halves are not doweled.
An oil hole in the groove of each upper bearing shell,
midway between the parting lines, registers with a
Main bearing trouble is ordinarily indicated by low or no
vertical oil passage in the cylinder block. Lubricating oil,
oil pressure. All of the main bearing load is carried on
under pressure, passes from the cylinder block oil
the lower bearings; therefore, wear will occur on the
gallery by way of the bearing shells to the drilled
lower bearing shells first. The condition of the lower
passages in the crankshaft, then to the connecting rods
bearing shells may be observed by removing the main
and connecting rod bearings.
bearing caps.
Fig. 3. Removing Upper Main Bearing Shell (Except
Fig. 1. Main Bearing Shells, Bearing Caps and
Rear Main)
Crankshaft Thrust Washers
10-2-59
