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Sec. 4
Page 62
GM COACH MAINTENANCE MANUAL
AIR BRAKES
available in standard and three oversizes, each
size for use with certain oversize linings and
drums as explained below:
NOTE:
To avoid misunderstanding the
term "oversize" as applied to linings and
drums, the following example applies:
1/8" Oversize Brake Drum
Inside diameter (I.D.) of the brake drum has
been increased 1/8"; that is, 1/16" of metal has
been removed around the circumference of the
drum.
1/8" Oversize Lining
Linings are 1/16" thicker than standard, thus
the total increased thickness of linings on both
shoes compensates for the 1/8" increased diam-
eter of drum.
Standard Rollers (1.250" O.D.)
Standard
rollers
are
used when installing
standard linings with standard drums, 1/16" over-
size drums, and 1/8" oversize lining with 1/8"
oversize drums.
If
the drums have been previous-
ly refaced and oversize rollers installed, when
lining becomes worn, the oversize rollers must
be removed and standard rollers installed.
1/8" (1.375") Oversize Rollers
1/8" oversize rollers are used with the instal-
lation of standard lining with worn or trued drums
which are 1/32" oversize. l/8
11
oversize rollers
should be installed, then the lining circle ground
to the drum diameter.
1/
4" (1. 500) Oversize Rollers
When 1/16" oversize linings have been installed
with 1/16" oversize drums and standard rollers,
the standard rollers should be removed and 1/4"
oversize rollers installed before the lining be-
comes worn to a point where the brake cam is
no longer effective with the standard rollers.
1/2" (1. 750") Oversize Rollers
When 1/8" oversize linings and 1/8" oversize
drums have been used, the standard rollers should
be removed when the linings become worn approx-
imately 50% and the 1/2" oversize rollers in-
stalled.
IMPORTANT:
Whenever installing new
lining, regardless of the size, make sure
standard rollers are installed in shoes,
except as explained under
11
1/8
11
Oversize
Rollers" above.
Brake drums should never be machined to more
than 1/8" beyond original diameter.
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