Owner's manual

No. 908500
90850050
Brake Drum Micrometer
Operating Instructions and
Parts Identification
AMMCO Tools, Inc. • Hennessy Industries • 1601 J.P. Hennessy Drive, LaVergne, TN 37086-3565
[phone] • [phone] • [phone] FAX
8500 8500-50
Unit of Measure Inch Metric
Drum Diameter 6" to 16.125" 15 cm to 41 cm
Scale Graduations .001" .1 mm
(Increments)
Graduated Shaft 1" and .125" 1 cm and 2 mm
Optional Shaft 16" – 26" 39 cm – 66 cm
Measuring the Drum (Figure 2)
1. Loosen 2 lock screws and move both dial and
anvil along the shaft until the “whole” number of
the drum diameter is visible at each arrow.
NOTE: Metric shafts have even numbers on one
side, odd on the other. Inch shafts have identical
scales.
2. Place micrometer inside the drum across the
greatest diameter to be measured. Hold anvil
steady and move dial back and forth against the
braking surface to obtain the highest reading.
Calibration (Figure 3)
The micrometer is calibrated at the factory. Check
the calibration using a No. 909582 Checking Gauge
or a standard outside micrometer. To recalibrate,
loosen the jam nut (#22) and adjust set screw (#21)
until the correct reading is obtained. Tighten jam nut
while holding set screw stationary.
Drum Condition
Drums should always be carefully inspected for
cracks, worn or loose bearing races, scoring, heat
checks taper, bell-mouth, and out-of-roundness.
Drum out-of-round conditions are checked by taking
micrometer readings at 2 or more positions around
the drum. Scoring, out-of-roundness, bell-mouth,
and taper should be corrected by machining.
Correcting most of the above conditions will require
cutting material from the drum wall. If the drum
diameter after machining is larger than the
manufacturer’s specified rebore limit it must be
replaced.
Drums that are too thin are apt to be weak and
springy, and will not dissipate heat well, resulting in
a loss of braking power. Defective drums should be
scrapped.
Graduated Shaft
Lock Screw
Dial
Anvil
Lock Screw
Arrows for
Reading Scale
Dial
Lock Screw
Arrow
Scale
Figure 1 - Basic Parts Identification
11.000" Standard
27.94 cm Metric Equiv.
No. 909582
Gauge
28
29
Figure 3 -
Calibration
Figure 2 - Measuring the Drum