AMMCO 8500 Brake Drum Micrometer User 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

11.000 in.
Diameter

11.375 in.
Diameter

METRIC

INCH

(Examples For Reading The Inch Micrometer)

11.390 in.
Diameter

27.00 cm
Diameter

11.000 in. + 0.375 in. + 0.015 in. = 11.390 in.

11.000 in. + 0.375 in. = 11.375 in.

11.000 in.

27.00 cm