Lycoming IO-390-D Series Maintenance Manual page 270

(1) Torque Method (measure the torque required to slip the belt at the small pulley as
follows):
(a) Hold and secure the propeller to prevent rotation of the crankshaft.
(b) Apply a torque indicating wrench to the nut that attaches the pulley to the alternator
and turn it clockwise.
(c) Record the torque value on the torque indicating wrench to slip the belt(s) at the small
pulley.
(d) Adjust the belt tension as per the required torque values that correspond to a new or
used belt identified in Table 1.
Width of Belt
3/8 inch
Twin 3/8 inch
½ inch
11 millimeter
* A belt is considered used if it has been installed on the engine and the engine has been operated.
(2) Deflection Method
(a) Attach the hook of a small spring-scale to the alternator belt at the approximate mid-
point between the ring gear support and the alternator.
(b) Pull on the scale until a reading of 14 lb (6.4 kg), is shown for a new belt, 10 lb. (4.5
kg) for a used belt.
(c) Measure the distance the alternator belt has moved with the 10 or 14 lb. (4.5 to 6.4
kg) load applied.
(d) The distance (deflection) is to be 5/16 in. (7.94 mm). If less than 5/16 in. (7.94 mm),
the alternator belt tension is too tight.
(e) Adjust the belt tension as required for 5/16 in. (7.94 mm) distance (deflection) per the
"Alternator Belt Tension Adjustment" procedure in this chapter.
(3) Belt Tension Dial Gage Method (using Lycoming tool number ST-131 (Figure 2):
(a) Extend the hook on the Belt Tension Dial
Gage ST-131 (Figure 2) to its extreme position
by depressing the handle.
(b) Put the hook over the alternator belt with the
nose piece centered on the alternator belt.
NOTICE: In the next step, release the handle of
the Belt Tension Dial Gage (ST-131) quickly.
If the handle is released slowly, internal
friction will cause an inaccurate reading.
72-70
Page 244
Table 1
Required Torque (Belt Tension)
Torque at Pulley
Condition
of Belt
ft.-lb
New 11 to 13
New 22 to 26
New 13 to 15
New 22 to 24
IO-390-D Series Engine Maintenance Manual
IO-390-D Series Engine Maintenance Manual
Condition
of Belt
Nm
15 to 18 Used*
30 to 35 Used*
18 to 20 Used*
30 to 33 Used*
Belt Tension Dial Gage - ST-131
© 2020 Avco Corporation. All Rights Reserved
Torque at Pulley
ft.-lb Nm
7 to 9 10 to 12
14 to 18 19 to 24
9 to 11 12 to 15
15 to 17 20 to 23
Figure 2
March 2020