Tuthill PD Plus 1215 - Adjusting Interlobe Clearance

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8.2.4 ADJUSTING INTERLOBE CLEARANCE
21. The timing gears are made up of two
pieces. The outer gear shells are
fastened to the inner hubs with six cap
screws and located with two dowel pins.
By adding shims between a gear shell
and the inner hub the gear is moved
axially relative to the inner hub which is
mounted on the rotor shaft. Being a helical
gear, it rotates as it is moved out and
the rotor turns changing the clearances
between rotor lobes. Adding .012” (.30
mm) shim thickness will change the
rotor lobe clearance by .003” (.076 mm).
The timing shim is formed from a
number of .003 (.075 mm) shims, which
have been laminated together. They are
easily peeled off as necessary.
Use feeler gauges to check the
clearance at AA (right-hand reading)
and BB (left-hand reading). See Figure
10. The clearances should be adjusted
so they are as equal between all lobes
as possible. Usually within .003” to
.004” (.076 mm to .10 mm). For best
results use feeler gauges no longer than .006” (.15 mm).
EXAMPLE: If AA reading is .030” (.76 mm) and BB reading is .022” (.56 mm), by removing .016” (.40
mm) shim, the readings will change one-quarter of this amount or .004” (.10 mm). AA reading will drop
to .026” (.66 mm) and BB reading will increase to .026” (.66 mm).
To determine the amount of shim to add or remove from the driven gear, simply subtract the two
readings .030” (.76 mm - .022” (55 mm) = .008” (.21 mm) and double the result .008 (.21 mm) × 2 =
.016 (.42 mm).
22. To determine whether to add or remove this amount the following rule will always apply: If the right
reading (AA) is greater than the left side reading (BB), remove this amount.
If the right side reading (AA) is greater than the left side reading (BB), remove this amount. If the right
side reading (AA) is less than the left side reading (BB), then add this amount.
NOTE
If the results require you to remove shim from the driven gear and there
are no shims left under this gear, go to the drive gear and add this amount
for the same result. When removing or replacing a gear for shimming the
timing mark should be matched and on center. Either gear may be pulled
in this position.
Figure 10 - Checking Rotor Interlobe Clearance
A
BB
B
A
B
A
A
A
A
B
A
B
B
A
B
LONG FEELER GAUGE
LONG FEELER GAUGE
RECORD A-A
READING HERE
RECORD A-A
READING HERE
RECORD B-B
READING HERE
3 LOBE
2 LOBE
RECORD B-B
READING HERE
B
B
B
B
B
B
B
B
B
B
B
B
A
A
A
A
A
A
A
A
A
A
A
A
DRIVE
DRIVEN
DRIVE
DRIVEN