1967 FIAT 500 Shifter

CHAPTER 6
GEARBOX AND DIFFERENTIAL UNIT
6:1
6:2
6:3
6:4
6:5
6:6Description
Removal of gearbox/differential unit
Dismantling — gearbox
Reassembly — gearbox
Dismantling — differential and final drive
Reassembly— differential and final drive6:7
6:8
6:9
6:10
6:11
6:12Final drive gear set adjustment
Replacement of gearbox/differential unit
Swing axle shafts and slip joints
Gear shift control mechanism
Modifications
Fault diagnosis
6 :1 Description
The gearbox and differential unit are housed in an
aluminium alloy casing made up of t w o parts which split
at the differential centre line. The output drive is trans-
mitted by means of t w o half axle drive shafts coupled
through wheel shafts and slip joints to the rear wheels (see
FIG 6 : 1)
The gearbox provides four forward-speeds and a reverse,
the fourth-speed being considered as an overdrive. All
gears except first and reverse gears, which are straight
toothed sliding gears, are in constant mesh and have heli-
cal teeth. No synchromesh is provided, gear engagement
being completed by sliding shifter sleeves of the dog
clutch type. The first-speed gear slides on the outside of
the third and fourth shifter sleeve, whilst the reverse gear
train moves on a fixed shaft. The clutch shaft is integral
with the third, top and combined and first and second gear
pinions, whilst the second gear pinion is splined at the
front end. An internal splined
coupling sleeve joins the
clutch shaft and the input shaft and transmits the power
from the engine to the gearbox. The mainshaft and
differential drive pinion are combined together and besides
carrying the constant mesh gears and the sliding first gear
pinion, it also has attached the sliding sleeves and hubs.
F50065 The speedometer drive pinion is located at the front end.
The gearbox/differential casing is attached to the engine
by six studs which locate in the crankcase. The front of the
unit rests upon the rubber mounted support which is
secured to the body floor.
The complete unit comprises three detachable parts
made up as follows:
1 The support for the connection of the gearbox to the
engine which also acts as part of the differential front
end and clutch/flywheel housing at rear.
2 A central body which is divided into two compartments.
The front compartment contains first, third, fourth and
reverse gears w i t h the primary shaft, layshaft and reverse
shaft as well as the necessary gear selection striker rods
and forks. The rear of this compartment also forms a half
cover for the differential unit.
3 The gearbox casing cover, which contains the second-
speed gears together with the relative fork, gear
selection rod and speedometer drive gears.
The central part of the casing is provided with a top
inspection cover. The primary shaft is coupled to the clutch
shaft through an internally splined sleeve and t w o pins
which are retained by rings. The primary shaft together
with the first, third and fourth driving gears form a cluster.

PRIMARY SHAFT-TO-CLUTCH
SHAFT MOUNTING PINCLUTCH SHAFT SEAL PRIMARY SHAFT WITH
1st, 3 r d A N D 4 t h SPEED
GEAR CLUSTER
2 n d SPEED
DRIVE GEARPRIMARY SHAFT
FRONT BALL BEARINGPRIMARY SHAFT
REAR BALL BEARING
PRIMARY
SHAFT-TO-CLUTCH
i SHAFT JOINING SLEEVEJOINING SLEEVELOCKRING
SPEED SELECTOR LEVER
CLUTCH SHAFT
LAYSHAFT WITH BEVEL PINION
SPEEDOMETER DRIVE GEAR
2 n d SPEED ENGAGEMENT
SLIDING SLEEVE
2 N D SPEED DRIVEN GEAR
LAYSHAFT FRONT BALL
BEARING
3 r d SPEED DRIVEN GEARHUB FOR
SLEEVE
3rd AND 4th/
SPEED ENGAGEMENT
SLIDING SLEEVE4 t h SPEED
DRIVEN GEARLAYSHAFT REAR ROLLER BEARING
LAYSHAFT-TO-PINION
ADJUSTMENT WASHER 1st SPEED AND)
REVERSE SLIDING GEAR
FIG 6:1Gearbox-differential unit longitudinal section
On the primary shaft extension outside the main central
body is mounted the second-speed driving gear. The
primary shaft is supported at both ends by ballbearings
whilst the clutch shaft rotates in a bronze spigot bush which
is press fitted into the end of the crankshaft. An oil seal is
fitted in the clutch shaft passage seat in the c o n n e c t i o n
support. The layshaft, together with the drive pinion of the
differential unit is supported at the front by a ballbearing
race. The layshaft carries the pinion adjustment shim, the
fourth-speed driven gear and its bushing, the hub and
relevant engagement sleeve for the t h i r d - and fourth-
speed w i t h first-speed and reverse-speed, the third-speed
gear and bushing and, outside the casing the second gear
and bushing, the hub with relevant second-speed engage-
ment sleeve and the speedometer drive gear.
The gears are operated by a gear selection lever mounted
on the tunnel situated between the two front seats. This
operates through a rod actuating the selector and shifting
mechanism which is housed in the gearbox casing cover.
Refer to FIG 6 :3 where it will be seen th a t the positioning
of the striker rods is ensured by a spring-loaded ball-
bearing. The risk of engaging two gears at the same time is
eliminated by three rollers which slide in suitable slots
machined in the rod (see FIG 6 : 4).
The differential unit and the final drive gear train are
housed in the t w o semi-covers which
have previously
been described, the final drive pinion being integral with
the layshaft. The differential casing may be split into t w o
halves. The bevel gear is fitted on one of the halves and is
66
secured by the same screws joining the two halves
together. The differential side gears are internally splined
into which are located the splined axle shaft ends which
are able to slide by means of a specially designed slip joint.
Two specially shaped rubber oil boots are inserted on the
swing axle shafts and fit over the side gear extensions to
ensure that there is no loss of lubricant or the ingress
of dirt.
The transmission and differential unit to suit the station
wagon differs from the sedan in that the transmission to
engine mounting plate and the bellhousing are modified to
suit the engine repositioned on its side.
6 : 2 Removal of gearbox/differential unit:
The complete unit may be removed whilst the engine
remains in situ and to remove this unit proceed as follows:
1 Disconnect the battery positive cable and jack up the
rear of t h e vehicle and place on firmly based stands.
2 Disconnect all control cables and wiring from the
starter motor and the clutch lever. Remove the starter
motor, the clutch lever reaction spring and the clutch
cable support on the gearbox.
3 Remove the three bolts securing each of the driving
shaft flanges to the flexible coupling at the wheel ends
and carefully take out the inner spring.
4 Disconnect the shifter rod from the gearshift lever at the
gearbox extension and also the
speedometer drive
cable. Remove also the flywheel undercover. Place a
suitable support under the engine.

1st A N D 2 n d SPEED
STRIKER R O D LOCK
ROLLER1st A N D 2 n d SPEED
STRIKER R O D 3 r d A N D 4 t h SPEED
STRIKER R O D 3 r d A N D 4 t h SPEED
STRIKER R O D LOCK -
ROLLER REVERSE STRIKER
ROD LOCK ROLLER
REVERSESTRIKER R O D
FIG 6 : 4 Detail of gearbox section through striker rod
lock rollers
2 n d SPEED DRIVENGEAR2 n d SPEED ENGAGEMENT SLIDING SLEEVE 2 n d SPEED FORK 1st A N D 2 n d
SPEED STRIKER R O D REVERSE GEAR3rd AND 4th
SPEED STRIKER R O DREVERSE STRIKER R O D2 n d SPEED DRIVE GEAR SPEED SELECTOR LEVER
FIG 6 : 5 Cross section of gearbox through striker rods
with the indication of the gear selector lever angular
displacement
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AXLE SHAFT-TO-JOINT SLEEVE
OIL BOOT
RING GEAR
FIG 6:6 Differential unit complete with axle shafts
BEARING CONEADJUSTER LOCK RING BEARING HOUSING DIFFERENTIAL CASE
OIL BOOT RETAINER
AXLE SHAFT
1 Remove the speedometer drive support together with
its gears. Remove the front cover together with its
gasket and the gear selection rod (see FIG 6:2).
2 Remove the lock ring and carefully slide the splined
sleeve from the axle shafts. Remove the t w o roller
bearing housings together with the relevent rubber
boot fastening cover, the rubber boots and the adjuster
ring lock rings.
3 Detach the transmission mounting on engine support
and carefully remove the differential case assembly
complete w i t h the ring gear and axle shafts.
4 Open the fasteners and loosen all the shifter fork lock
bolts.
5 Engage t w o gears at the same time so locking the
input shaft and the mainshaft together. Remove the
cotter pins and the nuts on the front end of the input
shaft and the mainshaft.
6 Remove the retaining cover located on the lefthand
side of the gearbox and carefully take out the shifter
rod positioning balls and springs from their bores.
Remove the upper shifter fork shaft and shifter fork,
the middle shifter fork shaft together with its interlock
pin and also the third and fourth gear shifter fork.
Remove the plain washer and speedometer drive
driving gear from the layshaft.
7 Remove the second gear engagement sliding sleeve
together with the shifter fork, shifter fork shaft, sliding
sleeve hub, second-speed driven gear with relevent
bush and the first gear engagement
fork. Take very
great care not to drop the safety rollers whilst extract-
ing the striker rods.
8 Remove the second-speed driving gear from the input
shaft. Release the reverse shaft retaining screw and
remove the shaft together with the reverse gear cluster.
Remove the layshaft front bearing retaining plate.
9 Push the input shaft forwards until both ballbearing
races are pressed from their seating and the coupling
sleeve located between the input shaft and the clutch
shaft can be reached from above. Carefully remove the
retaining ring and a connection pin at the input shaft
side of the coupling sleeve and carefully withdraw the
clutch shaft and coupling sleeve.
10 Remove the input shaft front bearing race. Tilt the
primary shaft in the casing and remove it from the
gearbox housing. Remove the rear bearing. Carefully

3 Check the layshaft and the primary shaft between
centres and using a dial gauge ensure that there is no
distortion. The out of true reading for bearing seats
should be less than .0008 inch. Inspect the splines and
ensure that the side and outerfaces are not indented.
Replace if there are signs of wear. Also inspect the
reverse shaft to ensure that it is perfectly smooth and
not showing signs of pitting or partial seizure.
4 Carefully inspect all gear teeth for signs of excessive
wear or damage. The contact faces must be smooth and
show no signs of indentation marks. Also check that
the backlash between the gears, when correctly
mounted on the surface plate is not greater than .0039
inch. The maximum wear limit is .0079 inch. Check that
the quick engagement front teeth of the second-, third-
and fourth-speed gears are not damaged or excessively
worn. Renew if there are signs of gearchange lever
misuse.
5 Inspect the sliding sleeves and the relevent hubs, ensure
that all the working surfaces are perfectly smooth. The
clearance between the mating parts should not be
greater than .0059 inch. Also inspect the meshing teeth
and ensure that they are in good condition.
6 Check that the gear shifting selector forks are not dis-
torted and that the striker rods freely slide in their seats
in the gearbox casing.
7 Carefully inspect that all the oil seals are in perfect con-
dition and if there is any doubt they should be renewed.
8 Inspect the striker rod locking balls and the safety rollers
and ensure that they slide freely in their seatings. Also
check the ball load spring efficiency and if this shows
signs of weakness then it should be renewed. Any
faults with these items will cause difficulty in engaging
gears or autodisengagement during engine acceleration.
6 : 4 Reassembly—gearbox
To reassemble the components to the gearbox ensure
that all components are thoroughly clean and then pro-
ceed as follows:
1 Press t h e pinion rear roller bearing onto the layshaft.
Install the layshaft in the casing, and at the same time
fit the adjustment shim, the fourth-speed driven gear
together w i t h its bush, the sleeve and hub for third and
fourth gear engagement, the first-speed and reverse
driven gear and the third-speed driven gear with its
hub.
2 Install the front layshaft ballbearing securing the
bearing retainment plate w i t h the screws and finally
lock the screws in position.
3 Insert the primary shaft together with the first, third
and fourth driving gear train in the gearbox housing
and slide t h e rear bearing onto the shaft. Push the
input shaft forward to enable the clutch shaft coupling
to be inserted in t h e rear and to be pulled up the
primary shaft by means of the pin. Do not forget the
retaining ring.
4 Slide the primary shaft backwards so pushing the rear
bearing into its bore. Refit the front bearing onto the
primary shaft.
5 On the layshaft insert the second-speed driven shaft
and bush, the second gear sliding dog sleeve and the
hub. Also insert the second-speed striker rod and the
first-speed fork. Do not forget to fit the safety roller
into its seating.
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6 Slide the second-speed driving gear onto the primary
shaft. Place the engagement fork on the third- and
fourth-speed engagement sliding sleeve, and insert
the striker rod, lock the relevent fork and fit the safety
roller.
7 Insert the third safety roller and install the reverse shaft
and insert the reverse gear cluster onto it and secure
the shaft by tightening its block bolt. Install the reverse
striker rod w i t h its relevent fork.
8 Now refit the speedometer drive worm gear onto the
layshraft. Tighten the nuts on the primary shaft and the
layshaft, not forgetting to interpose the plain washer
between the speedometer driving worm and the nut,
to a torque wrench setting of 1 8.1 Ib ft to 25.3 Ib ft
(primary shaft) and 28.9 to 36.2 Ib ft (layshaft). It is
recommended that the input shaft and layshaft are
locked together by engaging t w o gears at the same
time. Finally, fit the splitpins.
9 Lock the shifter forks onto their shafts by tightening
their respective bolts and bend up the special locking
plates. Install the three positioning balls and relevant
springs into their bores and refit the cover plate.
10 Install the casing cover together with its relevant
gasket. This will enable correct positioning of the shift
shaft lever so that it engages in the outer ends of the
shifter shafts protruding from the gearbox casing.
11 Install the speedometer drive support on the gearbox
casing and refit the upper gearbox cover.
For subsequent differential operations and adjustments
see Sections 6:6.
6:5 Dismantling—differential and final drive
For the removal of the differential and final drive unit
from the gearbox refer to Section 6:3. To dismantle the
differential assembly
proceed as follows:
1 Remove the retaining ring 2 (see FIG 6 : 7) retaining
the splined sleeve 4 and slide the sleeve off the drive
shaft.
2 Remove t h e retaining covers 7 and the oil boots all
together with the bushes and seals, adjuster ring lock
rings 13, adjuster rings housing 14, and bearing
adjuster and roller bearing outer races.
3 Separate the differential housing halves by unscrewing
and removing the six bolts that keep both the halves and
the crownwheel together. It is advisable to mark the
respective components to ensure correct matching
upon reassembly.
4 Remove the differential pinion shaft retainer ring, also
the differential pinions 10 by removing the shaft and
withdrawing the two axle shafts 1.
5 Remove the differential side gears 6 and relevent thrust
rings 8.
6 Using a press remove the differential bearing inner
races from their seats on the carrier halves.
7 Thoroughly clean all parts and dry using a compressed
air jet.
Inspection:
The components of the differential unit must be
thoroughly inspected for correct operation and to deter-
mine the presence of any wear, damage or other irregulari-
ties. To check the items concerned proceed as follows:
1 Check thoroughly the idle pinion carrier shaft for signs

FIG 6:19 Modified items in transmission-differential assembly
Key to Fig 6:19 1 Reverse shifter shaft 2 Third and fourth shifter shaft 3 First and second shifter shaft
4 Countershaft pinion (and ring gear) 5 - 7 Transmission-differential case 6 Housing 8 Clutch shaft
FIG 6:20 End sectional view of transmission — differential assembly
The items having undergone design modifications, besides and final drive gears 2 Seal 3 Bushing 4 Oil
those shown in Fig 6 :19 are the following: 1 Differential case boot 5 Axle shaft 6 Joint casing
77F500