Specifications

Chapter 1 Engine, clutch and gearbox
17
Gearbox
Reduction ratios
Primary drive
1st
2nd
3rd
4th
5th
Final drive
Selector fork claw end thickness ...
Wear limit
Selector fork bore diameter
Wear limit
Selector fork shaft diameter
Wear limit
Kickstart shaft diameter - at pinion
Up to engine number 1020010
Engine number 1020011 on ...
Wear limit
Kickstart pinion bore diameter
Up to engine number 1020010
Wear limit
Engine number 1020011 on ...
Wear limit
1 General description
The engine unit employed in the Honda CG125 models is of
the single cylinder air cooled four stroke type. Unlike most other
machines in the Honda range, the CG125 utilises a pushrod
operated valve arrangement.
The unit is of all-alloy construction, employing vertically
split crankcases which house both the crankshaft assembly and
the gear clusters. The cylinder head and cylinder barrel are also
of light alloy, the latter incorporating a steel liner in which the
cylinder bore is machined.
Lubrication is provided by a small trochoidal oil pump
feeding the major engine components. The lubricating oil is con-
tained in the lower portion of the crankcase which forms a com-
bined sump and an oil bath for the gearbox components.
2 Operations with the engine unit in the frame
It is not necessary to remove the engine unit from the frame
unless the crankshaft assembly and/or the gearbox internals
require attention. Most operations can be accomplished with
the engine in place, such as:
1 Removal and replacement of the cylinder head.
2 Removal and replacement of the cylinder barrel and piston.
3 Removal and replacement of the camshaft.
4 Removal and replacement of the generator.
5 Removal and replacement of the contact breaker assembly.
6 Removal and replacement of the clutch assembly.
7 Removal and replacement of the centrifugal oil filter.
8 Removal and replacement of the oil pump.
When several operations need to be undertaken
simultaneously, it will probably be advantageous to remove the
complete engine unit from the frame, an operation that should
take approximately one hour, working at a leisurely pace. This
will give the advantage of better access and more working
space.
3 Operations with engine removed
1 Removal and replacement of the crankshaft assembly.
2 Removal and replacement of the gear cluster, selectors and
gearbox main bearings.
4.055:1 (18/73T)
2.769:1 (13/36T)
1.882:1 (17/32T)
1.450:1 (20/29T)
1.174:1 (23/27T)
1.000:1 (25/25T)
2.267:1 (15/34T)
4.93 - 5.00 mm (0.1941 - 0.1969 in)
4.70 mm (0.1850 in)
12.00 - 12.02 mm (0.4724 - 0.4732 in)
12.05 mm (0.4744 in)
1 1.98 - 1 1.99 mm (0.4717 - 0.4721 in)
1 1.96 mm (0.4709 in)
N/Av
19.959 - 19.980 mm (0.7858 - 0.7866 in)
19.900 mm (0.7835 in)
24.900 - 24.920 mm (0.9803 - 0.9811 in)
24.940 mm (0.9819 in)
20.000 - 20.021 mm (0.7874 - 0.7882 in)
20.050 mm (0.7894 in)
4 Method of engine/gearbox removal
As mentioned previously, the engine and gearbox are of unit
construction, and it is necessary to remove the unit complete, in
order to gain access to the internal components. Separation and
reassembly are only possible with the engine unit removed from
the frame. It is recommended that the procedure detailed below
is adhered to, as in certain instances, components are much
easier to remove whilst the unit is supported by the frame.
5 Removing the engine/gearbox unit
1 Place the machine securely on its centre stand, ensuring
that there is no likelihood of it falling over during engine
removal. Engine removal can be made much easier if the
machine is raised about two feet by means of a stand. A stout
table can be modified for this purpose, or alternatively, a few
substantial planks and some concrete blocks will suffice. This
procedure is by no means essential, but will greatly ease the
discomfort of squatting or kneeling down to work.
2 Place a container of at least one litre beneath the engine
unit, then remove the drain plug and leave the oil to drain while
further dismantling is carried out. Turn the fuel tap to the off
position and prise off the fuel pipe from the stub at the base of
the tap. A small screwdriver can be used to ease the pipe off
without straining it.
3 Release the seat mounting bolts, which are located
immediately above the rear indicator lamps, and lift the seat
away from the frame. Disengage the rear of the petrol tank from
the rubber block which retains it. The tank can then be pulled
upwards and back to release the front mounting blocks.
4 Slacken off the clutch cable adjuster nuts, and release the
cable from the actuating arm on the top of the engine casing.
Disengage the cable and lodge it clear of the engine. The spark
plug lead should also be detached and placed out of the way on
the top frame tube.
5 Unscrew the carburettor top and withdraw the throttle
valve assembly. There is no need to disconnect the cable, but
the assembly should be positioned where it will not get
damaged during engine removal. Disconnect the rubber intake
hose, then remove the two nuts which hold the carburettor
flange to the cylinder head. The carburettor body should be
lifted away, together with the heat shield and spacer or inlet
stub. Pull off the crankcase breather pipe.
6 Remove the two nuts which retain the exhaust pipe to the
cylinder head, sliding the flange clear and removing the two