Motorcycle Owners Club

Honda CG125 but also some universal motorcycle information

Honda CG125 Valve Clearance

< Servicing

New valves will change their clearances while they run in so check every 2500 miles,
when run in you will find they do not need adjusting so often, so check every 5000 miles.

It's not recommended to ignore checking the valve clearance, but if you do,
listen (helmet off) to the valves at the top of the engine when it's running,
if they are excessively noisy (clatter, clatter) the clearance is too big and needs adjustment.
If one of the valves goes quiet its clearance is too small and must be adjusted immediately to prevent damage.
Performance drop, bad idling, problems starting or compression drop could mean the valve clearance is too big or small,
check immediately to prevent damage.
Valve clearances normally increase due to mechanical wear, but can sometimes decrease.
The Honda CG125 is a pushrod type so valve clearance could reduce (but may increase), as the engine warms up.
If the valve clearance is to big the only damage you are likely to have done is increased the wear rate of the valve.
Valves wear out (normally takes a very long time and high mileage) and can be replaced,
when the gap is too large they wear out far quicker.

1. Leave bike overnight to cool down, this is essential since the valve clearance changes with a hot engine.

If you do not have the two covers in the picture above (you will have something more like Picture),
skip step 2 below and go straight to step 3.

2. If you really cannot do this step 2 then try step 4 instead.
Remove the two round covers (use 10 mm and 6 mm Allen keys, hex, 6 sided Picture)
on the left hand side of the engine case (see picture above).
(If they will not undo, warm the bike up and try again, once they are off leave the bike overnight to cool down again).
The 10 mm one covers the nut for turning over the engine.
The 6 mm one covers the marks for valve clearance and timing (T is for valve and F for timing).

Turn engine over anticlockwise with a 14 mm socket (Picture)
until a small on its side T appears near the outside edge of the disc (don't mistake F for T).
If you do not have a 14 mm socket
you can use rear wheel + 5th gear instead (wheel needs to be turning to change between gears)
(Never turn the rear wheel in the direction that would make the bike go backwards when the bikes in gear,
that would turn the engine over in reverse which it is not designed to do)
You can remove the spark plug to make turning over the engine easier but it's not really needed.

Use a torch and magnifying glass since T is very small and stamped in to the metal,
do not use a house light bulb since it's too powerful and will hide it with glare.
The black outside edge in the drawing above could be curved away from you making it invisible under the wrong light
(grey area is not curved so will be visible).
Direct sunlight or strong daylight could also make it impossible to see
(take bike in to a building with low or no daylight or at least find as much shade as possible)

Line the mark (vertical line) below (maybe next to) the T up with the grove at the top of the hole
(only turn anticlockwise since engines don't like going in reverse)
(if your eyesight is to bad ask someone else).

3. Skip all of this step 3 (go to step 4) if you were able to do step 2.
If you really cannot do this step 3 then try step 4 instead.
Remove the gear change lever
Remove the left hand crankcase cover Picture (needs an 8 mm long and thin socket).
(if the nuts will not undo, warm the bike up and try again, once they are off leave bike overnight to cool down again)
(if the engine cover does not come off after removing all the nuts, tap the cover to break the seal).
If the covers gasket does not come off in one piece (due to old age) you will have to replace it
(you can smear a very small amount of light grease around it to help next time).
If the nuts and threads are suffering from corrosion you can use grease (even better is copper grease)
to stop them getting any worse.


Vertical line next to the T

Find the mark at around 12 to 1 o'clock,
Turn the engine over anti clockwise with a 17 mm socket (Picture),
you may be able to use a spanner or your hand (by hand requires the spark plug removed).
Line the vertical line next to the T with the mark (don't mistake F for T).

4. Skip all of this step 4 if you managed step 2 or 3.
This step is not as good as step 2 or 3 so only do this step as a last resort.
You need to get the piston to the highest point in the engine; this is called Top Dead Centre (TDC).

Never turn the rear wheel in the direction that would make the bike go backwards when the bikes in gear,
that would turn the engine over in reverse.
Put the bike in to 5th gear (the back wheel needs to be turning to change between gears).
Remove the spark plug and put something like a thin screw driver down the spark plug hole.
Turn the back wheel and watch the piston move up and down
(make sure that whatever you put in the hole does not get trapped or damage anything or fall in to the engine).
When the piston moves upwards it will push whatever you put in the hole upwards,
after the piston reaches Top Dead Centre (TDC) it will go downwards.
It's essential to get the piston to the highest point in the engine to get the correct valve clearance.

Ignore all of Step 5 if you have the front drum brake model.
5. Valve Clearance Step 5

6. Remove the valve cover, it's on the very top of the engine, two bolts on the right hand side and one on the left (10mm),
there's a rubber gasket inside a grove that's inside the lid
(do not lose or damage / trap it, if you drop it in dirt clean it then soak it in engine oil).
You will find there is a technique to getting the valve cover off;
it gets caught on the bikes metal frame and also the valves inside.
You may think it's impossible to get out, but there is a way,
keep trying all the different angles till you find the technique, brute force does not work.
Front Disc Brake model, go to the right hand side of bike, lift valve cover up and tilt it forwards slightly, tilt cover towards you.

7. Check that you can rock the valves, if you cannot;
you are on the wrong engine stroke, simply repeat step 2 or 3 (whichever one you did).
If you cannot work out if the valves are rocking, they will only rock if there's some valve clearance.
If you're on the wrong stroke there's no valve clearance (see picture below).

Valve Screw
Valve Nut
Valve Clearance

       2nd Valve                                                            Feeler Gauge

Alternative Picture

8. The valve clearance should be set to 0.08mm (0.003 in) on both valves,
this applies to all Honda CG125 models, there are only 2 valves.

A Feeler Gauge (Picture) is available from most car motor shops;
a feeler gauge is just a set of metal plates of various thicknesses.

The valve clearance is the gap between the round metal rods (it's easy to assume it's a single rod since the gap is so small).
If the valve clearance is correct, you will feel very slight resistance from the feeler gauge when moving it through the gap.
If you find the valves need to be adjusted,
it's worth trying to turn the engine over several times (repeat step 2 or 3 twice then check and repeat)
to see if it is then correct, there's always a slight variation every time you turn the engine over.
To adjust a valve, loosen the nut (10mm) on the valve you wish to adjust,
use pliers (or hand) to turn the top of the screw thread (or obtain the proper tool)
until the clearance is at the correct amount, then re tighten the nut.
Recheck the valve clearance after re tightening the nut, if it has gone out, loosen the nut again and readjust.
If the valve clearance changes every time you tighten the nut, you may have to hold the screw when tightening the nut
or set the clearance slightly wrong so when the nut is tightened the clearance becomes correct.

9. Reassemble everything in reverse order, but make sure the rubber air hose is not trapped on the inside when
attaching the air box to the valve cover (does not apply to the front drum brake model since it does not have one).

< Servicing