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Honda CG125 but also some universal motorcycle information

Honda CG125 How to Increase Engine Size

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Please also see Increasing Engine Size

The information below is from someone who emailed it to me.

Big Bore CG125 Engines

I first found out that it is possible to overbore and over stroke small Honda OHC engines from a website called
From this website I found that it was possible to raise the cubic capacity of TL/CB125 engines
to 150cc by increasing the bore to 61mm and to go further by altering the crank to get about 210cc.
Since the cg125 engine is similar to the OHC engines
(But better because there is no cam chain and cam bearings to go wrong)
I obtained cg125 top end to overbore from a motorcycle scrapyard.



Bore (mm)


Engine size CC






Compression ratio about 7.5/1





Crankcase mouth needs opening 1mm





Compression ratio about 9.5/1 (untried)





Low compression

All of the above pistons use a 15mm diameter pin

As you can see from the above chart, the most sensible choice is the Honda cb550 or cbx550 route.
As the cg125 cast iron liner is only 71mm in outer diameter,
boring out to 61mm or even 63mm leaves little in the way of metal behind and would probably fail.
The solution is to machine out the cg125 liner and press in a Honda 550 or750 liner coated with araldite,
and then bore out to the correct bore for the piston.
According to Dave at PISTONBROKE (Dave does this sort of thing often)
the mouth of the cg/cb125 crankcase needs opening by about 1mm to accommodate
the lower part of a cb750 liner, also by doing so risks breaking into an oil way on cg125 engines.


The Honda cg125 piston has a 15mm gudgeon pin and a slightly domed crown,
the CB550 also uses a 15mm gudgeon pin but the piston is flat toped and .5mm
smaller crown height meaning lower compression.
I have recently found out that Honda CBX550 pistons may have a domed crown
thus making it a better option by keeping the compression ratio at about 9.5/1.
Higher compression means more power, but good fuel is needed.
Lower compression means slightly less power but poorer quality fuel works fine.
I decided to bore out to 58.75mm (all I could obtain from David Silver spares was a CB550 .25mm oversized piston kit)

How it was done

Because I work in an engineering factory I had the barrel bored out on a Dean Smith & Grace centre lathe.
A friend at work clamped the barrel in a four jaw lathe chuck,
adjusted the jaws until the bore was true using a dial gauge and bored it out to58.73mm.
Then using fine grade emory cloth, the tooling marks where polished out taking the bore size out to the desired 58.75mm.
The cylinder head was cleaned and the intake and exhaust passageways polished using a dremel type rotary tool.
The valves where reground in using grinding paste.
The barrel, piston and head was fitted on to the engine using standard gaskets.

Teething Problems

I started first time but ran rough until I sorted out the carb.
The carburettor needle has 3 grooves with the circlip fitted to the middle one;
I found that if I moved the circlip to the top groove the engine worked well.


I have found that there is a noticeable increase in power as the bike can climb hills better
and where as before it would only do 55mph on the flat (I am quite heavy)
I now can get 63mph.
A lighter rider may perhaps get 68-70mph but that would be quite scary on my 1978 cb100n with cg125 engine.
I consider the modifications to have been worthwhile because I can maintain a higher average speed when riding the bike.
In America, according to the tl125 website,
they bore out the crank pin holes then press in offset bushes to lengthen the stroke of the engine.
An aluminium thick washer is fitted between the barrel and crankcase mouth
so that with the extra stroke the piston does not kiss the valves.
I don’t think this mod would make a reliable engine because a bush could come adrift and cause mechanical mayhem.

Someone else emailed me and says

The second line states the cast iron barrel is 71mm OD when it's only 62.5mm OD measured from bottom of barrel.
The correct place to measure is at the bottom, not at the top.

There is a site where you can get CB550 pistons up to 1mm over size page

This will then give you the choice of going up to 59.5mm, 3mm over size.
You can increase the compression by having the head ground or if you wish to keep your head standard, use the
head gasket off a W model (1998) onwards which all use a metal head gasket which is approx. 0.5mm thinner.

I have 2 early cg engines 2 Brazil's, One W model, One M model, One ES4 engine.
All engines to M model (pre 2004) have 15mm gudgeon pin but the ES4 model (2004 on) has a 13mm gudgeon pin
The barrel (M and ES) (2001 onwards) has an oil way in the base, so you cannot use an early barrel on these later engines.
Also the cylinder head has a different shape combustion chamber
and will not fit on early models as the domed piston will hit the cylinder head.

2 oil way holes on the M model 2001 to date Picture
The new style flat top piston M model 2001 to date (please note 13mm gudgeon pin es4 models) Picture
The oil way in bottom of barrel M model 2001 to date Picture
The new style head reshaped to run with flat top piston M model 2001 to date Picture

Please note any engine mods can shorten engine life / if done wrong will break your engine.
Also note all pre 2004 models with drum front brake can be a bit lacking on stopping power,
even with new shoes and properly set up if you are in the 19 stone bracket. It is like riding 2 up on the bike.

I'm in the lucky passion of having 3 cg125 and lots of spares.
To start with I will put an end to some of the myths 1976-80 approx. models did not run bigger valve heads.
They did have a carb with a lot bigger slide which helped with performance at the cost of fuel if ridden flat out all the time.
Fuel would drop to 75-80mpg.

The problem of doing a general guide of how to do mods
and results is Honda over the years has changed air box intake size (more restrictive) and the jet sizes.
Slow running jet 36-40 Main jet 80-110, needle size and slide size.
Standard type carb, then carb with accelerator pump and back to standard carb for ES4 2004 model on.
The compression ratio also changes on years from 9:1-9.5:1.
The 1997-2003 W and M1 models had the most restrictive air box intake.

Mods done to my 1999 cg125 W model.

Cylinder head .010 ground off head, Head polished and ported,
1mm over size piston which gives you 128.5cc. Needle lifted up one notch on slide.
A little harder to kick over. I weigh in at 19 stone so a lighter rider of 12 stone would probably get higher speeds.
After running in the bore will do 70mph/75mph on flat
so if we allow 10% for speedo error would be more realistic 63mph-68mph.

The reason for myself to improve performance was to get a bit more lower down power (torque)
as my 25 mile travel is 30-40mph limits with a couple of 50mph.
As a result of mods I can go down to 20mph in top gear on flat
and pull away up to 40mph at half throttle in approx. 8-10 seconds.
With added bonus of 135mpg+ winter riding, in summer 145mpg and the best ever of 155mpg.
The lowest was 115mpg giving it some right stick.
My carb has slow running jet 36 and main off 82.

Remember any modes to improve performance if ridden hard will shorten engine life and ruin your fuel economy.

You may find this article of interest

Please note the Chinese clone Honda CG125 overbore and big bore kits will not fit on genuine Honda CG125 engines.
I have emailed a few Honda CG owners who were disappointed when they bought their kits and did not fit.

The reason the Chinese CG150cc kit does not fit is the liner is OD 67.5mm (too big)
and the Honda CG crank cases are only OD 64mm (to small).
So to be made to fit would take full engine strip and major machine work to crank case,
and the danger of breaking into one of the oil ways.
Remember this work will not be cheap and not all Honda Cg125 crank case will be able to be machined bigger.
Honda changed the design of crank case over the years.

Please also see Increasing Engine Size

< Performance