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Honda CG125 but also some universal motorcycle information

Honda CG125 Increasing Engine Size

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Please also see How to Increase Engine Size

History of the Engine

The Honda CG125 is 124cc in engine size.

It's an Overhead Valve design (push rod) from 1976 to 2008.

It's basically a Honda CB125 engine, except it is Overhead Valve instead of Overhead Cam.

An Overhead Cam is more complicated and needs more servicing (servicing is critical) but gives better engine performance.

The Overhead Valve system is needed for the third world,
who demanded easy servicing (also survive if engine never serviced), easy to repair and cheap.

The Honda CB125 and Honda CG125 engines are used in other Honda motorcycles (not sure about 2009 models onwards).
As a general rule, as long as it's a Honda Motorcycle (not sure about scooters),
4 stroke, Air Cooled (not liquid cooled) & single cylinder,
if it says overhead valve (OHV) it's the CG125 engine, if it says overhead cam (OHC) it's the CB125.

The 125cc engine size history is complicated,
a long time ago the learner laws of the UK and several other countries was 250cc.
250cc engines over time increased in power output, until they were too powerful for the UK and the several other countries,
so they changed the law to 125cc.

Back then 125cc was an uncommon size, but due to the law,
125cc engines became common in the countries with the 125cc learner law.
In countries without the learner law, 125cc has never caught on (except third world countries that use whatever is cheapest).
Before the 125cc learner law 150cc was common.

Engine manufacturers hate to build and design new engines, so they normally just convert the engine to the size required.

Reducing the engine size is the easiest and safest since the engine will have less power;
this means less strain on all the other components of the engine and bike.
All you have to do to reduce the engine size,
is put a sleeve or larger sleeve in the cylinder and change the piston to a smaller one.
You can also change the crank so the piston does not move as far up and down the cylinder (called the Stroke).

Manufacturers often increase the engine size above the original design of the engine and this normally has side effects.
The side effects often include,
increased vibration and components not lasting as long (they may change or improve some components to compensate).
To increase the engine size you need to bore out the cylinder (barrel) or change the sleeve or change the cylinder
(the size is called the Bore).
Sometimes they are able to change the crank so the piston moves further up and down the cylinder (called the Stroke).

All Honda CG125 & Honda CB125 engines have always been (1976 to 2008)
Bore x Stroke 56.5 x 49.5 mm

I then wondered if there was a Honda CB150 in the past and found there was,
the Honda CB125 maybe a converted Honda CB150.

That made me wonder if there are still Honda CB or CG150 being made today, this is what I have found.

In brazil (Portuguese language) were the latest Honda CG125 are made,
they also put a 150cc Overhead Cam engine in developing just over 13hp (125cc = 11hp) and called it the Honda CG150.
Its Bore x Stroke is 57.3 x 57.8 mm, 149.2 cc, 9.5 :1 compression, peak power 8000 rpm, peak torque 6500 rpm.
From 2009 onwards,
Brazil have continued to make the Honda CG125
but it looks like it's probably using the same 125cc engine as the India Honda CBF125 (see below).
So the Honda CG150 and CG125 currently being made in Brazil
should be called CB150 and CB125 since they are overhead cam engines.
I guess the CG name is so big in Brazil they did not want to lose it.
I assume the old Overhead Valve engine was abandoned due to emission laws.

India has started making the next generation of Honda third world bikes;
its first is the Unicorn.
It's a bit heavy, but notice the mono shock suspension,
fuel economy we can only dream of, Overhead Cam, 150cc engine etc.
It looks like it's called a Honda CBF150 in other countries.
The engine may or may not be totally different to the Honda CG and CB engines.
I would not be surprised if it's the same engine as the 150cc Brazil Honda CG150 (see above).

India has also released a Honda CBF125 also called the Honda Stunner (in 2008), it has an Overhead Cam.
It has been converted to fuel injection for the UK and European market
to replace the Honda CG125 due to the Euro 3 emission law in 2009.
The Honda CBF125 Bore and Stroke is 52.4 x 57.8mm which is totally different sizes to the CG125 and CB125 engines.
The Honda CBF125 engine may or may not be a totally different engine to the CG125 and CB125 engines.
But notice the Honda CBF125 Stroke is identical to the Brazil Honda CG150 mentioned above.

Engine performance is not only about power output.

You also need to look at the torque output; this is the amount of pulling power,
very important when going uphill or head wind or heavy loads.
The larger the engine size the more torque output you will get.

You also want to look at the state of tune, you can tune an engine to give maximum power output (high state of tune),
but the engine will only produce that power at the top of the rev range
and below that the power and torque output will be very low.
The Honda CG125 is in a low state of tune, this means the maximum power output is low,
but that means you will get far more power and torque in the low to mid-range of the rev band
than a 125cc engine in a high state of tune.

Please also see How to Increase Engine Size

< Performance