Installing Athena 166cc bore up kit on a Honda CBR125R

Athena Honda CBR125R

In November 2015, I decided to change the original piston and cylinder of my Honda CBR125R. There are some bore up kits on the market, at least two: one sold by Malossi, and one sold by Athena. Unfortunately, in Japan, the Malossi bore up kit is not available, so I chose the one sold by Athena. As you can see in the picture, the kit that I bought does not include the ECU additional module; the reason why I did not buy is that I made it by myself. You can buy the full kit, which includes also the ECU module (the product code is P400210100026), which is compatible with all CBR125R models starting from 2007 until the most recent one (2015); these models have FI (Fuel Injection). Actually, this kit is also compatible with previous models (carburator instead of FI), but you will not need to install the ECU, so I recommed you to buy the cheaper kit which does not include the ECU, unless you need it.

For more info on this kit, you can download these PDF files, which are also available on the Athena website: P400210100026 and P400210100026_EN. The piston diameter is 67mm (original is 58mm), and the compression ratio is 11.4:1 (original is 11:1). The maximum torque is 15Nm at 7170 rpm, and the maximum power is 16.8hp. Torque and power curves are visible in the PDF documents above.

The best way to install the bore up kit, which includes a new cylinder, piston, and other parts, is to remove the engine and bring it in a clean room, where you can sit down and work without any stress. I do not recommend to install the kit directly on the motorcycle (without removing the engine) because the space is not much, and you will have a lot of stress. In addition to that, since you need to remove many engine parts, it is better to perform this in a clean room, slowly, and with the right tools.

First of all, make sure that you have all the tools that you will need, and print the installation manual, which you can download in PDF format: CBR125R_athena_instructions. I also recommend you to print the necessary pages from the Honda CBR125R Service Manual: Honda CBR125R Service Manual. The first step is to remove the engine from the motorcycle frame. It will require 1-2 hours, if it is the first time, since you need first to dry the engine oil and the engine coolant and coolant hoses. Then, you also need to remove the clutch cable, remove the air filter and throttle body, cables, and so on. After removing the engine, your motorcycle will look somehow like this.

Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R

And the engine unmounted will look like this. Be careful to protect the holes with some paper/plastic sheet, so that dirt cannot enter. In particular, you should be careful about the intake hole, exhaust hole, and cooling fluid circuit holes.

Athena Honda CBR125R
Athena Honda CBR125R

Then, bring the engine in your room, where you can work slowly. The engine is not heavy (25kg), so you can easily transport it. First of all, remove the camshaft cover.

Athena Honda CBR125R
Athena Honda CBR125R

Then, remove the camshaft holder block, and then the cylinder head.

Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R

The, you have to remove the cylinder, and finally the piston. In the picture you can see the difference between the original piston (right) and the Athena piston (left).

Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R
Athena Honda CBR125R

From now on, you just need to repeat the reverse procedure: mount the new piston and cylinder, mount the original cylinder head, camshaft holder block and cover. Before doing it, you should also check the valve clearance using a special tool (thickness gauge). Then you mount again the engine on the motorcycle, connect the cables, wires, put the coolant fluid and the engine oil. You should also install the ECU additional module, in case your motorcycle is Fuel Injection type instead of carburetor type (for this, please check the official Installation Instruction provided with the kit).

Unfortunately, when I did the procedure above, the camshaft chain felt inside the crankshaft case (damn!). It was somehow locked inside, I could not remove it without also removing the flywheel. In order to extract the chain from the crankcase, I had to remove the flywheel, and to do that I had to buy a special tool from Rakuten Japan (フライホイールプーラー グロム・MSX125用 STRAIGHT/19-636 (STRAIGHT/ストレート)). It cost me 1,580Yen (12 Euro), and I had to wait few days to receive it at home, so I recommend you to be very careful that the chain does not fall inside the crankshaft.

Athena Honda CBR125R
Athena Honda CBR125R

After removing and re-installing the chain, I could finally install the Athena cylinder. This is how it looked like.

Athena Honda CBR125R
Athena Honda CBR125R

Also, before mounting the engine cylinder head, I cleaned it a bit since it was very dirty.

Athena Honda CBR125R
Athena Honda CBR125R

This is the engine after completely mounting it (except for the camshaft holder cover, and the flywheel cover).

Athena Honda CBR125R
Athena Honda CBR125R

In case you did not have my trouble, it is not necessary to remove the flywheel cover. Unfortunately, I had to do it, due to the chain fallen and locked into the engine basement. So, when I mounted the cover again, I first made the new gasket (you need to buy gasket paper and cut it with scissors to the right shape). It will take a lot of time to do that (30 minutes to 1 hour), so I recommend you to buy the "ready to use" gaskets from the Athena website. But if you are really into DIY, you can still do it by yourself. The following picture shows how the gasket looked like (in the middle of the cutting process - I still needed to completely finish the job, in this picture). I recommend you to take your time when you do this, because if you cut it wrong, the cover will not be completely sealed to the crankcase, and your motorcycle will leak engine oil from the crankcase.

Athena Honda CBR125R
Athena Honda CBR125R

Author: Davide Cavaliere

I am an Italian Electrical Engineer graduated at Politecnico di Milano. My interests are motorcycles and cars, electronics, programming, Internet of Things, and Japanese culture.

12 thoughts on “Installing Athena 166cc bore up kit on a Honda CBR125R”

  1. Hi I am very interested in buying one of these kits myself as I have 2006 cbr repsol 125 and love the little bike it's took me ages too find a review worth reading mine is the carburetted already have an upgraded exhaust system on its the top end speed is between 60-70 as it is and it's boring what were the power gains and top very interested in hearing this I'm in two minds of ordering the kit

    1. Hello Chris,
      Thank you for the comment. If you have the carburetor model, the engine is exactly the same as for the "electronic fuel injection" model (my motorcycle, in the article). However, you should buy the kit without fuel injection controller, since it is a bit cheaper (you do not need the fuel injection electronic control unit for your carburetor model).
      The complete job requires about 1 day, maybe less if you are skilled.
      At the end, you should adapt the carburetor settings by changing the jet size.

  2. Can sum1 help iv inatalled this kit to my cbr 125 r and and the ecu that come with the bike has fueiling issues it cant come its a athena 166cc

    1. When you install a 166cc kit, more air flows into the cylinder, due to the increased amount of volume (166cm3 instead of 125cm3). But, the original ECU, which is a Keihin, does not know about your upgrade, and continues to inject the standard amount of gasoline, by electronic fuel injection. Therefore, the result is that you have a too high amount of air, and therefore the mixture of air and fuel is too lean (too much air compared to fuel).
      You need to install an additional ECU to regulate the fuel to the proper stoichiometric ratio.
      For more info, please have a look at my project, Fuelino. This additional piggyback unit allows you to customize the fuel injection, achieving the best air-fuel ratio.

  3. Howdy, I've been looking at your fuelino project and custom ECU for a bit while now and seriously can't understand a dime about what you are talking about. I'm familiar with electronics but this is way out of my league.
    I bought a CBR 125 2007 with a 166cc Malossi kit pre installed. It started a bit bad and just completely stopped when you werent revving the engine, so I decided to switch the cylinder back to the original 125. But now its getting kinda boring and I wanna change it back. Could you help me with an easier guide or step by step instructions on how to install or adjust the ECU/CDI. I don't really want to buy a new one from their website, as it costs 250 American dollars (I live in Norway but I'm assuming you don't know the currency here).

    One a sidenote worth mentioning: The engine ran perfecty well without the ECU when it was 166cc, but it didn't get enogh fuel on lower RPM, do you maybe think it doesn't need programming, maybe increasing the intake valve will help it stay alive? It doesn't need to go perfect, it just needs to work and not die instantly when i stop revving.

    Help would be much appriciated, friend 🙂

  4. Is it worth the money and effort? I can do the job myself, but is it any downside reguarding cost, quality and performance? Best regards from Thom

  5. Hi there, just wondering what code is this as i cant find a 2007 - 2010 only with ecu and you use stock valve head ? .... thanks

  6. hi I have ordered this kit i will be installing soon the only thing I am concerned about is no one had said about how to run the new cylinder and piston if you could let me know would be very grateful as this will be the first time i run in a new cylinder

    1. This page shows how to mount it. After that, remember that you need an electronic control unit to modify the injection signal. You can create a Fuelino by yourself, or buy a Power Commander or similar ECUs for higher price.

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