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.
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.
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.
Then, remove the camshaft holder block, and then the cylinder head.
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).
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.
After removing and re-installing the chain, I could finally install the Athena cylinder. This is how it looked like.
Also, before mounting the engine cylinder head, I cleaned it a bit since it was very dirty.
This is the engine after completely mounting it (except for the camshaft holder cover, and the flywheel cover).
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.