Here is another tip.
On all my other motorcycles, I always install the piston on the connecting rod first. Then I lower the cylinder onto the piston, while compressing the piston rings. I tried using this method with the NC250 engine, and it did
not work. The piston rings of the RX3 are
very difficult to compress, and I actually snapped one of the piston rings as I tried to lower the cylinder onto the piston.
Therefore, I ordered another, complete set of piston rings, and used the alternative method of installing the piston into the cylinder first.
Here is a summary of that method.
First of all, install a circlip into one side of the piston. Using this method, you will only need to wrestle with a single circlip when the cylinder is resting over the engine case.
Then install the new piston rings in the piston. Apply motor oil to the piston rings and the piston skirts. I also spread a little oil inside the cylinder.
Turn the cylinder upside down on a workbench, and turn the piston upside down, making sure the arrow on top of the piston is facing the exhaust side of the cylinder. Using this method, it is very easy to compress each piston ring and push the piston into the bottom of the cylinder.
Push the piston into the cylinder, but leave enough room at the bottom to install the piston pin later.
Make sure you install a new cylinder base gasket on the engine crankcase before you install the cylinder/piston assembly.
Take the cylinder/piston assembly to the engine. Tilt the engine to the left, so the cylinder will not easily drop onto the engine case. Now you can install the cylinder/piston assembly over the cylinder bolts, and lower it to the proximity of the connecting rod.
Apply oil to the piston pin, and insert the piston pin through the piston and the connecting rod until it bottoms on the single circlip you installed earlier. Stuff a rag around the connecting rod so you don't accidentally drop the final piston circlip inside the engine.
Install the final circlip. Now you can remove the rag surrounding the connecting rod.
Finally, push the piston farther into the cylinder, and lower the cylinder onto the engine.
This method is
very easy to perform while the engine is removed from the frame, and I might try using this method on my other bikes in the future.