Aprilia RS 50 Tech Tips
Removing the cylinder head:
With the coolant completely drained and the exhaust removed you are now ready to proceed.
This is really the easy part. Using a set of pliers, remove the OEM hose clamps that go to the large and small hoses attached to the cylinder head (see picture 1). Now, just twist and pull on the hoses until they come off. Some water will drain at this point, which is fine. Next using a proper 13/16” spark plug socket remove the stock spark plug (circled green in picture 1).
Now using a deep-well socket, remove the water spigot, which is where the small coolant hose attached to (see picture 2). If you are using a Top Performance big-bore kit, you will reuse this part, so set it aside. If you are using any other big-bore kit, this will be a leftover part.
Next, using the proper size deep-well socket remove the temperature sending unit from the stock head (circled red in picture 1). The temperature sending unit is the only part of the head that has an electrical wire running to it. Now that the head is bare, you now need to remove the four nuts that hold down the head and cylinder to the crankcase. Note, sometimes the entire stud comes out instead of the nut alone…this is not a big deal, you just have to locate a stud installer tool or use a pair of M7 nuts to re-install the cylinder stud. With the nuts removed you simply pull up on the head and it will come off. Make sure to collect all of the four small and two large o-rings (see picture 3).
Removing the cylinder:
Try to soak up any remaining coolant in the water jacket of the cylinder. You really do not want a ton of water going into the crankcase. Next, pull up on the cylinder itself, it should slide off with some very light effort. Try to pull it up as even and square as possible (see picture 4).
The friction you feel is the piston rings pressing on the bore, which is completely normal. Also, keep a hand underneath to catch the connecting rod, so it does not bang against the crankcase when the cylinder comes off. After the cylinder is off, place a few clean rags into opening in the crankcase. You DO NOT want anything falling into the crankcase.
Removing the piston and wristpin:
The piston in held onto the connecting rod by the wristpin and two small circlips (highlighted in picture 5).
The circlips fit into small grooves in the piston, and keep the wristpin from falling out. Using a pair of needle nose pliers grab the tab on the circlip that retains the wristpin and gently pull on it (see picture 5).
It helps to give the pliers a twisting motion too, as if you are tightening the circlip onto itself. Make sure you have clean rags stuffed in the crankcase, just in case, you drop the circlip. You DO NOT want it to fall into the motor.
With just one circlip removed, you can then use a properly sized socket and extension to push the wristpin out from the other side (see picture 6).
There is a special tool for this, but the wristpins on the RS50 come out very easy. Hold the piton very firmly with one hand while removing the wristpin. You do not want to put any lateral strain on the connecting rod and big-end bearing. The piston will now come off of the connecting rod. You will also notice at this point, the small-end bearing (see picture 7).
You can either replace it at this time or simply re-use it. Finally, make sure to remove all traces of base gasket from the top of the crankcase.
Source: Aprilia Forum
