Introduction:
Steps one through five must be done prior to proceeding.  Note, it does not matter if the gearbox oil is drained oil filled.  Begin by laying out all the parts on a clean dry surface (see picture 1). You should have at the very least a big-bore piston,  piston ring(s), a wristpin, two new piston circlips, a new base gasket, and a new head gasket.

Preparing the cylinder head:
Start by preparing the new cylinder head.  First, put a very light coating of gasket sealer on the temperature probe and thread into the new head.  Snug tightly.  If you are using a Top Performance big-bore kit, you need to also install the small water spigot that you removed from the stock cylinder head (circled blue in picture 2).  The next step is installing the original stock thermostat on the new cylinder head.  Simply unscrew the two Phillips screws and reinstall on the new head. 

VERY IMPORTANT....Notice the bleed hole in the flange (see picture 3), locate the thermostat in a position that allows fluid passage.  It will appear like it is only 50% open, and this is perfectly normal.

Preparing the piston and cylinder:
Start by determining which way the ring goes.  Every piston ring will have a stamp mark near the ring gap.  It is usually a stamped "N" or "T".  This MUST be installed facing upwards (see detailed picture 4).  Also, locate the "locating pin" in the piston groove.  The ring gap will go here.

To install the piston ring, slip one end of the ring into the piston groove, and work towards the other end.  The ring gap will be centered over a locating pin in the piston groove.  Try to open the ring as little as possible.  Be very careful during this step.  The ring and piston will eventually be going up and down over 11,000 times per minute.

Now would also be a good time to learn about the arrow mark on the top of the piston.  This always points towards the exhaust port.  Since the exhaust exits out the front on the RS50, the arrow will be facing forward in the bike.

Next, install one circlip into the piston.  It does not really matter which one.

The cylinder preparation consists of installing the exhaust o-ring into the exhaust port (see picture 5).  It rests in a small groove in the exhaust port.  Applying two-stroke oil to the o-ring will aid exhaust installation later.  Lubricate it well.

We also like to coat the cylinder walls with two-stroke oil as well.  Rub some oil all around the walls with your finger.  You can not really over-oil it.

Installing the piston:
Start by thoroughly and heavily oiling the small-end bearing. Also, thoroughly oil the wristpin.  With the piston facing the correct direction (see above), slide the piston over the connecting rod.  Insert the well-lubed wristpin through the piston with your fingers.  Finally, install the second circlip. 

Now is a good time to double check that both circlips are fully seated in their respective grooves in the piston.  You do not want them coming out!!!

Installing the cylinder:
Start by installing the new base gasket. Note, it can only be installed one way.

The next few steps are probably the hardest of all.  It is VERY important that they are done correctly.  The job is easiest, if you place the crank and piston at top dead center (TDC). In other words, pull the piston up as high as it will go out of the crankcase.  The bike must be in neutral.

To start, you are going to have to fully compress the piston ring in order to install the cylinder over it.  You are also going to have to make sure the ring gap is located on the "locating pin" in the piston groove (see close-up picture 6).  Squeeze the piston ring between your thumb and first finger.  With the other hand lower the cylinder over the piston.  Once the piston is inside the cylinder a little way, it should lower with VERY LITTLE effort.  DO NOT force it down!!!  Try to lower the cylinder down evenly and squarely.  If it does not go on easily, one of two things is happening.  Either the ring gap is not centered on the "locating pin", or you are not fully compressing the ring during the cylinder installation process.  This job maybe the easiest with two people.  One person is in charge of compressing the ring and locating the ring gap, while the other person slowly lowers the cylinder onto the studs (see picture 7).

Push the cylinder down fully onto the base gasket.

Installing the cylinder head:
Start by installing all of the head gaskets.  On the Top Performance kit, one small o-ring is installed over each cylinder stud.  The Malossi kit has these small o-rings integrated into the inner head gasket.  It should be obvious how the inner and outer head gaskets are arranged.  Do not forget the outer head o-ring, which seals the coolant inside the cylinder cooling jacket.  Lower the new cylinder head onto the cylinder studs and ensure that all o-rings are seated properly.  Install the washers and the cylinder head nuts (see picture 8).  Initially torque all four nuts to 10Nm in a criss-cross pattern.  Finally, torque them to 14-16Nm.  It is important to torque them in stages like this to insure flat and even pressure.  Install the spark plug and torque to 20-25Nm.  Reinstall the big coolant hose on the top of the head.  Use a new hose clamp.  The small hose comes from the T-fitting on the right side coolant hose.  On Malossi and other kits with no small hose connection on the cylinder head, special steps must be taken.  A straight hose-to-hose connector will have to be obtained from an auto parts in order to remove the T-fitting from that radiator hose.  We do not re-install the hoses for the carburetor manifold plumbing.

Note:
If you do not feel comfortable with these steps, please seek the advise of a professional.  Anyone with 2-stroke dirt bike experience would be more than qualified to help.

Proceed to Step 8: Installing the Carburetor

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