APRILIA RS 125 : Aprilia RS125 Top end cylinder barrel head and piston rebuild replacement

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Top end removal pictorial (video)

Tools required:

5 and 6mm allen key
11mm ring spanner
13mm spanner
screw driver
stanley knife blade
mallet
needle nose pliers
10mm spanner

Additional key items:

2 stroke oil THE BEST QUALITY
clean rag
New Spark plug

Full Top end rebuild items:

New cylinder or reconditioned cylinder
New piston
New piston rings
New little end bearing
New C clips
Top end gasket set

First off you need to remove the tank , once you have that off, remove the airbox (not easy)this will give little more space to work in you can remove the top engine mount .

Now you have even more room to work in you can go ahead and remove the carb/reed block/manifold assembly in one go, you need to remove the 5x 5mm allen key bolts from the manifold then the whole lot will slide out and can be moved to the side of the bike out of the way , this was you can keep the throttle cable and oi line connected to carb.

undo coolant drain bolt the coolant ,as you will be removing the cylinder head and water jacket and these a filled with coolant and can be messy . There will be a drain screw on the LHS crank case cover, at the front, it will say DRAIN above it.
Once you have drain coolant out , remove the HT lead and the spark plug.

Now remove the 4x5mm water jacket bolts so you can remove the water jacket, which will reveal the cylinder head, there will be 5x13mm bolts, which may be tight to undo, undo these each diagonally opposite to stop any possible cylinder head warping. Dispose of any gaskets or o rings, also clean the surfaces where the gaskets have been very carefully with a Stanley knife blade.

Now you need to remove you PV from the cylinder, or blanking plate if you have a restricted bike (2x5mm allen key bolts).

Now the cylinder itself is ready to be removed ,try to avoid small parts falling into the crank case if you do engine removal and swilling out with old diesel will be required
, first remove the 4 bolts at the base of the cylinder (11mm spanner, also remove them in a diagonal fashion) once they are removed, lift the cylinder off the crank case making sure its going up straight and not at an angle, lift it about 25mm and stuff a rag into the crank case tightly t avoid any debris falling into the crank case.

Once the cylinder is out of the way you can begin to remove the C clips which are are at the side of the piston (set in about 5mm) once these are gone remove the piston pin / gudgeon pin this can be tight so be carful when tapping and removing it brace piston if tapping is required once pin is removed, the piston will come off the con rod and you can remove the little end bearing.

Put a new little end bearing in the conrod (oil it before hand with 2 stroke oil, also oil the top of the con rod, now oil the new wrist/gudgeon/piston pin and place that inside the bearing. Next put the piston rings onto the piston, Every piston ring will have a stamp mark near the ring gap, This MUST be installed facing upwards. Also, locate the "locating pin" in the piston groove. The ring gap will go here, do this for both piston rings.

Now your piston is ready to go on the conrod, there will be an arrow on top of the piston, this faces the exhaust port (forwards facing on most bikes), place the piston over the con rod end and get the piston "holes" inline with the wrist pic which is already installed, once you have it inline place the C clips inside the "holes" so it locks the piston to the conrod and wrist pin, once there in fully make sure the gaps in the clips are not near the little groove or indentation in the wrist pin/piston holes else they may fall out

Oil the piston with 2 stroke oil, especially around the rings. Now get your new cylinder or reconditioned one and lube the bore with 2 stroke. Now that your cylinder is ready place your base gasket onto the crank case cover (i would you the medium thickness one but this can be altered for squish readings when more advanced ) get your cylinder ready , hold the piston rings tight so they compress and slide the cylinder onto the piston, very slowly at first it will be difficult, once you get the rings into the bore it will slide straight on , once its on just work your way from step 6 to 1 in reverse order.