Overhauling the cylinder/piston components
Overhauling the cylinder
Check that the walls of the cylinder are perfectly smooth. Measure the cylinder diameter at 50 mm from the top face and determine the size class to which it belongs in accordance with the values specified in Section "Cylinder/piston".
Repeat measurement of the diameter at three heights "A" (10 mm from the upper surface), "B" (50 mm from the upper surface) and "C" (100 mm from the upper surface) and in two directions at 90° between them; check that the measurements of taper and ovality fall within the range specified in Section "Cylinder/piston".
In the event of damage or excessive wear the cylinder must be replaced as it has a silicon carbide coating (which provides the cylinder walls with excellent anti-friction and anti-wear properties) and therefore cannot be ground.
The cylinders are marked with letters (stamped between the two oil return ways) indicating their size class.
Always match cylinders with pistons from the same size class.
Overhauling the piston
Clean the piston crown and piston ring grooves, removing any carbon deposits.
Visually inspect the piston and check its dimensions carefully: there must be no signs of scuffing, scoring, cracks, or other damage.
The piston diameter must be measured at 7.5 mm up from the bottom of the skirt and in perpendicular direction to the gudgeon pin axis.
The pistons must always be replaced as a pair.
Checking the piston-cylinder clearance
The pistons are marked with a symbol (selected letter (L) punched onto the piston crown) that indicates the size class to which they belong.
Always match cylinders with pistons from the same size class.
(F): arrow that indicates the exhaust side.
For the correct values, refer to chapter "Cylinder/piston".
Overhauling the gudgeon pins
Gudgeon pins must be perfectly smooth without signs of scoring, steps, or blueing due to overheating.
The well-lubricated gudgeon pin must slide smoothly inside the piston without stiffness.
For the coupling clearance values with the piston and the connecting rod, see chapter "Cylinder/Piston".
If a new gudgeon pin is fitted, you must also change the connecting rod small end bush.
Overhauling the piston rings
The piston rings must not show any signs of scuffing or scoring.
Spare pistons are supplied complete with piston rings and gudgeon pin.
Checking the piston ring-grooves clearance
The maximum permissible wear limit is 0.15 mm for the top ring (1st ring "S") and 0.10 mm for the others (2nd ring "T" and oil scraper ring "R").
The piston rings must always be fitted with markings (M) facing upwards.
Checking the piston ring/cylinder clearance
Insert the piston ring 50 mm from the top face of the cylinder; make sure that the ring is positioned perfectly square to the cylinder axis by checking with a gauge at several points around the ring that the top surface of the ring is 50 mm from the cylinder top face.
Measure the piston ring gap (A):
   
Distance (A) mm
 
Wear limit
 
Upper piston ring
 
0.2 to 0.4
 
0.8
 
Intermediate piston ring
 
0.3 to 0.5
 
0.8
 
Oil scraper ring
 
0.2 to 0.7
 
1.0