The result was successful, as the tapered pistons had enough clearance near the top to avoid seizure there, but with smaller clearance and reduced taper along the skirt. This guided the piston closely, not letting it hammer itself in its bore. When warmed up, such tapered and oval pistons assumed a cylindrical shape that resulted in a large area of intimate contact with the cylindrical bore, providing best piston cooling.
When you reach the piston ring belt close to the piston crown that is in contact with hot combustion gasthat taper increases a bunch.
Rihanna
Why? Gilera at the time was trying to manufacture everything in-house, including pistons. But the company lacked the special tooling required to provide the tapered-and-oval shape described above. That being so, it made its pistons as simple lathe projects: truly round and without taper.
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An easy-to-access air filter sits above the engine
The need for this complex shape was made clear by the mystery piston troubles Gilera had in 1947 with its first post–World War II 500-four roadrace engine. Sometimes its pistons would seize. Other times they broke up for no apparent reason.
Because the piston’s crown is heated by combustion, that is its hottest part and expands the most, and so requires the greatest clearance. The lower parts of the piston, being farther from the heat, run cooler and so expand less, requiring less clearance. As a result, the piston is tapered—largest at the bottom of the skirt and smallest in the ring belt.
This needs for taper is magnified in the ring belt, where a lot of clearance is required to keep that part of the piston from expanding enough to reduce the clearance to zero, making seizure likely.