If you pick up a full-skirted piston from an older air-cooled engine, and measure its diameter in several places and orientations, you will see that pistons are neither round nor straight. Instead they are deliberately made oval so that their diameter is slightly greater front-to-back than it is from side-to-side (parallel with the wrist pin). When setting piston clearance it is normal to measure diameter at the bottom of the piston skirt. But as you measure higher up, you find that the piston tapers inward slightly. And when you reach the piston ring belt—close to the piston crown that is in contact with hot combustion gas—that taper increases a bunch.
Sometimes its pistons would seize. Other times they broke up for no apparent reason.
The reasons for these deviations from being cylindrical are as follows:
- During the power stroke, gas pressure pushes the piston downward but the angle of the connecting rod, transmitting that force to the rotating crankshaft, pushes the piston quite firmly against the rear cylinder wall. Then on compression, the resistance of the fuel-air mixture to being compressed above the piston again acts through the connecting rod, which is now angled to press the piston against the front cylinder wall. But there is no force at all pushing the piston against the sides of the cylinder. Even so, because of the extra mass of the wrist-pin bosses, pistons naturally expand more from side-to-side than they do back-to-front. To compensate for this, pistons are made slightly oval, measuring slightly smaller in diameter side-to-side than back-to-front.
- 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 need 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 clearance to zero, making seizure likely.
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.
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.
When such pistons were heated by combustion, their crowns expanded the most, so in order to avoid seizing in the ring belt (which runs hot because it’s just below the crown) Gilera had to provide ring-belt clearance over the full length of the piston, making it very loose in the bore. If there was enough clearance to prevent seizure in the piston ring belt, the resultant clattering produced cracking and piston breakup. If the clearance were reduced to prevent that, the pistons seized.
The answer was to have its pistons made by a specialist firm that had the dedicated production equipment to make oval pistons with the necessary two-stage taper. Gilera chose Borgo, who remains in the piston business to this day.
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.